Basic work on a publish/subscribe abstraction

The goal here is to provide a way to decouple pieces of the code
that want to learn "when something happens" from those that realize
that it has happened.

The implementation here consists of a generic backend, plus a set of
macros to define and implement a set of type-safe frontends.

1 files changed, 177 insertions, 0 deletions

diff --git a/src/common/pubsub.h b/src/common/pubsub.h
new file mode 100644
index 0000000000..09e492ec4f
--- /dev/null
+++ b/src/common/pubsub.h
@@ -0,0 +1,177 @@
+/* Copyright (c) 2016, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file pubsub.h
+ * \brief Macros to implement publish/subscribe abstractions.
+ *
+ * To use these macros, call DECLARE_PUBSUB_TOPIC() with an identifier to use
+ * as your topic.  Below, I'm going to assume you say DECLARE_PUBSUB_TOPIC(T).
+ *
+ * Doing this will declare the following types:
+ *   typedef struct T_event_data_t T_event_data_t; // you define this struct
+ *   typedef struct T_subscriber_data_t T_subscriber_data_t; // this one too.
+ *   typedef struct T_subscriber_t T_subscriber_t; // opaque
+ *   typedef int (*T_subscriber_fn_t)(T_event_data_t*, T_subscriber_data_t*);
+ *
+ * and it will declare the following functions:
+ *     const T_subscriber_t *T_subscribe(T_subscriber_fn_t,
+ *                                       T_subscriber_data_t *,
+ *                                       unsigned flags,
+ *                                       unsigned priority);
+ *     int T_unsubscribe(const T_subscriber_t *)
+ *
+ * Elsewhere you can say DECLARE_NOTIFY_PUBSUB_TOPIC(static, T), which declares:
+ *    static int T_notify(T_event_data_t *, unsigned notify_flags);
+ *    static void T_clear(void);
+ *
+ * And in some C file, you would define these functions with:
+ *    IMPLEMENT_PUBSUB_TOPIC(static, T).
+ *
+ * The implementations will be small typesafe wrappers over generic versions
+ * of the above functions.
+ *
+ * To use the typesafe functions, you add any number of subscribers with
+ * T_subscribe().  Each has an associated function pointer, data pointer,
+ * and priority. Later, you can invoke T_notify() to declare that the
+ * event has occurred. Each of the subscribers will be invoked once.
+ **/
+
+#ifndef TOR_PUBSUB_H
+#define TOR_PUBSUB_H
+
+#include "torint.h"
+
+/**
+ * Flag for T_subscribe: die with an assertion failure if the event
+ * have ever been published before.  Used when a subscriber must absolutely
+ * never have missed an event.
+ */
+#define SUBSCRIBE_ATSTART (1u<<0)
+
+#define DECLARE_PUBSUB_STRUCT_TYPES(name)                               \
+  /* You define this type. */                                           \
+  typedef struct name ## _event_data_t name ## _event_data_t;           \
+  /* You define this type. */                                           \
+  typedef struct name ## _subscriber_data_t name ## _subscriber_data_t;
+
+#define DECLARE_PUBSUB_TOPIC(name)                                      \
+  /* This type is opaque. */                                            \
+  typedef struct name ## _subscriber_t name ## _subscriber_t;           \
+  /* You declare functions matching this type. */                       \
+  typedef int (*name ## _subscriber_fn_t)(                              \
+                                    name ## _event_data_t *data,        \
+                                    name ## _subscriber_data_t *extra); \
+  /* Call this function to subscribe to a topic. */                     \
+  const name ## _subscriber_t *name ## _subscribe(                      \
+                         name##_subscriber_fn_t subscriber,             \
+                         name##_subscriber_data_t *extra_data,          \
+                         unsigned flags,                                \
+                         unsigned priority);                            \
+  /* Call this function to unsubscribe from a topic. */                 \
+  int name ## _unsubscribe(const name##_subscriber_t *s);
+
+#define DECLARE_NOTIFY_PUBSUB_TOPIC(linkage, name)                          \
+  /* Call this function to notify all subscribers. Flags not yet used. */   \
+  linkage int name ## _notify(name ## _event_data_t *data, unsigned flags); \
+  /* Call this function to release storage held by the topic. */            \
+  linkage void name ## _clear(void);
+
+/**
+ * Type used to hold a generic function for a subscriber.
+ *
+ * [Yes, it is safe to cast to this, so long as we cast back to the original
+ * type before calling.  From C99: "A pointer to a function of one type may be
+ * converted to a pointer to a function of another type and back again; the
+ * result shall compare equal to the original pointer."]
+*/
+typedef int (*pubsub_subscriber_fn_t)(void *, void *);
+
+/**
+ * Helper type to implement pubsub abstraction. Don't use this directly.
+ * It represents a subscriber.
+ */
+typedef struct pubsub_subscriber_t {
+  /** Function to invoke when the event triggers. */
+  pubsub_subscriber_fn_t fn;
+  /** Data associated with this subscriber. */
+  void *subscriber_data;
+  /** Priority for this subscriber. Low priorities happen first. */
+  unsigned priority;
+  /** Flags set on this subscriber. Not yet used.*/
+  unsigned subscriber_flags;
+} pubsub_subscriber_t;
+
+/**
+ * Helper type to implement pubsub abstraction. Don't use this directly.
+ * It represents a topic, and keeps a record of subscribers.
+ */
+typedef struct pubsub_topic_t {
+  /** List of subscribers to this topic. May be NULL. */
+  struct smartlist_t *subscribers;
+  /** Total number of times that pubsub_notify_() has ever been called on this
+   * topic. */
+  uint64_t n_events_fired;
+  /** True iff we're running 'notify' on this topic, and shouldn't allow
+   * any concurrent modifications or events. */
+  unsigned locked;
+} pubsub_topic_t;
+
+const pubsub_subscriber_t *pubsub_subscribe_(pubsub_topic_t *topic,
+                                             pubsub_subscriber_fn_t fn,
+                                             void *subscriber_data,
+                                             unsigned subscribe_flags,
+                                             unsigned priority);
+int pubsub_unsubscribe_(pubsub_topic_t *topic, const pubsub_subscriber_t *sub);
+void pubsub_clear_(pubsub_topic_t *topic);
+typedef int (*pubsub_notify_fn_t)(pubsub_subscriber_t *subscriber,
+                                  void *notify_data);
+int pubsub_notify_(pubsub_topic_t *topic, pubsub_notify_fn_t notify_fn,
+                   void *notify_data, unsigned notify_flags);
+
+#define IMPLEMENT_PUBSUB_TOPIC(notify_linkage, name)                    \
+  static pubsub_topic_t name ## _topic_ = { NULL, 0, 0 };               \
+  const name ## _subscriber_t *                                         \
+  name ## _subscribe(name##_subscriber_fn_t subscriber,                 \
+                     name##_subscriber_data_t *extra_data,              \
+                     unsigned flags,                                    \
+                     unsigned priority)                                 \
+  {                                                                     \
+    const pubsub_subscriber_t *s;                                       \
+    s = pubsub_subscribe_(&name##_topic_,                               \
+                          (pubsub_subscriber_fn_t)subscriber,           \
+                          extra_data,                                   \
+                          flags,                                        \
+                          priority);                                    \
+    return (const name##_subscriber_t *)s;                              \
+  }                                                                     \
+  int                                                                   \
+  name ## _unsubscribe(const name##_subscriber_t *subscriber)           \
+  {                                                                     \
+    return pubsub_unsubscribe_(&name##_topic_,                          \
+                               (const pubsub_subscriber_t *)subscriber); \
+  }                                                                     \
+  static int                                                            \
+  name##_call_the_notify_fn_(pubsub_subscriber_t *subscriber,           \
+                             void *notify_data)                         \
+  {                                                                     \
+    name ## _subscriber_fn_t fn;                                        \
+    fn = (name ## _subscriber_fn_t) subscriber->fn;                     \
+    return fn(notify_data, subscriber->subscriber_data);                \
+  }                                                                     \
+  notify_linkage int                                                    \
+  name ## _notify(name ## _event_data_t *event_data, unsigned flags)    \
+  {                                                                     \
+    return pubsub_notify_(&name##_topic_,                               \
+                          name##_call_the_notify_fn_,                   \
+                          event_data,                                   \
+                          flags);                                       \
+  }                                                                     \
+  notify_linkage void                                                   \
+  name ## _clear(void)                                                  \
+  {                                                                     \
+    pubsub_clear_(&name##_topic_);                                      \
+  }
+
+#endif /* TOR_PUBSUB_H */
+