i3bar: Add protocol for workspace buttonsi3bar-ws-protocol

Closes #3818 (parent issue)
Fixes #1808
Fixes #2333
Fixes #2617
Fixes #3548

25 files changed, 886 insertions, 292 deletions

diff --git a/docs/i3bar-workspace-protocol b/docs/i3bar-workspace-protocol
new file mode 100644
index 00000000..31acfd90
--- /dev/null
+++ b/docs/i3bar-workspace-protocol
@@ -0,0 +1,184 @@
+i3bar workspace buttons protocol
+================================
+
+This document explains the protocol in which i3bar expects input for
+configuring workspace buttons. This feature is a available since i3 version
+4.23.
+
+Programs defined by the +workspace_command+ configuration option for i3bar can
+modify the workspace buttons displayed by i3bar. The command should constantly
+print in its standard output a stream of messages following the protocol
+defined in this page.
+
+If you are looking for the status line protocol instead, see https://i3wm.org/docs/i3bar-protocol.html.
+
+== The protocol
+
+Each message should be a newline-delimited JSON array. The array is in the same
+format as the +GET_WORKSPACES+ ipc event, see
+https://i3wm.org/docs/ipc.html#_workspaces_reply.
+
+As an example, this is the output of the +i3-msg -t get_workspaces+ command:
+------------------------------
+[
+  {
+    "id": 94131549984064,
+    "num": 1,
+    "name": "1",
+    "visible": false,
+    "focused": false,
+    "output": "HDMI-A-0",
+    "urgent": false
+  },
+  {
+    "id": 94131550477584,
+    "num": 2,
+    "name": "2",
+    "visible": true,
+    "focused": true,
+    "output": "HDMI-A-0",
+    "urgent": false
+  },
+  {
+    "id": 94131550452704,
+    "num": 3,
+    "name": "3:some workspace",
+    "visible": false,
+    "focused": false,
+    "output": "HDMI-A-0",
+    "urgent": false
+  }
+]
+------------------------------
+
+Please note that this example was pretty printed for human consumption, with
+the +"rect"+ field removed. Workspace button commands should output each array
+in one line.
+
+Each element in the array represents a workspace. i3bar creates one workspace
+button for each element in the array. The order of these buttons is the same as
+the order of the elements in the array.
+
+In general, we recommend subscribing to the +workspace+ and +output+
+https://i3wm.org/docs/ipc.html#_workspace_event[events],
+fetching the current workspace information with +GET_WORKSPACES+, modifying the
+JSON array in the response according to your needs and then printing it to the
+standard output. However, you are free to build a new message from the ground
+up.
+
+=== Workspace objects in detail
+
+The documentation of +GET_WORKSPACES+ should be sufficient to understand the
+meaning of each property but here we provide extra notes for each property and
+its meaning with respect to i3bar.
+
+All properties but +name+ are optional.
+
+id (integer)::
+    If it is included it will be used to switch to that workspace when you
+    click the corresponding button. If it's not provided, the +name+ will be
+    used. You can use the +id+ field to present workspaces under a modified
+    name.
+num (integer)::
+    The only use of a workspace's number is if the +strip_workspace_numbers+
+    setting is enabled.
+name (string)::
+    The only required property. If an +id+ is provided you can freely change
+    the +name+ as you wish, effectively renaming the buttons of i3bar.
+visible (boolean)::
+    Defaults to +true+ if not included.
+focused (boolean)::
+    Defaults to +false+ if not included. Generally, exactly one of the
+    workspaces should be +focused+. If not, no button will have the
+    +focused_workspace+ color.
+urgent (boolean)::
+    Defaults to +false+ if not included.
+rect (map)::
+    Not used by i3bar but will be ignored.
+output (string)::
+    Defaults to the primary output if not included.
+
+== Examples
+
+These example scripts require the https://stedolan.github.io/jq/[jq] utility to
+be installed but otherwise just use the standard +i3-msg+ utility included with
+i3. However, you can write your own scripts in your preferred language, with
+the help of one of the
+https://i3wm.org/docs/ipc.html#_see_also_existing_libraries[pre-existing i3
+libraries]
+
+=== Base configuration
+
+------------------------------
+bar {
+  …
+  workspace_command /path/to/your/script.sh
+  …
+}
+------------------------------
+
+=== Re-create the default behaviour of i3bar
+
+Not very useful by itself but this will be the basic building block of all the
+following scripts. This one does not require +jq+.
+
+------------------------------
+#!/bin/sh
+i3-msg -t subscribe -m '["workspace", "output"]' | {
+    # Initially print the current workspaces before we receive any events. This
+    # avoids having an empty bar when starting up.
+    i3-msg -t get_workspaces;
+    # Then, while we receive events, update the workspace information.
+    while read; do i3-msg -t get_workspaces; done;
+}
+------------------------------
+
+=== Hide workspace named +foo+ unless if it is focused.
+
+------------------------------
+#!/bin/sh
+i3-msg -t subscribe -m '["workspace", "output"]' | {
+    i3-msg -t get_workspaces;
+    while read; do i3-msg -t get_workspaces; done;
+} | jq --unbuffered -c '[ .[] | select(.name != "foo" or .focused) ]'
+------------------------------
+
+Important! Make sure you use the +--unbuffered+ flag with +jq+, otherwise you
+might not get the changes in real-time but whenever they are flushed, which
+might mean that you are getting an empty bar until enough events are written.
+
+=== Show empty workspaces +foo+ and +bar+ on LVDS1 even if they do not exist at the moment.
+
+------------------------------
+#!/bin/sh
+i3-msg -t subscribe -m '["workspace", "output"]' | {
+    i3-msg -t get_workspaces;
+    while read; do i3-msg -t get_workspaces; done;
+} | jq --unbuffered -c '
+    def fake_ws(name): {
+        name: name,
+        output: "LVDS1",
+    };
+    . + [ fake_ws("foo"), fake_ws("bar") ] | unique_by(.name)
+'
+------------------------------
+
+=== Sort workspaces in reverse alphanumeric order
+
+------------------------------
+#!/bin/sh
+i3-msg -t subscribe -m '["workspace", "output"]' | {
+    i3-msg -t get_workspaces;
+    while read; do i3-msg -t get_workspaces; done;
+} | jq --unbuffered -c 'sort_by(.name) | reverse'
+------------------------------
+
+=== Append "foo" to the name of each workspace
+
+------------------------------
+#!/bin/sh
+i3-msg -t subscribe -m '["workspace", "output"]' | {
+    i3-msg -t get_workspaces;
+    while read; do i3-msg -t get_workspaces; done;
+} | jq --unbuffered -c '[ .[] | .name |= . + " foo" ]'
+------------------------------
diff --git a/docs/userguide b/docs/userguide
index 7fe0b9af..1bc5c73b 100644
--- a/docs/userguide
+++ b/docs/userguide
@@ -1611,6 +1611,30 @@ bar {
 }
 -------------------------------------------------
 
+[[workspace_command]]
+=== Workspace buttons command
+
+Since i3 4.23, i3bar can run a program and use its +stdout+ output to define
+the workspace buttons displayed on the left hand side of the bar. With this
+feature, you can, for example, rename the buttons of workspaces, hide specific
+workspaces, always show a workspace button even if the workspace does not exist
+or change the order of the buttons.
+
+Also see <<status_command>> for the statusline option and
+https://i3wm.org/docs/i3bar-workspace-protocol.html for the detailed protocol.
+
+*Syntax*:
+------------------------
+workspace_command <command>
+------------------------
+
+*Example*:
+-------------------------------------------------
+bar {
+    workspace_command /path/to/script.sh
+}
+-------------------------------------------------
+
 === Display mode
 
 You can either have i3bar be visible permanently at one edge of the screen
diff --git a/i3bar/include/child.h b/i3bar/include/child.h
index ae523bc0..e77b51e3 100644
--- a/i3bar/include/child.h
+++ b/i3bar/include/child.h
@@ -11,7 +11,7 @@
 
 #include <config.h>
 
-#include <stdbool.h>
+#include <ev.h>
 
 #define STDIN_CHUNK_SIZE 1024
 
@@ -40,6 +40,18 @@ typedef struct {
      */
     bool click_events;
     bool click_events_init;
+
+    /**
+     * stdin- and SIGCHLD-watchers
+     */
+    ev_io *stdin_io;
+    ev_child *child_sig;
+    int stdin_fd;
+
+    /**
+     * Line read from child that did not include a newline character.
+     */
+    char *pending_line;
 } i3bar_child;
 
 /*
@@ -50,36 +62,66 @@ void clear_statusline(struct statusline_head *head, bool free_resources);
 
 /*
  * Start a child process with the specified command and reroute stdin.
- * We actually start a $SHELL to execute the command so we don't have to care
- * about arguments and such
+ * We actually start a shell to execute the command so we don't have to care
+ * about arguments and such.
+ *
+ * If `command' is NULL, such as in the case when no `status_command' is given
+ * in the bar config, no child will be started.
  *
  */
 void start_child(char *command);
 
 /*
+ * Same as start_child but starts the configured client that manages workspace
+ * buttons.
+ *
+ */
+void start_ws_child(char *command);
+
+/*
+ * Returns true if the status child process is alive.
+ *
+ */
+bool status_child_is_alive(void);
+
+/*
+ * Returns true if the workspace child process is alive.
+ *
+ */
+bool ws_child_is_alive(void);
+
+/*
  * kill()s the child process (if any). Called when exit()ing.
  *
  */
-void kill_child_at_exit(void);
+void kill_children_at_exit(void);
 
 /*
- * kill()s the child process (if any) and closes and
- * free()s the stdin- and SIGCHLD-watchers
+ * kill()s the child process (if any) and closes and free()s the stdin- and
+ * SIGCHLD-watchers
  *
  */
 void kill_child(void);
 
 /*
+ * kill()s the workspace child process (if any) and closes and free()s the
+ * stdin- and SIGCHLD-watchers.
+ * Similar to kill_child.
+ *
+ */
+void kill_ws_child(void);
+
+/*
  * Sends a SIGSTOP to the child process (if existent)
  *
  */
-void stop_child(void);
+void stop_children(void);
 
 /*
  * Sends a SIGCONT to the child process (if existent)
  *
  */
-void cont_child(void);
+void cont_children(void);
 
 /*
  * Whether or not the child want click events
@@ -92,3 +134,14 @@ bool child_want_click_events(void);
  *
  */
 void send_block_clicked(int button, const char *name, const char *instance, int x, int y, int x_rel, int y_rel, int out_x, int out_y, int width, int height, int mods);
+
+/*
+ * When workspace_command is enabled this function is used to re-parse the
+ * latest received JSON from the client.
+ */
+void repeat_last_ws_json(void);
+
+/*
+ * Replaces the workspace buttons with an error message.
+ */
+void set_workspace_button_error(const char *message);
diff --git a/i3bar/include/configuration.h b/i3bar/include/configuration.h
index 24079c5d..c9bae7c3 100644
--- a/i3bar/include/configuration.h
+++ b/i3bar/include/configuration.h
@@ -62,6 +62,7 @@ typedef struct config_t {
     bool strip_ws_name;
     char *bar_id;
     char *command;
+    char *workspace_command;
     char *fontname;
     i3String *separator_symbol;
     TAILQ_HEAD(tray_outputs_head, tray_output_t) tray_outputs;
@@ -79,17 +80,17 @@ typedef struct config_t {
 extern config_t config;
 
 /**
- * Start parsing the received bar configuration JSON string
+ * Parse the received bar configuration JSON string
  *
  */
-void parse_config_json(char *json);
+void parse_config_json(const unsigned char *json, size_t size);
 
 /**
- * Start parsing the received bar configuration list. The only usecase right
- * now is to automatically get the first bar id.
+ * Parse the received bar configuration list. The only usecase right now is to
+ * automatically get the first bar id.
  *
  */
-void parse_get_first_i3bar_config(char *json);
+void parse_get_first_i3bar_config(const unsigned char *json, size_t size);
 
 /**
  * free()s the color strings as soon as they are not needed anymore.
diff --git a/i3bar/include/mode.h b/i3bar/include/mode.h
index e8e4296d..4646b9f4 100644
--- a/i3bar/include/mode.h
+++ b/i3bar/include/mode.h
@@ -24,7 +24,7 @@ struct mode {
 typedef struct mode mode;
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_mode_json(char *json);
+void parse_mode_json(const unsigned char *json, size_t size);
diff --git a/i3bar/include/outputs.h b/i3bar/include/outputs.h
index 4685e51e..560abe53 100644
--- a/i3bar/include/outputs.h
+++ b/i3bar/include/outputs.h
@@ -22,10 +22,10 @@ SLIST_HEAD(outputs_head, i3_output);
 extern struct outputs_head* outputs;
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_outputs_json(char* json);
+void parse_outputs_json(const unsigned char* json, size_t size);
 
 /*
  * Initiate the outputs list
diff --git a/i3bar/include/workspaces.h b/i3bar/include/workspaces.h
index ff61450c..6c8e7145 100644
--- a/i3bar/include/workspaces.h
+++ b/i3bar/include/workspaces.h
@@ -18,10 +18,10 @@ typedef struct i3_ws i3_ws;
 TAILQ_HEAD(ws_head, i3_ws);
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_workspaces_json(char *json);
+void parse_workspaces_json(const unsigned char *json, size_t size);
 
 /*
  * free() all workspace data structures
@@ -38,7 +38,6 @@ struct i3_ws {
     bool visible;             /* If the ws is currently visible on an output */
     bool focused;             /* If the ws is currently focused */
     bool urgent;              /* If the urgent hint of the ws is set */
-    rect rect;                /* The rect of the ws (not used (yet)) */
     struct i3_output *output; /* The current output of the ws */
 
     TAILQ_ENTRY(i3_ws) tailq; /* Pointer for the TAILQ-Macro */
diff --git a/i3bar/src/child.c b/i3bar/src/child.c
index df4c6601..20858f68 100644
--- a/i3bar/src/child.c
+++ b/i3bar/src/child.c
@@ -10,6 +10,7 @@
 #include "common.h"
 #include "yajl_utils.h"
 
+#include <ctype.h> /* isspace */
 #include <err.h>
 #include <errno.h>
 #include <ev.h>
@@ -27,14 +28,30 @@
 #include <yajl/yajl_parse.h>
 
 /* Global variables for child_*() */
-i3bar_child child = {0};
-#define DLOG_CHILD DLOG("%s: pid=%ld stopped=%d stop_signal=%d cont_signal=%d click_events=%d click_events_init=%d\n", \
-                        __func__, (long)child.pid, child.stopped, child.stop_signal, child.cont_signal, child.click_events, child.click_events_init)
-
-/* stdin- and SIGCHLD-watchers */
-ev_io *stdin_io;
-int stdin_fd;
-ev_child *child_sig;
+i3bar_child status_child = {0};
+i3bar_child ws_child = {0};
+
+#define DLOG_CHILD(c)                                                                                                              \
+    do {                                                                                                                           \
+        if ((c).pid == 0) {                                                                                                        \
+            DLOG("%s: child pid = 0\n", __func__);                                                                                 \
+        } else if ((c).pid == status_child.pid) {                                                                                  \
+            DLOG("%s: status_command: pid=%ld stopped=%d stop_signal=%d cont_signal=%d click_events=%d click_events_init=%d\n",    \
+                 __func__, (long)(c).pid, (c).stopped, (c).stop_signal, (c).cont_signal, (c).click_events, (c).click_events_init); \
+        } else if ((c).pid == ws_child.pid) {                                                                                      \
+            DLOG("%s: workspace_command: pid=%ld stopped=%d stop_signal=%d cont_signal=%d click_events=%d click_events_init=%d\n", \
+                 __func__, (long)(c).pid, (c).stopped, (c).stop_signal, (c).cont_signal, (c).click_events, (c).click_events_init); \
+        } else {                                                                                                                   \
+            ELOG("%s: unknown child, this should never happen "                                                                    \
+                 "pid=%ld stopped=%d stop_signal=%d cont_signal=%d click_events=%d click_events_init=%d\n",                        \
+                 __func__, (long)(c).pid, (c).stopped, (c).stop_signal, (c).cont_signal, (c).click_events, (c).click_events_init); \
+        }                                                                                                                          \
+    } while (0)
+#define DLOG_CHILDREN             \
+    do {                          \
+        DLOG_CHILD(status_child); \
+        DLOG_CHILD(ws_child);     \
+    } while (0)
 
 /* JSON parser for stdin */
 yajl_handle parser;
@@ -127,7 +144,7 @@ __attribute__((format(printf, 1, 2))) static void set_statusline_error(const cha
     TAILQ_INSERT_TAIL(&statusline_head, message_block, blocks);
 
 finish:
-    FREE(message);
+    free(message);
     va_end(args);
 }
 
@@ -135,22 +152,27 @@ finish:
  * Stop and free() the stdin- and SIGCHLD-watchers
  *
  */
-static void cleanup(void) {
-    if (stdin_io != NULL) {
-        ev_io_stop(main_loop, stdin_io);
-        FREE(stdin_io);
-        close(stdin_fd);
-        stdin_fd = 0;
-        close(child_stdin);
-        child_stdin = 0;
+static void cleanup(i3bar_child *c) {
+    DLOG_CHILD(*c);
+
+    if (c->stdin_io != NULL) {
+        ev_io_stop(main_loop, c->stdin_io);
+        FREE(c->stdin_io);
+
+        if (c->pid == status_child.pid) {
+            close(child_stdin);
+            child_stdin = 0;
+        }
+        close(c->stdin_fd);
     }
 
-    if (child_sig != NULL) {
-        ev_child_stop(main_loop, child_sig);
-        FREE(child_sig);
+    if (c->child_sig != NULL) {
+        ev_child_stop(main_loop, c->child_sig);
+        FREE(c->child_sig);
     }
 
-    memset(&child, 0, sizeof(i3bar_child));
+    FREE(c->pending_line);
+    memset(c, 0, sizeof(i3bar_child));
 }
 
 /*
@@ -362,15 +384,13 @@ static int stdin_end_array(void *context) {
  * Returns NULL on EOF.
  *
  */
-static unsigned char *get_buffer(ev_io *watcher, int *ret_buffer_len) {
-    int fd = watcher->fd;
-    int n = 0;
+static unsigned char *get_buffer(int fd, int *ret_buffer_len) {
     int rec = 0;
     int buffer_len = STDIN_CHUNK_SIZE;
     unsigned char *buffer = smalloc(buffer_len + 1);
     buffer[0] = '\0';
     while (1) {
-        n = read(fd, buffer + rec, buffer_len - rec);
+        const ssize_t n = read(fd, buffer + rec, buffer_len - rec);
         if (n == -1) {
             if (errno == EAGAIN) {
                 /* finish up */
@@ -390,10 +410,11 @@ static unsigned char *get_buffer(ev_io *watcher, int *ret_buffer_len) {
 
         if (rec == buffer_len) {
             buffer_len += STDIN_CHUNK_SIZE;
-            buffer = srealloc(buffer, buffer_len);
+            buffer = srealloc(buffer, buffer_len + 1);
         }
     }
-    if (*buffer == '\0') {
+    buffer[rec] = '\0';
+    if (buffer[0] == '\0') {
         FREE(buffer);
         rec = -1;
     }
@@ -443,13 +464,14 @@ static bool read_json_input(unsigned char *input, int length) {
  * in statusline
  *
  */
-static void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+static void stdin_io_cb(int fd) {
     int rec;
-    unsigned char *buffer = get_buffer(watcher, &rec);
-    if (buffer == NULL)
+    unsigned char *buffer = get_buffer(fd, &rec);
+    if (buffer == NULL) {
         return;
+    }
     bool has_urgent = false;
-    if (child.version > 0) {
+    if (status_child.version > 0) {
         has_urgent = read_json_input(buffer, rec);
     } else {
         read_flat_input((char *)buffer, rec);
@@ -463,22 +485,23 @@ static void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
  * whether this is JSON or plain text
  *
  */
-static void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+static void stdin_io_first_line_cb(int fd) {
     int rec;
-    unsigned char *buffer = get_buffer(watcher, &rec);
-    if (buffer == NULL)
+    unsigned char *buffer = get_buffer(fd, &rec);
+    if (buffer == NULL) {
         return;
+    }
     DLOG("Detecting input type based on buffer *%.*s*\n", rec, buffer);
     /* Detect whether this is JSON or plain text. */
     unsigned int consumed = 0;
     /* At the moment, we don’t care for the version. This might change
      * in the future, but for now, we just discard it. */
-    parse_json_header(&child, buffer, rec, &consumed);
-    if (child.version > 0) {
-        /* If hide-on-modifier is set, we start of by sending the
-         * child a SIGSTOP, because the bars aren't mapped at start */
+    parse_json_header(&status_child, buffer, rec, &consumed);
+    if (status_child.version > 0) {
+        /* If hide-on-modifier is set, we start of by sending the status_child
+         * a SIGSTOP, because the bars aren't mapped at start */
         if (config.hide_on_modifier) {
-            stop_child();
+            stop_children();
         }
         draw_bars(read_json_input(buffer + consumed, rec - consumed));
     } else {
@@ -489,9 +512,133 @@ static void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int rev
         read_flat_input((char *)buffer, rec);
     }
     free(buffer);
-    ev_io_stop(main_loop, stdin_io);
-    ev_io_init(stdin_io, &stdin_io_cb, stdin_fd, EV_READ);
-    ev_io_start(main_loop, stdin_io);
+}
+
+static bool isempty(char *s) {
+    while (*s != '\0') {
+        if (!isspace(*s)) {
+            return false;
+        }
+        s++;
+    }
+    return true;
+}
+
+static char *append_string(const char *previous, const char *str) {
+    if (previous != NULL) {
+        char *result;
+        sasprintf(&result, "%s%s", previous, str);
+        return result;
+    }
+    return sstrdup(str);
+}
+
+static char *ws_last_json;
+
+static void ws_stdin_io_cb(int fd) {
+    int rec;
+    unsigned char *buffer = get_buffer(fd, &rec);
+    if (buffer == NULL) {
+        return;
+    }
+
+    gchar **strings = g_strsplit((const char *)buffer, "\n", 0);
+    for (int idx = 0; strings[idx] != NULL; idx++) {
+        if (ws_child.pending_line == NULL && isempty(strings[idx])) {
+            /* In the normal case where the buffer ends with '\n', the last
+             * string should be empty */
+            continue;
+        }
+
+        if (strings[idx + 1] == NULL) {
+            /* This is the last string but it is not empty, meaning that we have
+             * read data that is incomplete, save it for later. */
+            char *new = append_string(ws_child.pending_line, strings[idx]);
+            free(ws_child.pending_line);
+            ws_child.pending_line = new;
+            continue;
+        }
+
+        free(ws_last_json);
+        ws_last_json = append_string(ws_child.pending_line, strings[idx]);
+        FREE(ws_child.pending_line);
+
+        parse_workspaces_json((const unsigned char *)ws_last_json, strlen(ws_last_json));
+    }
+
+    g_strfreev(strings);
+    free(buffer);
+
+    draw_bars(false);
+}
+
+static void common_stdin_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+    if (watcher == status_child.stdin_io) {
+        if (status_child.version == (uint32_t)-1) {
+            stdin_io_first_line_cb(watcher->fd);
+        } else {
+            stdin_io_cb(watcher->fd);
+        }
+    } else if (watcher == ws_child.stdin_io) {
+        ws_stdin_io_cb(watcher->fd);
+    } else {
+        ELOG("Got callback for unknown watcher fd=%d\n", watcher->fd);
+    }
+}
+
+/*
+ * When workspace_command is enabled this function is used to re-parse the
+ * latest received JSON from the client.
+ */
+void repeat_last_ws_json(void) {
+    if (ws_last_json) {
+        DLOG("Repeating last workspace JSON\n");
+        parse_workspaces_json((const unsigned char *)ws_last_json, strlen(ws_last_json));
+    }
+}
+
+/*
+ * Wrapper around set_workspace_button_error to mimic the call of
+ * set_statusline_error.
+ */
+__attribute__((format(printf, 1, 2))) static void set_workspace_button_error_f(const char *format, ...) {
+    char *message;
+    va_list args;
+    va_start(args, format);
+    if (vasprintf(&message, format, args) == -1) {
+        goto finish;
+    }
+
+    set_workspace_button_error(message);
+
+finish:
+    free(message);
+    va_end(args);
+}
+
+/*
+ * Replaces the workspace buttons with an error message.
+ */
+void set_workspace_button_error(const char *message) {
+    free_workspaces();
+
+    char *name = NULL;
+    sasprintf(&name, "Error: %s", message);
+
+    i3_output *output;
+    SLIST_FOREACH (output, outputs, slist) {
+        i3_ws *fake_ws = scalloc(1, sizeof(i3_ws));
+        /* Don't set the canonical_name field to make this workspace unfocusable. */
+        fake_ws->name = i3string_from_utf8(name);
+        fake_ws->name_width = predict_text_width(fake_ws->name);
+        fake_ws->num = -1;
+        fake_ws->urgent = fake_ws->visible = true;
+        fake_ws->output = output;
+
+        TAILQ_INSERT_TAIL(output->workspaces, fake_ws, tailq);
+    }
+
+    free(name);
 }
 
 /*
@@ -501,27 +648,45 @@ static void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int rev
  *
  */
 static void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
-    int exit_status = WEXITSTATUS(watcher->rstatus);
+    const int exit_status = WEXITSTATUS(watcher->rstatus);
 
     ELOG("Child (pid: %d) unexpectedly exited with status %d\n",
-         child.pid,
+         watcher->pid,
          exit_status);
 
+    void (*error_function_pointer)(const char *, ...) = NULL;
+    const char *command_type = "";
+    i3bar_child *c = NULL;
+    if (watcher->pid == status_child.pid) {
+        command_type = "status_command";
+        error_function_pointer = set_statusline_error;
+        c = &status_child;
+    } else if (watcher->pid == ws_child.pid) {
+        command_type = "workspace_command";
+        error_function_pointer = set_workspace_button_error_f;
+        c = &ws_child;
+    } else {
+        ELOG("Unknown child pid, this should never happen\n");
+        return;
+    }
+    DLOG_CHILD(*c);
+
     /* this error is most likely caused by a user giving a nonexecutable or
      * nonexistent file, so we will handle those cases separately. */
-    if (exit_status == 126)
-        set_statusline_error("status_command is not executable (exit %d)", exit_status);
-    else if (exit_status == 127)
-        set_statusline_error("status_command not found or is missing a library dependency (exit %d)", exit_status);
-    else
-        set_statusline_error("status_command process exited unexpectedly (exit %d)", exit_status);
+    if (exit_status == 126) {
+        error_function_pointer("%s is not executable (exit %d)", command_type, exit_status);
+    } else if (exit_status == 127) {
+        error_function_pointer("%s not found or is missing a library dependency (exit %d)", command_type, exit_status);
+    } else {
+        error_function_pointer("%s process exited unexpectedly (exit %d)", command_type, exit_status);
+    }
 
-    cleanup();
+    cleanup(c);
     draw_bars(false);
 }
 
 static void child_write_output(void) {
-    if (child.click_events) {
+    if (status_child.click_events) {
         const unsigned char *output;
         size_t size;
         ssize_t n;
@@ -535,7 +700,7 @@ static void child_write_output(void) {
         yajl_gen_clear(gen);
 
         if (n == -1) {
-            child.click_events = false;
+            status_child.click_events = false;
             kill_child();
             set_statusline_error("child_write_output failed");
             draw_bars(false);
@@ -543,6 +708,41 @@ static void child_write_output(void) {
     }
 }
 
+static pid_t sfork(void) {
+    const pid_t pid = fork();
+    if (pid == -1) {
+        ELOG("Couldn't fork(): %s\n", strerror(errno));
+        exit(EXIT_FAILURE);
+    }
+    return pid;
+}
+
+static void spipe(int pipedes[2]) {
+    if (pipe(pipedes) == -1) {
+        err(EXIT_FAILURE, "pipe(pipe_in)");
+    }
+}
+
+static void exec_shell(char *command) {
+    execl(_PATH_BSHELL, _PATH_BSHELL, "-c", command, (char *)NULL);
+}
+
+static void setup_child_cb(i3bar_child *child) {
+    /* We set O_NONBLOCK because blocking is evil in event-driven software */
+    fcntl(child->stdin_fd, F_SETFL, O_NONBLOCK);
+
+    child->stdin_io = smalloc(sizeof(ev_io));
+    ev_io_init(child->stdin_io, &common_stdin_cb, child->stdin_fd, EV_READ);
+    ev_io_start(main_loop, child->stdin_io);
+
+    /* We must cleanup, if the child unexpectedly terminates */
+    child->child_sig = smalloc(sizeof(ev_child));
+    ev_child_init(child->child_sig, &child_sig_cb, child->pid, 0);
+    ev_child_start(main_loop, child->child_sig);
+
+    DLOG_CHILD(*child);
+}
+
 /*
  * Start a child process with the specified command and reroute stdin.
  * We actually start a shell to execute the command so we don't have to care
@@ -553,8 +753,9 @@ static void child_write_output(void) {
  *
  */
 void start_child(char *command) {
-    if (command == NULL)
+    if (command == NULL) {
         return;
+    }
 
     /* Allocate a yajl parser which will be used to parse stdin. */
     static yajl_callbacks callbacks = {
@@ -568,69 +769,77 @@ void start_child(char *command) {
         .yajl_end_array = stdin_end_array,
     };
     parser = yajl_alloc(&callbacks, NULL, &parser_context);
-
     gen = yajl_gen_alloc(NULL);
 
     int pipe_in[2];  /* pipe we read from */
     int pipe_out[2]; /* pipe we write to */
+    spipe(pipe_in);
+    spipe(pipe_out);
 
-    if (pipe(pipe_in) == -1)
-        err(EXIT_FAILURE, "pipe(pipe_in)");
-    if (pipe(pipe_out) == -1)
-        err(EXIT_FAILURE, "pipe(pipe_out)");
-
-    child.pid = fork();
-    switch (child.pid) {
-        case -1:
-            ELOG("Couldn't fork(): %s\n", strerror(errno));
-            exit(EXIT_FAILURE);
-        case 0:
-            /* Child-process. Reroute streams and start shell */
-
-            close(pipe_in[0]);
-            close(pipe_out[1]);
+    status_child.pid = sfork();
+    if (status_child.pid == 0) {
+        /* Child-process. Reroute streams and start shell */
+        close(pipe_in[0]);
+        close(pipe_out[1]);
 
-            dup2(pipe_in[1], STDOUT_FILENO);
-            dup2(pipe_out[0], STDIN_FILENO);
+        dup2(pipe_in[1], STDOUT_FILENO);
+        dup2(pipe_out[0], STDIN_FILENO);
 
-            setpgid(child.pid, 0);
-            execl(_PATH_BSHELL, _PATH_BSHELL, "-c", command, (char *)NULL);
-            return;
-        default:
-            /* Parent-process. Reroute streams */
+        setpgid(status_child.pid, 0);
+        exec_shell(command);
+        return;
+    }
+    /* Parent-process. Reroute streams */
+    close(pipe_in[1]);
+    close(pipe_out[0]);
 
-            close(pipe_in[1]);
-            close(pipe_out[0]);
+    status_child.stdin_fd = pipe_in[0];
+    child_stdin = pipe_out[1];
+    status_child.version = -1;
 
-            stdin_fd = pipe_in[0];
-            child_stdin = pipe_out[1];
+    setup_child_cb(&status_child);
+}
 
-            break;
+/*
+ * Same as start_child but starts the configured client that manages workspace
+ * buttons.
+ *
+ */
+void start_ws_child(char *command) {
+    if (command == NULL) {
+        return;
     }
 
-    /* We set O_NONBLOCK because blocking is evil in event-driven software */
-    fcntl(stdin_fd, F_SETFL, O_NONBLOCK);
+    ws_child.stop_signal = SIGSTOP;
+    ws_child.cont_signal = SIGCONT;
 
-    stdin_io = smalloc(sizeof(ev_io));
-    ev_io_init(stdin_io, &stdin_io_first_line_cb, stdin_fd, EV_READ);
-    ev_io_start(main_loop, stdin_io);
+    int pipe_in[2]; /* pipe we read from */
+    spipe(pipe_in);
 
-    /* We must cleanup, if the child unexpectedly terminates */
-    child_sig = smalloc(sizeof(ev_child));
-    ev_child_init(child_sig, &child_sig_cb, child.pid, 0);
-    ev_child_start(main_loop, child_sig);
+    ws_child.pid = sfork();
+    if (ws_child.pid == 0) {
+        /* Child-process. Reroute streams and start shell */
+        close(pipe_in[0]);
+        dup2(pipe_in[1], STDOUT_FILENO);
 
-    atexit(kill_child_at_exit);
-    DLOG_CHILD;
+        setpgid(ws_child.pid, 0);
+        exec_shell(command);
+        return;
+    }
+    /* Parent-process. Reroute streams */
+    close(pipe_in[1]);
+    ws_child.stdin_fd = pipe_in[0];
+
+    setup_child_cb(&ws_child);
 }
 
 static void child_click_events_initialize(void) {
-    DLOG_CHILD;
+    DLOG_CHILD(status_child);
 
-    if (!child.click_events_init) {
+    if (!status_child.click_events_init) {
         yajl_gen_array_open(gen);
         child_write_output();
-        child.click_events_init = true;
+        status_child.click_events_init = true;
     }
 }
 
@@ -639,7 +848,7 @@ static void child_click_events_initialize(void) {
  *
  */
 void send_block_clicked(int button, const char *name, const char *instance, int x, int y, int x_rel, int y_rel, int out_x, int out_y, int width, int height, int mods) {
-    if (!child.click_events) {
+    if (!status_child.click_events) {
         return;
     }
 
@@ -706,35 +915,85 @@ void send_block_clicked(int button, const char *name, const char *instance, int
     child_write_output();
 }
 
+static bool is_alive(i3bar_child *c) {
+    return c->pid > 0;
+}
+
+/*
+ * Returns true if the status child process is alive.
+ *
+ */
+bool status_child_is_alive(void) {
+    return is_alive(&status_child);
+}
+
+/*
+ * Returns true if the workspace child process is alive.
+ *
+ */
+bool ws_child_is_alive(void) {
+    return is_alive(&ws_child);
+}
+
 /*
  * kill()s the child process (if any). Called when exit()ing.
  *
  */
-void kill_child_at_exit(void) {
-    DLOG_CHILD;
+void kill_children_at_exit(void) {
+    DLOG_CHILDREN;
+    cont_children();
 
-    if (child.pid > 0) {
-        if (child.cont_signal > 0 && child.stopped)
-            killpg(child.pid, child.cont_signal);
-        killpg(child.pid, SIGTERM);
+    if (is_alive(&status_child)) {
+        killpg(status_child.pid, SIGTERM);
+    }
+    if (is_alive(&ws_child)) {
+        killpg(ws_child.pid, SIGTERM);
     }
 }
 
+static void cont_child(i3bar_child *c) {
+    if (is_alive(c) && c->cont_signal > 0 && c->stopped) {
+        c->stopped = false;
+        killpg(c->pid, c->cont_signal);
+    }
+}
+
+static void kill_and_wait(i3bar_child *c) {
+    DLOG_CHILD(*c);
+    if (!is_alive(c)) {
+        return;
+    }
+
+    cont_child(c);
+    killpg(c->pid, SIGTERM);
+    int status;
+    waitpid(c->pid, &status, 0);
+    cleanup(c);
+}
+
 /*
- * kill()s the child process (if existent) and closes and
- * free()s the stdin- and SIGCHLD-watchers
+ * kill()s the child process (if any) and closes and free()s the stdin- and
+ * SIGCHLD-watchers
  *
  */
 void kill_child(void) {
-    DLOG_CHILD;
+    kill_and_wait(&status_child);
+}
 
-    if (child.pid > 0) {
-        if (child.cont_signal > 0 && child.stopped)
-            killpg(child.pid, child.cont_signal);
-        killpg(child.pid, SIGTERM);
-        int status;
-        waitpid(child.pid, &status, 0);
-        cleanup();
+/*
+ * kill()s the workspace child process (if any) and closes and free()s the
+ * stdin- and SIGCHLD-watchers.
+ * Similar to kill_child.
+ *
+ */
+void kill_ws_child(void) {
+    kill_and_wait(&ws_child);
+}
+
+static void stop_child(i3bar_child *c) {
+    if (c->stop_signal > 0 && !c->stopped) {
+        c->stopped = true;
+        killpg(c->pid, c->stop_signal);
     }
 }
 
@@ -742,26 +1001,21 @@ void kill_child(void) {
  * Sends a SIGSTOP to the child process (if existent)
  *
  */
-void stop_child(void) {
-    DLOG_CHILD;
-
-    if (child.stop_signal > 0 && !child.stopped) {
-        child.stopped = true;
-        killpg(child.pid, child.stop_signal);
-    }
+void stop_children(void) {
+    DLOG_CHILDREN;
+    stop_child(&status_child);
+    stop_child(&ws_child);
 }
 
 /*
  * Sends a SIGCONT to the child process (if existent)
  *
  */
-void cont_child(void) {
-    DLOG_CHILD;
+void cont_children(void) {
+    DLOG_CHILDREN;
 
-    if (child.cont_signal > 0 && child.stopped) {
-        child.stopped = false;
-        killpg(child.pid, child.cont_signal);
-    }
+    cont_child(&status_child);
+    cont_child(&ws_child);
 }
 
 /*
@@ -769,5 +1023,5 @@ void cont_child(void) {
  *
  */
 bool child_want_click_events(void) {
-    return child.click_events;
+    return status_child.click_events;
 }
diff --git a/i3bar/src/config.c b/i3bar/src/config.c
index ccea937d..cebd5d5d 100644
--- a/i3bar/src/config.c
+++ b/i3bar/src/config.c
@@ -188,11 +188,17 @@ static int config_string_cb(void *params_, const unsigned char *val, size_t _len
     }
 
     if (!strcmp(cur_key, "status_command")) {
-        DLOG("command = %.*s\n", len, val);
+        DLOG("status_command = %.*s\n", len, val);
         sasprintf(&config.command, "%.*s", len, val);
         return 1;
     }
 
+    if (!strcmp(cur_key, "workspace_command")) {
+        DLOG("workspace_command = %.*s\n", len, val);
+        sasprintf(&config.workspace_command, "%.*s", len, val);
+        return 1;
+    }
+
     if (!strcmp(cur_key, "font")) {
         DLOG("font = %.*s\n", len, val);
         FREE(config.fontname);
@@ -396,16 +402,15 @@ static yajl_callbacks outputs_callbacks = {
 };
 
 /*
- * Start parsing the received bar configuration JSON string
+ * Parse the received bar configuration JSON string
  *
  */
-void parse_config_json(char *json) {
-    yajl_handle handle = yajl_alloc(&outputs_callbacks, NULL, NULL);
-
+void parse_config_json(const unsigned char *json, size_t size) {
     TAILQ_INIT(&(config.bindings));
     TAILQ_INIT(&(config.tray_outputs));
 
-    yajl_status state = yajl_parse(handle, (const unsigned char *)json, strlen(json));
+    yajl_handle handle = yajl_alloc(&outputs_callbacks, NULL, NULL);
+    yajl_status state = yajl_parse(handle, json, size);
 
     /* FIXME: Proper error handling for JSON parsing */
     switch (state) {
@@ -418,6 +423,11 @@ void parse_config_json(char *json) {
             break;
     }
 
+    if (config.disable_ws && config.workspace_command) {
+        ELOG("You have specified 'workspace_buttons no'. Your 'workspace_command %s' will be ignored.\n", config.workspace_command);
+        FREE(config.workspace_command);
+    }
+
     yajl_free(handle);
 }
 
@@ -427,16 +437,16 @@ static int i3bar_config_string_cb(void *params_, const unsigned char *val, size_
 }
 
 /*
- * Start parsing the received bar configuration list. The only usecase right
- * now is to automatically get the first bar id.
+ * Parse the received bar configuration list. The only usecase right now is to
+ * automatically get the first bar id.
  *
  */
-void parse_get_first_i3bar_config(char *json) {
+void parse_get_first_i3bar_config(const unsigned char *json, size_t size) {
     yajl_callbacks configs_callbacks = {
         .yajl_string = i3bar_config_string_cb,
     };
     yajl_handle handle = yajl_alloc(&configs_callbacks, NULL, NULL);
-    yajl_parse(handle, (const unsigned char *)json, strlen(json));
+    yajl_parse(handle, json, size);
     yajl_free(handle);
 }
 
diff --git a/i3bar/src/ipc.c b/i3bar/src/ipc.c
index 3ab4738c..95130209 100644
--- a/i3bar/src/ipc.c
+++ b/i3bar/src/ipc.c
@@ -24,14 +24,24 @@ ev_io *i3_connection;
 
 const char *sock_path;
 
-typedef void (*handler_t)(char *);
+typedef void (*handler_t)(const unsigned char *, size_t);
+
+/*
+ * Returns true when i3bar is configured to read workspace information from i3
+ * via JSON over the i3 IPC interface, as opposed to reading workspace
+ * information from the workspace_command via JSON over stdout.
+ *
+ */
+static bool i3_provides_workspaces(void) {
+    return !config.disable_ws && config.workspace_command == NULL;
+}
 
 /*
  * Called, when we get a reply to a command from i3.
  * Since i3 does not give us much feedback on commands, we do not much
  *
  */
-static void got_command_reply(char *reply) {
+static void got_command_reply(const unsigned char *reply, size_t size) {
     /* TODO: Error handling for command replies */
 }
 
@@ -39,9 +49,9 @@ static void got_command_reply(char *reply) {
  * Called, when we get a reply with workspaces data
  *
  */
-static void got_workspace_reply(char *reply) {
+static void got_workspace_reply(const unsigned char *reply, size_t size) {
     DLOG("Got workspace data!\n");
-    parse_workspaces_json(reply);
+    parse_workspaces_json(reply, size);
     draw_bars(false);
 }
 
@@ -50,7 +60,7 @@ static void got_workspace_reply(char *reply) {
  * Since i3 does not give us much feedback on commands, we do not much
  *
  */
-static void got_subscribe_reply(char *reply) {
+static void got_subscribe_reply(const unsigned char *reply, size_t size) {
     DLOG("Got subscribe reply: %s\n", reply);
     /* TODO: Error handling for subscribe commands */
 }
@@ -59,12 +69,12 @@ static void got_subscribe_reply(char *reply) {
  * Called, when we get a reply with outputs data
  *
  */
-static void got_output_reply(char *reply) {
+static void got_output_reply(const unsigned char *reply, size_t size) {
     DLOG("Clearing old output configuration...\n");
     free_outputs();
 
     DLOG("Parsing outputs JSON...\n");
-    parse_outputs_json(reply);
+    parse_outputs_json(reply, size);
     DLOG("Reconfiguring windows...\n");
     reconfig_windows(false);
 
@@ -73,8 +83,19 @@ static void got_output_reply(char *reply) {
         kick_tray_clients(o_walk);
     }
 
-    if (!config.disable_ws) {
+    if (i3_provides_workspaces()) {
         i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_WORKSPACES, NULL);
+    } else if (config.workspace_command) {
+        /* Communication with the workspace child is one-way. Since we called
+         * free_outputs() and free_workspaces() we have lost our workspace
+         * information which will result in no workspace buttons. A
+         * well-behaving client should be subscribed to output events as well
+         * and re-send the output information to i3bar. Even in that case
+         * though there is a race condition where the child can send the new
+         * workspace information after the output change before i3bar receives
+         * the output event from i3. For this reason, we re-parse the latest
+         * received JSON. */
+        repeat_last_ws_json();
     }
 
     draw_bars(false);
@@ -84,10 +105,10 @@ static void got_output_reply(char *reply) {
  * Called when we get the configuration for our bar instance
  *
  */
-static void got_bar_config(char *reply) {
+static void got_bar_config(const unsigned char *reply, size_t size) {
     if (!config.bar_id) {
         DLOG("Received bar list \"%s\"\n", reply);
-        parse_get_first_i3bar_config(reply);
+        parse_get_first_i3bar_config(reply, size);
 
         if (!config.bar_id) {
             ELOG("No bar configuration found, please configure a bar block in your i3 config file.\n");
@@ -106,13 +127,14 @@ static void got_bar_config(char *reply) {
     i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_OUTPUTS, NULL);
 
     free_colors(&(config.colors));
-    parse_config_json(reply);
+    parse_config_json(reply, size);
 
     /* Now we can actually use 'config', so let's subscribe to the appropriate
      * events and request the workspaces if necessary. */
     subscribe_events();
-    if (!config.disable_ws)
+    if (i3_provides_workspaces()) {
         i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_WORKSPACES, NULL);
+    }
 
     /* Initialize the rest of XCB */
     init_xcb_late(config.fontname);
@@ -121,6 +143,7 @@ static void got_bar_config(char *reply) {
     init_colors(&(config.colors));
 
     start_child(config.command);
+    start_ws_child(config.workspace_command);
 }
 
 /* Data structure to easily call the reply handlers later */
@@ -143,7 +166,7 @@ handler_t reply_handlers[] = {
  * Called, when a workspace event arrives (i.e. the user changed the workspace)
  *
  */
-static void got_workspace_event(char *event) {
+static void got_workspace_event(const unsigned char *event, size_t size) {
     DLOG("Got workspace event!\n");
     i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_WORKSPACES, NULL);
 }
@@ -152,7 +175,7 @@ static void got_workspace_event(char *event) {
  * Called, when an output event arrives (i.e. the screen configuration changed)
  *
  */
-static void got_output_event(char *event) {
+static void got_output_event(const unsigned char *event, size_t size) {
     DLOG("Got output event!\n");
     i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_OUTPUTS, NULL);
 }
@@ -161,9 +184,9 @@ static void got_output_event(char *event) {
  * Called, when a mode event arrives (i3 changed binding mode).
  *
  */
-static void got_mode_event(char *event) {
+static void got_mode_event(const unsigned char *event, size_t size) {
     DLOG("Got mode event!\n");
-    parse_mode_json(event);
+    parse_mode_json(event, size);
     draw_bars(false);
 }
 
@@ -183,11 +206,11 @@ static bool strings_differ(char *a, char *b) {
  * Called, when a barconfig_update event arrives (i.e. i3 changed the bar hidden_state or mode)
  *
  */
-static void got_bar_config_update(char *event) {
+static void got_bar_config_update(const unsigned char *event, size_t size) {
     /* check whether this affect this bar instance by checking the bar_id */
     char *expected_id;
     sasprintf(&expected_id, "\"id\":\"%s\"", config.bar_id);
-    char *found_id = strstr(event, expected_id);
+    char *found_id = strstr((const char *)event, expected_id);
     FREE(expected_id);
     if (found_id == NULL)
         return;
@@ -201,10 +224,12 @@ static void got_bar_config_update(char *event) {
     DLOG("Received bar config update \"%s\"\n", event);
 
     char *old_command = config.command;
+    char *old_workspace_command = config.workspace_command;
     config.command = NULL;
+    config.workspace_command = NULL;
     bar_display_mode_t old_mode = config.hide_on_modifier;
 
-    parse_config_json(event);
+    parse_config_json(event, size);
     if (old_mode != config.hide_on_modifier) {
         reconfig_windows(true);
     }
@@ -214,13 +239,21 @@ static void got_bar_config_update(char *event) {
     init_colors(&(config.colors));
 
     /* restart status command process */
-    if (strings_differ(old_command, config.command)) {
+    if (!status_child_is_alive() || strings_differ(old_command, config.command)) {
         kill_child();
         clear_statusline(&statusline_head, true);
         start_child(config.command);
     }
     free(old_command);
 
+    /* restart workspace command process */
+    if (!ws_child_is_alive() || strings_differ(old_workspace_command, config.workspace_command)) {
+        free_workspaces();
+        kill_ws_child();
+        start_ws_child(config.workspace_command);
+    }
+    free(old_workspace_command);
+
     draw_bars(false);
 }
 
@@ -284,7 +317,7 @@ static void got_data(struct ev_loop *loop, ev_io *watcher, int events) {
 
     /* Now that we know, what to expect, we can start read()ing the rest
      * of the message */
-    char *buffer = smalloc(size + 1);
+    unsigned char *buffer = smalloc(size + 1);
     rec = 0;
 
     while (rec < size) {
@@ -304,10 +337,11 @@ static void got_data(struct ev_loop *loop, ev_io *watcher, int events) {
     /* And call the callback (indexed by the type) */
     if (type & (1UL << 31)) {
         type ^= 1UL << 31;
-        event_handlers[type](buffer);
+        event_handlers[type](buffer, size);
     } else {
-        if (reply_handlers[type])
-            reply_handlers[type](buffer);
+        if (reply_handlers[type]) {
+            reply_handlers[type](buffer, size);
+        }
     }
 
     FREE(header);
@@ -377,9 +411,9 @@ void destroy_connection(void) {
  *
  */
 void subscribe_events(void) {
-    if (config.disable_ws) {
-        i3_send_msg(I3_IPC_MESSAGE_TYPE_SUBSCRIBE, "[ \"output\", \"mode\", \"barconfig_update\" ]");
-    } else {
+    if (i3_provides_workspaces()) {
         i3_send_msg(I3_IPC_MESSAGE_TYPE_SUBSCRIBE, "[ \"workspace\", \"output\", \"mode\", \"barconfig_update\" ]");
+    } else {
+        i3_send_msg(I3_IPC_MESSAGE_TYPE_SUBSCRIBE, "[ \"output\", \"mode\", \"barconfig_update\" ]");
     }
 }
diff --git a/i3bar/src/main.c b/i3bar/src/main.c
index 4e93bb02..ce5257bf 100644
--- a/i3bar/src/main.c
+++ b/i3bar/src/main.c
@@ -185,12 +185,12 @@ int main(int argc, char **argv) {
     ev_signal_start(main_loop, sig_int);
     ev_signal_start(main_loop, sig_hup);
 
+    atexit(kill_children_at_exit);
+
     /* From here on everything should run smooth for itself, just start listening for
      * events. We stop simply stop the event loop, when we are finished */
     ev_loop(main_loop, 0);
 
-    kill_child();
-
     clean_xcb();
     ev_default_destroy();
 
diff --git a/i3bar/src/mode.c b/i3bar/src/mode.c
index 13d02110..aea43ab4 100644
--- a/i3bar/src/mode.c
+++ b/i3bar/src/mode.c
@@ -16,7 +16,6 @@
 
 /* A datatype to pass through the callbacks to save the state */
 struct mode_json_params {
-    char *json;
     char *cur_key;
     char *name;
     bool pango_markup;
@@ -96,26 +95,17 @@ static yajl_callbacks mode_callbacks = {
 };
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_mode_json(char *json) {
-    /* FIXME: Fasciliate stream processing, i.e. allow starting to interpret
-     * JSON in chunks */
+void parse_mode_json(const unsigned char *json, size_t size) {
     struct mode_json_params params;
-
     mode binding;
-
     params.cur_key = NULL;
-    params.json = json;
     params.mode = &binding;
 
-    yajl_handle handle;
-    yajl_status state;
-
-    handle = yajl_alloc(&mode_callbacks, NULL, (void *)&params);
-
-    state = yajl_parse(handle, (const unsigned char *)json, strlen(json));
+    yajl_handle handle = yajl_alloc(&mode_callbacks, NULL, (void *)&params);
+    yajl_status state = yajl_parse(handle, json, size);
 
     /* FIXME: Proper error handling for JSON parsing */
     switch (state) {
diff --git a/i3bar/src/outputs.c b/i3bar/src/outputs.c
index 168f3eef..5aca53cd 100644
--- a/i3bar/src/outputs.c
+++ b/i3bar/src/outputs.c
@@ -18,10 +18,8 @@
 
 /* A datatype to pass through the callbacks to save the state */
 struct outputs_json_params {
-    struct outputs_head *outputs;
     i3_output *outputs_walk;
     char *cur_key;
-    char *json;
     bool in_rect;
 };
 
@@ -263,21 +261,17 @@ void init_outputs(void) {
 }
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_outputs_json(char *json) {
+void parse_outputs_json(const unsigned char *json, size_t size) {
     struct outputs_json_params params;
     params.outputs_walk = NULL;
     params.cur_key = NULL;
-    params.json = json;
     params.in_rect = false;
 
-    yajl_handle handle;
-    yajl_status state;
-    handle = yajl_alloc(&outputs_callbacks, NULL, (void *)&params);
-
-    state = yajl_parse(handle, (const unsigned char *)json, strlen(json));
+    yajl_handle handle = yajl_alloc(&outputs_callbacks, NULL, (void *)&params);
+    yajl_status state = yajl_parse(handle, json, size);
 
     /* FIXME: Proper errorhandling for JSON-parsing */
     switch (state) {
@@ -291,6 +285,7 @@ void parse_outputs_json(char *json) {
     }
 
     yajl_free(handle);
+    free(params.cur_key);
 }
 
 /*
@@ -319,12 +314,14 @@ void free_outputs(void) {
  *
  */
 i3_output *get_output_by_name(char *name) {
-    i3_output *walk;
     if (name == NULL) {
         return NULL;
     }
+    const bool is_primary = !strcasecmp(name, "primary");
+
+    i3_output *walk;
     SLIST_FOREACH (walk, outputs, slist) {
-        if (!strcmp(walk->name, name)) {
+        if ((is_primary && walk->primary) || !strcmp(walk->name, name)) {
             break;
         }
     }
diff --git a/i3bar/src/workspaces.c b/i3bar/src/workspaces.c
index bd56f5d0..10c9fcf0 100644
--- a/i3bar/src/workspaces.c
+++ b/i3bar/src/workspaces.c
@@ -19,7 +19,8 @@ struct workspaces_json_params {
     struct ws_head *workspaces;
     i3_ws *workspaces_walk;
     char *cur_key;
-    char *json;
+    bool need_output;
+    bool parsing_rect;
 };
 
 /*
@@ -71,26 +72,23 @@ static int workspaces_integer_cb(void *params_, long long val) {
         return 1;
     }
 
+    /* rect is unused, so we don't bother to save it */
     if (!strcmp(params->cur_key, "x")) {
-        params->workspaces_walk->rect.x = (int)val;
         FREE(params->cur_key);
         return 1;
     }
 
     if (!strcmp(params->cur_key, "y")) {
-        params->workspaces_walk->rect.y = (int)val;
         FREE(params->cur_key);
         return 1;
     }
 
     if (!strcmp(params->cur_key, "width")) {
-        params->workspaces_walk->rect.w = (int)val;
         FREE(params->cur_key);
         return 1;
     }
 
     if (!strcmp(params->cur_key, "height")) {
-        params->workspaces_walk->rect.h = (int)val;
         FREE(params->cur_key);
         return 1;
     }
@@ -156,15 +154,16 @@ static int workspaces_string_cb(void *params_, const unsigned char *val, size_t
         sasprintf(&output_name, "%.*s", len, val);
 
         i3_output *target = get_output_by_name(output_name);
+        i3_ws *ws = params->workspaces_walk;
         if (target != NULL) {
-            params->workspaces_walk->output = target;
-
-            TAILQ_INSERT_TAIL(params->workspaces_walk->output->workspaces,
-                              params->workspaces_walk,
-                              tailq);
+            ws->output = target;
+            TAILQ_INSERT_TAIL(ws->output->workspaces, ws, tailq);
         }
 
+        params->need_output = false;
         FREE(output_name);
+        FREE(params->cur_key);
+
         return 1;
     }
 
@@ -172,28 +171,42 @@ static int workspaces_string_cb(void *params_, const unsigned char *val, size_t
 }
 
 /*
- * We hit the start of a JSON map (rect or a new output)
+ * We hit the start of a JSON map (rect or a new workspace)
  *
  */
 static int workspaces_start_map_cb(void *params_) {
     struct workspaces_json_params *params = (struct workspaces_json_params *)params_;
 
-    i3_ws *new_workspace = NULL;
-
     if (params->cur_key == NULL) {
-        new_workspace = smalloc(sizeof(i3_ws));
+        i3_ws *new_workspace = scalloc(1, sizeof(i3_ws));
         new_workspace->num = -1;
-        new_workspace->name = NULL;
-        new_workspace->visible = 0;
-        new_workspace->focused = 0;
-        new_workspace->urgent = 0;
-        memset(&new_workspace->rect, 0, sizeof(rect));
-        new_workspace->output = NULL;
 
         params->workspaces_walk = new_workspace;
+        params->need_output = true;
+        params->parsing_rect = false;
+    } else {
+        params->parsing_rect = true;
+    }
+
+    return 1;
+}
+
+static int workspaces_end_map_cb(void *params_) {
+    struct workspaces_json_params *params = (struct workspaces_json_params *)params_;
+    i3_ws *ws = params->workspaces_walk;
+    const bool parsing_rect = params->parsing_rect;
+    params->parsing_rect = false;
+
+    if (parsing_rect || !ws || !ws->name || !params->need_output) {
         return 1;
     }
 
+    ws->output = get_output_by_name("primary");
+    if (ws->output == NULL) {
+        ws->output = SLIST_FIRST(outputs);
+    }
+    TAILQ_INSERT_TAIL(ws->output->workspaces, ws, tailq);
+
     return 1;
 }
 
@@ -216,43 +229,42 @@ static yajl_callbacks workspaces_callbacks = {
     .yajl_integer = workspaces_integer_cb,
     .yajl_string = workspaces_string_cb,
     .yajl_start_map = workspaces_start_map_cb,
+    .yajl_end_map = workspaces_end_map_cb,
     .yajl_map_key = workspaces_map_key_cb,
 };
 
 /*
- * Start parsing the received JSON string
+ * Parse the received JSON string
  *
  */
-void parse_workspaces_json(char *json) {
-    /* FIXME: Fasciliate stream processing, i.e. allow starting to interpret
-     * JSON in chunks */
-    struct workspaces_json_params params;
-
+void parse_workspaces_json(const unsigned char *json, size_t size) {
     free_workspaces();
 
-    params.workspaces_walk = NULL;
-    params.cur_key = NULL;
-    params.json = json;
-
-    yajl_handle handle;
-    yajl_status state;
-    handle = yajl_alloc(&workspaces_callbacks, NULL, (void *)&params);
-
-    state = yajl_parse(handle, (const unsigned char *)json, strlen(json));
+    struct workspaces_json_params params = {0};
+    yajl_handle handle = yajl_alloc(&workspaces_callbacks, NULL, (void *)&params);
+    yajl_status state = yajl_parse(handle, json, size);
 
     /* FIXME: Proper error handling for JSON parsing */
     switch (state) {
         case yajl_status_ok:
             break;
         case yajl_status_client_canceled:
-        case yajl_status_error:
-            ELOG("Could not parse workspaces reply!\n");
-            exit(EXIT_FAILURE);
+        case yajl_status_error: {
+            unsigned char *err = yajl_get_error(handle, 1, json, size);
+            ELOG("Could not parse workspaces reply, error:\n%s\njson:---%s---\n", err, json);
+            yajl_free_error(handle, err);
+
+            if (config.workspace_command) {
+                kill_ws_child();
+                set_workspace_button_error("Could not parse workspace_command's JSON");
+            } else {
+                exit(EXIT_FAILURE);
+            }
             break;
+        }
     }
 
     yajl_free(handle);
-
     FREE(params.cur_key);
 }
 
@@ -261,14 +273,14 @@ void parse_workspaces_json(char *json) {
  *
  */
 void free_workspaces(void) {
-    i3_output *outputs_walk;
     if (outputs == NULL) {
         return;
     }
-    i3_ws *ws_walk;
 
+    i3_output *outputs_walk;
     SLIST_FOREACH (outputs_walk, outputs, slist) {
         if (outputs_walk->workspaces != NULL && !TAILQ_EMPTY(outputs_walk->workspaces)) {
+            i3_ws *ws_walk;
             TAILQ_FOREACH (ws_walk, outputs_walk->workspaces, tailq) {
                 I3STRING_FREE(ws_walk->name);
                 FREE(ws_walk->canonical_name);
diff --git a/i3bar/src/xcb.c b/i3bar/src/xcb.c
index 0cda125c..4ff44d27 100644
--- a/i3bar/src/xcb.c
+++ b/i3bar/src/xcb.c
@@ -334,7 +334,7 @@ static void hide_bars(void) {
         }
         xcb_unmap_window(xcb_connection, walk->bar.id);
     }
-    stop_child();
+    stop_children();
 }
 
 /*
@@ -351,7 +351,7 @@ static void unhide_bars(void) {
     uint32_t mask;
     uint32_t values[5];
 
-    cont_child();
+    cont_children();
 
     SLIST_FOREACH (walk, outputs, slist) {
         if (walk->bar.id == XCB_NONE) {
@@ -500,6 +500,49 @@ static int predict_button_width(int name_width) {
                logical_px(config.ws_min_width));
 }
 
+static char *quote_workspace_name(const char *in) {
+    /* To properly handle workspace names with double quotes in them, we need
+     * to escape the double quotes. We allocate a large enough buffer (twice
+     * the unescaped size is always enough), then we copy character by
+     * character. */
+    const size_t namelen = strlen(in);
+    const size_t len = namelen + strlen("workspace \"\"") + 1;
+    char *out = scalloc(2 * len, 1);
+    memcpy(out, "workspace \"", strlen("workspace \""));
+    size_t inpos, outpos;
+    for (inpos = 0, outpos = strlen("workspace \"");
+         inpos < namelen;
+         inpos++, outpos++) {
+        if (in[inpos] == '"' || in[inpos] == '\\') {
+            out[outpos] = '\\';
+            outpos++;
+        }
+        out[outpos] = in[inpos];
+    }
+    out[outpos] = '"';
+    return out;
+}
+
+static void focus_workspace(i3_ws *ws) {
+    char *buffer = NULL;
+    if (ws->id != 0) {
+        /* Workspace ID has higher precedence since the workspace_command is
+         * allowed to change workspace names as long as it provides a valid ID. */
+        sasprintf(&buffer, "[con_id=%lld] focus workspace", ws->id);
+        goto done;
+    }
+
+    if (ws->canonical_name == NULL) {
+        return;
+    }
+
+    buffer = quote_workspace_name(ws->canonical_name);
+
+done:
+    i3_send_msg(I3_IPC_MESSAGE_TYPE_RUN_COMMAND, buffer);
+    free(buffer);
+}
+
 /*
  * Handle a button press event (i.e. a mouse click on one of our bars).
  * We determine, whether the click occurred on a workspace button or if the scroll-
@@ -620,37 +663,7 @@ static void handle_button(xcb_button_press_event_t *event) {
             return;
     }
 
-    /* To properly handle workspace names with double quotes in them, we need
-     * to escape the double quotes. Unfortunately, that’s rather ugly in C: We
-     * first count the number of double quotes, then we allocate a large enough
-     * buffer, then we copy character by character. */
-    int num_quotes = 0;
-    size_t namelen = 0;
-    const char *utf8_name = cur_ws->canonical_name;
-    for (const char *walk = utf8_name; *walk != '\0'; walk++) {
-        if (*walk == '"' || *walk == '\\')
-            num_quotes++;
-        /* While we’re looping through the name anyway, we can save one
-         * strlen(). */
-        namelen++;
-    }
-
-    const size_t len = namelen + strlen("workspace \"\"") + 1;
-    char *buffer = scalloc(len + num_quotes, 1);
-    memcpy(buffer, "workspace \"", strlen("workspace \""));
-    size_t inpos, outpos;
-    for (inpos = 0, outpos = strlen("workspace \"");
-         inpos < namelen;
-         inpos++, outpos++) {
-        if (utf8_name[inpos] == '"' || utf8_name[inpos] == '\\') {
-            buffer[outpos] = '\\';
-            outpos++;
-        }
-        buffer[outpos] = utf8_name[inpos];
-    }
-    buffer[outpos] = '"';
-    i3_send_msg(I3_IPC_MESSAGE_TYPE_RUN_COMMAND, buffer);
-    free(buffer);
+    focus_workspace(cur_ws);
 }
 
 /*
@@ -674,9 +687,9 @@ static void handle_visibility_notify(xcb_visibility_notify_event_t *event) {
     }
 
     if (num_visible == 0) {
-        stop_child();
+        stop_children();
     } else {
-        cont_child();
+        cont_children();
     }
 }
 
@@ -1945,10 +1958,10 @@ void reconfig_windows(bool redraw_bars) {
                 /* Unmap the window, and draw it again when in dock mode */
                 umap_cookie = xcb_unmap_window_checked(xcb_connection, walk->bar.id);
                 if (config.hide_on_modifier == M_DOCK) {
-                    cont_child();
+                    cont_children();
                     map_cookie = xcb_map_window_checked(xcb_connection, walk->bar.id);
                 } else {
-                    stop_child();
+                    stop_children();
                 }
 
                 if (config.hide_on_modifier == M_HIDE) {
diff --git a/include/config_directives.h b/include/config_directives.h
index 600226e9..f910d591 100644
--- a/include/config_directives.h
+++ b/include/config_directives.h
@@ -105,6 +105,7 @@ CFGFUN(bar_tray_output, const char *output);
 CFGFUN(bar_tray_padding, const long spacing_px);
 CFGFUN(bar_color_single, const char *colorclass, const char *color);
 CFGFUN(bar_status_command, const char *command);
+CFGFUN(bar_workspace_command, const char *command);
 CFGFUN(bar_binding_mode_indicator, const char *value);
 CFGFUN(bar_workspace_buttons, const char *value);
 CFGFUN(bar_workspace_min_width, const long width);
diff --git a/include/configuration.h b/include/configuration.h
index 99f4b64e..19d2f714 100644
--- a/include/configuration.h
+++ b/include/configuration.h
@@ -335,6 +335,10 @@ struct Barconfig {
      * Will be passed to the shell. */
     char *status_command;
 
+    /** Command that should be run to get the workspace buttons. Will be passed
+     * to the shell. */
+    char *workspace_command;
+
     /** Font specification for all text rendered on the bar. */
     char *font;
 
diff --git a/man/i3bar.man b/man/i3bar.man
index 479e10fc..761748f3 100644
--- a/man/i3bar.man
+++ b/man/i3bar.man
@@ -46,9 +46,12 @@ Be verbose.
 workspace switching buttons and a statusline generated by i3status(1) or
 similar. It is automatically invoked (and configured through) i3.
 
-i3bar supports colors via a JSON protocol starting from v4.2, see
+i3bar supports using a JSON protocol for setting the status line, see
 https://i3wm.org/docs/i3bar-protocol.html
 
+Since i3 4.23, i3bar supports another JSON protocol for setting workspace
+buttons. See https://i3wm.org/docs/i3bar-workspace-protocol.html.
+
 == ENVIRONMENT
 
 === I3SOCK
diff --git a/meson.build b/meson.build
index 25080cea..30beb055 100644
--- a/meson.build
+++ b/meson.build
@@ -86,6 +86,7 @@ if get_option('docs')
     'docs/wsbar',
     'docs/testsuite',
     'docs/i3bar-protocol',
+    'docs/i3bar-workspace-protocol',
     'docs/layout-saving',
   ]
   foreach m : doc_toc_inputs
@@ -135,6 +136,7 @@ else
 	'docs/wsbar.html',
 	'docs/testsuite.html',
 	'docs/i3bar-protocol.html',
+	'docs/i3bar-workspace-protocol.html',
 	'docs/layout-saving.html',
 	'docs/debugging.html',
       ],
diff --git a/parser-specs/config.spec b/parser-specs/config.spec
index 52bd3212..33708b52 100644
--- a/parser-specs/config.spec
+++ b/parser-specs/config.spec
@@ -528,6 +528,7 @@ state BAR:
   'set' -> BAR_IGNORE_LINE
   'i3bar_command'          -> BAR_BAR_COMMAND
   'status_command'         -> BAR_STATUS_COMMAND
+  'workspace_command'      -> BAR_WORKSPACE_COMMAND
   'socket_path'            -> BAR_SOCKET_PATH
   'mode'                   -> BAR_MODE
   'hidden_state'           -> BAR_HIDDEN_STATE
@@ -567,6 +568,10 @@ state BAR_STATUS_COMMAND:
   command = string
       -> call cfg_bar_status_command($command); BAR
 
+state BAR_WORKSPACE_COMMAND:
+  command = string
+      -> call cfg_bar_workspace_command($command); BAR
+
 state BAR_SOCKET_PATH:
   path = string
       -> call cfg_bar_socket_path($path); BAR
diff --git a/release-notes/changes/1-workspace_command b/release-notes/changes/1-workspace_command
new file mode 100644
index 00000000..7a6bba7b
--- /dev/null
+++ b/release-notes/changes/1-workspace_command
@@ -0,0 +1 @@
+add workspace_command option in i3bar
diff --git a/src/config.c b/src/config.c
index f06a3f8d..bf3ec6dc 100644
--- a/src/config.c
+++ b/src/config.c
@@ -105,6 +105,7 @@ static void free_configuration(void) {
         FREE(barconfig->outputs);
         FREE(barconfig->socket_path);
         FREE(barconfig->status_command);
+        FREE(barconfig->workspace_command);
         FREE(barconfig->i3bar_command);
         FREE(barconfig->font);
         FREE(barconfig->colors.background);
diff --git a/src/config_directives.c b/src/config_directives.c
index 9077fe98..81adf351 100644
--- a/src/config_directives.c
+++ b/src/config_directives.c
@@ -873,6 +873,11 @@ CFGFUN(bar_status_command, const char *command) {
     current_bar->status_command = sstrdup(command);
 }
 
+CFGFUN(bar_workspace_command, const char *command) {
+    FREE(current_bar->workspace_command);
+    current_bar->workspace_command = sstrdup(command);
+}
+
 CFGFUN(bar_binding_mode_indicator, const char *value) {
     current_bar->hide_binding_mode_indicator = !boolstr(value);
 }
diff --git a/src/ipc.c b/src/ipc.c
index 28a86092..d20090c6 100644
--- a/src/ipc.c
+++ b/src/ipc.c
@@ -827,6 +827,7 @@ static void dump_bar_config(yajl_gen gen, Barconfig *config) {
         ystr("top");
 
     YSTR_IF_SET(status_command);
+    YSTR_IF_SET(workspace_command);
     YSTR_IF_SET(font);
 
     if (config->bar_height) {
diff --git a/testcases/t/201-config-parser.t b/testcases/t/201-config-parser.t
index af50d81d..90fc8115 100644
--- a/testcases/t/201-config-parser.t
+++ b/testcases/t/201-config-parser.t
@@ -776,7 +776,7 @@ EOT
 $expected = <<'EOT';
 cfg_bar_start()
 cfg_bar_output(LVDS-1)
-ERROR: CONFIG: Expected one of these tokens: <end>, '#', 'set', 'i3bar_command', 'status_command', 'socket_path', 'mode', 'hidden_state', 'id', 'modifier', 'wheel_up_cmd', 'wheel_down_cmd', 'bindsym', 'position', 'output', 'tray_output', 'tray_padding', 'font', 'separator_symbol', 'binding_mode_indicator', 'workspace_buttons', 'workspace_min_width', 'strip_workspace_numbers', 'strip_workspace_name', 'verbose', 'height', 'padding', 'colors', '}'
+ERROR: CONFIG: Expected one of these tokens: <end>, '#', 'set', 'i3bar_command', 'status_command', 'workspace_command', 'socket_path', 'mode', 'hidden_state', 'id', 'modifier', 'wheel_up_cmd', 'wheel_down_cmd', 'bindsym', 'position', 'output', 'tray_output', 'tray_padding', 'font', 'separator_symbol', 'binding_mode_indicator', 'workspace_buttons', 'workspace_min_width', 'strip_workspace_numbers', 'strip_workspace_name', 'verbose', 'height', 'padding', 'colors', '}'
 ERROR: CONFIG: (in file <stdin>)
 ERROR: CONFIG: Line   1: bar {
 ERROR: CONFIG: Line   2:     output LVDS-1