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diff --git a/src/lib/dispatch/dispatch.h b/src/lib/dispatch/dispatch.h
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+/* Copyright (c) 2001, Matej Pfajfar.
+ * Copyright (c) 2001-2004, Roger Dingledine.
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2020, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_DISPATCH_H
+#define TOR_DISPATCH_H
+
+#include "lib/dispatch/msgtypes.h"
+
+/**
+ * \file dispatch.h
+ * \brief Low-level APIs for message-passing system.
+ *
+ * This module implements message dispatch based on a set of short integer
+ * identifiers.  For a higher-level interface, see pubsub.h.
+ *
+ * Each message is represented as a generic msg_t object, and is discriminated
+ * by its message_id_t.  Messages are delivered by a dispatch_t object, which
+ * delivers each message to its recipients by a configured "channel".
+ *
+ * A "channel" is a means of delivering messages.  Every message_id_t must
+ * be associated with exactly one channel, identified by channel_id_t.
+ * When a channel receives messages, a callback is invoked to either process
+ * the messages immediately, or to cause them to be processed later.
+ *
+ * Every message_id_t has zero or more associated receiver functions set up in
+ * the dispatch_t object.  Once the dispatch_t object is created, receivers
+ * can be enabled or disabled [TODO], but not added or removed.
+ *
+ * Every message_id_t has an associated datatype, identified by a
+ * msg_type_id_t.  These datatypes can be associated with functions to
+ * (for example) free them, or format them for debugging.
+ *
+ * To setup a dispatch_t object, first create a dispatch_cfg_t object, and
+ * configure messages with their types, channels, and receivers.  Then, use
+ * dispatch_new() with that dispatch_cfg_t to create the dispatch_t object.
+ *
+ * (We use a two-phase contruction procedure here to enable better static
+ * reasoning about publish/subscribe relationships.)
+ *
+ * Once you have a dispatch_t, you can queue messages on it with
+ * dispatch_send*(), and cause those messages to be delivered with
+ * dispatch_flush().
+ **/
+
+/**
+ * A "dispatcher" is the highest-level object; it handles making sure that
+ * messages are received and delivered properly.  Only the mainloop
+ * should handle this type directly.
+ */
+typedef struct dispatch_t dispatch_t;
+
+struct dispatch_cfg_t;
+
+dispatch_t *dispatch_new(const struct dispatch_cfg_t *cfg);
+
+/**
+ * Free a dispatcher.  Tor does this at exit.
+ */
+#define dispatch_free(d) \
+  FREE_AND_NULL(dispatch_t, dispatch_free_, (d))
+
+void dispatch_free_(dispatch_t *);
+
+int dispatch_send(dispatch_t *d,
+                  subsys_id_t sender,
+                  channel_id_t channel,
+                  message_id_t msg,
+                  msg_type_id_t type,
+                  msg_aux_data_t auxdata);
+
+int dispatch_send_msg(dispatch_t *d, msg_t *m);
+
+int dispatch_send_msg_unchecked(dispatch_t *d, msg_t *m);
+
+/* Flush up to <b>max_msgs</b> currently pending messages from the
+ * dispatcher.  Messages that are not pending when this function are
+ * called, are not flushed by this call.  Return 0 on success, -1 on
+ * unrecoverable error.
+ */
+int dispatch_flush(dispatch_t *, channel_id_t chan, int max_msgs);
+
+/**
+ * Function callback type used to alert some other module when a channel's
+ * queue changes from empty to nonempty.
+ *
+ * Ex 1: To cause messages to be processed immediately on-stack, this callback
+ * should invoke dispatch_flush() directly.
+ *
+ * Ex 2: To cause messages to be processed very soon, from the event queue,
+ * this callback should schedule an event callback to run dispatch_flush().
+ *
+ * Ex 3: To cause messages to be processed periodically, this function should
+ * do nothing, and a periodic event should invoke dispatch_flush().
+ **/
+typedef void (*dispatch_alertfn_t)(struct dispatch_t *,
+                                   channel_id_t, void *);
+
+int dispatch_set_alert_fn(dispatch_t *d, channel_id_t chan,
+                          dispatch_alertfn_t fn, void *userdata);
+
+#define dispatch_free_msg(d,msg)                                \
+  STMT_BEGIN {                                                  \
+    msg_t **msg_tmp_ptr__ = &(msg);                             \
+    dispatch_free_msg_((d), *msg_tmp_ptr__);                    \
+    *msg_tmp_ptr__= NULL;                                       \
+  } STMT_END
+void dispatch_free_msg_(const dispatch_t *d, msg_t *msg);
+
+char *dispatch_fmt_msg_data(const dispatch_t *d, const msg_t *msg);
+
+#endif /* !defined(TOR_DISPATCH_H) */