From: David M. Lee <dlee@digium.com>
Date: Mon, 30 Sep 2013 18:39:34 +0000 (+0000)
Subject: Optimize how Stasis forwards are dispatched
X-Git-Tag: 12.0.0-alpha2~24^2~43
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9d21631aeeade117fda1a2feddbbb13b4dbcca69;p=thirdparty%2Fasterisk.git

Optimize how Stasis forwards are dispatched

This patch optimizes how forwards are dispatched in Stasis.

Originally, forwards were dispatched as subscriptions that are invoked
on the publishing thread. This did not account for the vast number of
forwards we would end up having in the system, and the amount of work it
would take to walk though the forward subscriptions.

This patch modifies Stasis so that rather than walking the tree of
forwards on every dispatch, when forwards and subscriptions are changed,
the subscriber list for every topic in the tree is changed.

This has a couple of benefits. First, this reduces the workload of
dispatching messages. It also reduces contention when dispatching to
different topics that happen to forward to the same aggregation topic
(as happens with all of the channel, bridge and endpoint topics).

Since forwards are no longer subscriptions, the bulk of this patch is
simply changing stasis_subscription objects to stasis_forward objects
(which, admittedly, I should have done in the first place.)

Since this required me to yet again put in a growing array, I finally
abstracted that out into a set of ast_vector macros in
asterisk/vector.h.

Review: https://reviewboard.asterisk.org/r/2883/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@400180 65c4cc65-6c06-0410-ace0-fbb531ad65f3
---

diff --git a/apps/app_queue.c b/apps/app_queue.c
index e0889a5c37..50c7f526a4 100644
--- a/apps/app_queue.c
+++ b/apps/app_queue.c
@@ -10336,7 +10336,7 @@ static const struct ast_data_entry queue_data_providers[] = {
 };
 
 static struct stasis_message_router *agent_router;
-static struct stasis_subscription *topic_forwarder;
+static struct stasis_forward *topic_forwarder;
 
 static int unload_module(void)
 {
@@ -10364,7 +10364,7 @@ static int unload_module(void)
 		stasis_message_router_remove(message_router, queue_agent_ringnoanswer_type());
 	}
 	stasis_message_router_unsubscribe_and_join(agent_router);
-	topic_forwarder = stasis_unsubscribe(topic_forwarder);
+	topic_forwarder = stasis_forward_cancel(topic_forwarder);
 
 	STASIS_MESSAGE_TYPE_CLEANUP(queue_caller_join_type);
 	STASIS_MESSAGE_TYPE_CLEANUP(queue_caller_leave_type);
diff --git a/include/asterisk/stasis.h b/include/asterisk/stasis.h
index 09810ab561..70bb973faf 100644
--- a/include/asterisk/stasis.h
+++ b/include/asterisk/stasis.h
@@ -464,6 +464,8 @@ int stasis_subscription_is_done(struct stasis_subscription *subscription);
 struct stasis_subscription *stasis_unsubscribe_and_join(
 	struct stasis_subscription *subscription);
 
+struct stasis_forward;
+
 /*!
  * \brief Create a subscription which forwards all messages from one topic to
  * another.
@@ -477,9 +479,11 @@ struct stasis_subscription *stasis_unsubscribe_and_join(
  * \return \c NULL on error.
  * \since 12
  */
-struct stasis_subscription *stasis_forward_all(struct stasis_topic *from_topic,
+struct stasis_forward *stasis_forward_all(struct stasis_topic *from_topic,
 	struct stasis_topic *to_topic);
 
+struct stasis_forward *stasis_forward_cancel(struct stasis_forward *forward);
+
 /*!
  * \brief Get the unique ID for the subscription.
  *
diff --git a/include/asterisk/vector.h b/include/asterisk/vector.h
new file mode 100644
index 0000000000..f5d3e9a146
--- /dev/null
+++ b/include/asterisk/vector.h
@@ -0,0 +1,193 @@
+/*
+ * Asterisk -- An open source telephony toolkit.
+ *
+ * Copyright (C) 2013, Digium, Inc.
+ *
+ * David M. Lee, II <dlee@digium.com>
+ *
+ * See http://www.asterisk.org for more information about
+ * the Asterisk project. Please do not directly contact
+ * any of the maintainers of this project for assistance;
+ * the project provides a web site, mailing lists and IRC
+ * channels for your use.
+ *
+ * This program is free software, distributed under the terms of
+ * the GNU General Public License Version 2. See the LICENSE file
+ * at the top of the source tree.
+ */
+
+#ifndef _ASTERISK_VECTOR_H
+#define _ASTERISK_VECTOR_H
+
+/*! \file
+ *
+ * \brief Vector container support.
+ *
+ * A vector is a variable length array, with properties that can be useful when
+ * order doesn't matter.
+ *  - Appends are asymptotically constant time.
+ *  - Unordered removes are constant time.
+ *  - Search is linear time
+ *
+ * \author David M. Lee, II <dlee@digium.com>
+ * \since 12
+ */
+
+/*! \brief Define a vector structure */
+#define ast_vector(type)			\
+	struct {				\
+		type *elems;			\
+		size_t max;			\
+		size_t current;			\
+	}
+
+/*!
+ * \brief Initialize a vector
+ *
+ * If \a size is 0, then no space will be allocated until the vector is
+ * appended to.
+ *
+ * \param vec Vector to initialize.
+ * \param size Initial size of the vector.
+ *
+ * \return 0 on success.
+ * \return Non-zero on failure.
+ */
+#define ast_vector_init(vec, size) ({					\
+	size_t __size = (size);						\
+	size_t alloc_size = __size * sizeof(*(vec).elems);		\
+	(vec).elems = alloc_size ? ast_malloc(alloc_size) : NULL;	\
+	(vec).current = 0;						\
+	if ((vec).elems) {						\
+		(vec).max = __size;					\
+	} else {							\
+		(vec).max = 0;						\
+	}								\
+	alloc_size == 0 || (vec).elems != NULL ? 0 : -1;		\
+})
+
+/*!
+ * \brief Deallocates this vector.
+ *
+ * If any code to free the elements of this vector need to be run, that should
+ * be done prior to this call.
+ *
+ * \param vec Vector to deallocate.
+ */
+#define ast_vector_free(vec) do {		\
+	ast_free((vec).elems);			\
+	(vec).elems = NULL;			\
+	(vec).max = 0;				\
+	(vec).current = 0;			\
+} while (0)
+
+/*!
+ * \brief Append an element to a vector, growing the vector if needed.
+ *
+ * \param vec Vector to append to.
+ * \param elem Element to append.
+ *
+ * \return 0 on success.
+ * \return Non-zero on failure.
+ */
+#define ast_vector_append(vec, elem) ({					\
+	int res = 0;							\
+									\
+	if ((vec).current + 1 > (vec).max) {				\
+		size_t new_max = (vec).max ? 2 * (vec).max : 1;		\
+		typeof((vec).elems) new_elems = ast_realloc(		\
+			(vec).elems, new_max * sizeof(*new_elems));	\
+		if (new_elems) {					\
+			(vec).elems = new_elems;				\
+			(vec).max = new_max;				\
+		} else {						\
+			res = -1;					\
+		}							\
+	}								\
+									\
+	if (res == 0) {							\
+		(vec).elems[(vec).current++] = (elem);			\
+	}								\
+	res;								\
+})
+
+/*!
+ * \brief Remove an element from a vector by index.
+ *
+ * Note that elements in the vector may be reordered, so that the remove can
+ * happen in constant time.
+ *
+ * \param vec Vector to remove from.
+ * \param idx Index of the element to remove.
+ * \return The element that was removed.
+ */
+#define ast_vector_remove_unordered(vec, idx) ({		\
+	typeof((vec).elems[0]) res;				\
+	size_t __idx = (idx);					\
+	ast_assert(__idx < (vec).current);			\
+	res = (vec).elems[__idx];				\
+	(vec).elems[__idx] = (vec).elems[--(vec).current];	\
+	res;							\
+})
+
+
+/*!
+ * \brief Remove an element from a vector that matches the given comparison
+ *
+ * \param vec Vector to remove from.
+ * \param value Value to pass into comparator.
+ * \param cmp Comparator function/macros (called as \c cmp(elem, value))
+ * \return 0 if element was removed.
+ * \return Non-zero if element was not in the vector.
+ */
+#define ast_vector_remove_cmp_unordered(vec, value, cmp) ({		\
+	int res = -1;							\
+	size_t idx;							\
+	typeof(value) __value = (value);				\
+	for (idx = 0; idx < (vec).current; ++idx) {			\
+		if (cmp((vec).elems[idx], __value)) {			\
+			ast_vector_remove_unordered((vec), idx);	\
+			res = 0;					\
+			break;						\
+		}							\
+	}								\
+	res;								\
+})
+
+/*! \brief Default comparator for ast_vector_remove_elem_unordered() */
+#define AST_VECTOR_DEFAULT_CMP(a, b) ((a) == (b))
+
+/*!
+ * \brief Remove an element from a vector.
+ *
+ * \param vec Vector to remove from.
+ * \param elem Element to remove
+ * \return 0 if element was removed.
+ * \return Non-zero if element was not in the vector.
+ */
+#define ast_vector_remove_elem_unordered(vec, elem) ({	\
+	ast_vector_remove_cmp_unordered((vec), (elem),	\
+		AST_VECTOR_DEFAULT_CMP);		\
+})
+
+/*!
+ * \brief Get the number of elements in a vector.
+ *
+ * \param vec Vector to query.
+ * \return Number of elements in the vector.
+ */
+#define ast_vector_size(vec) (vec).current
+
+/*!
+ * \brief Get an element from a vector.
+ *
+ * \param vec Vector to query.
+ * \param idx Index of the element to get.
+ */
+#define ast_vector_get(vec, idx) ({		\
+	size_t __idx = (idx);			\
+	ast_assert(__idx < (vec).current);	\
+	(vec).elems[__idx];			\
+})
+
+#endif /* _ASTERISK_VECTOR_H */
diff --git a/main/cdr.c b/main/cdr.c
index fb02d33503..f7af298651 100644
--- a/main/cdr.c
+++ b/main/cdr.c
@@ -334,13 +334,13 @@ static struct ao2_container *active_cdrs_by_channel;
 static struct stasis_message_router *stasis_router;
 
 /*! \brief Our subscription for bridges */
-static struct stasis_subscription *bridge_subscription;
+static struct stasis_forward *bridge_subscription;
 
 /*! \brief Our subscription for channels */
-static struct stasis_subscription *channel_subscription;
+static struct stasis_forward *channel_subscription;
 
 /*! \brief Our subscription for parking */
-static struct stasis_subscription *parking_subscription;
+static struct stasis_forward *parking_subscription;
 
 /*! \brief The parent topic for all topics we want to aggregate for CDRs */
 static struct stasis_topic *cdr_topic;
@@ -3884,9 +3884,9 @@ static int process_config(int reload)
 
 static void cdr_engine_cleanup(void)
 {
-	channel_subscription = stasis_unsubscribe_and_join(channel_subscription);
-	bridge_subscription = stasis_unsubscribe_and_join(bridge_subscription);
-	parking_subscription = stasis_unsubscribe_and_join(parking_subscription);
+	channel_subscription = stasis_forward_cancel(channel_subscription);
+	bridge_subscription = stasis_forward_cancel(bridge_subscription);
+	parking_subscription = stasis_forward_cancel(parking_subscription);
 	stasis_message_router_unsubscribe_and_join(stasis_router);
 	ao2_cleanup(cdr_topic);
 	cdr_topic = NULL;
diff --git a/main/cel.c b/main/cel.c
index 6050fac757..36daf2066d 100644
--- a/main/cel.c
+++ b/main/cel.c
@@ -121,16 +121,16 @@ static struct stasis_topic *cel_topic;
 static struct stasis_topic *cel_aggregation_topic;
 
 /*! Subscription for forwarding the channel caching topic */
-static struct stasis_subscription *cel_channel_forwarder;
+static struct stasis_forward *cel_channel_forwarder;
 
 /*! Subscription for forwarding the channel caching topic */
-static struct stasis_subscription *cel_bridge_forwarder;
+static struct stasis_forward *cel_bridge_forwarder;
 
 /*! Subscription for forwarding the parking topic */
-static struct stasis_subscription *cel_parking_forwarder;
+static struct stasis_forward *cel_parking_forwarder;
 
 /*! Subscription for forwarding the CEL-specific topic */
-static struct stasis_subscription *cel_cel_forwarder;
+static struct stasis_forward *cel_cel_forwarder;
 
 struct stasis_message_type *cel_generic_type(void);
 STASIS_MESSAGE_TYPE_DEFN(cel_generic_type);
@@ -1394,10 +1394,10 @@ static void ast_cel_engine_term(void)
 	cel_aggregation_topic = NULL;
 	ao2_cleanup(cel_topic);
 	cel_topic = NULL;
-	cel_channel_forwarder = stasis_unsubscribe_and_join(cel_channel_forwarder);
-	cel_bridge_forwarder = stasis_unsubscribe_and_join(cel_bridge_forwarder);
-	cel_parking_forwarder = stasis_unsubscribe_and_join(cel_parking_forwarder);
-	cel_cel_forwarder = stasis_unsubscribe_and_join(cel_cel_forwarder);
+	cel_channel_forwarder = stasis_forward_cancel(cel_channel_forwarder);
+	cel_bridge_forwarder = stasis_forward_cancel(cel_bridge_forwarder);
+	cel_parking_forwarder = stasis_forward_cancel(cel_parking_forwarder);
+	cel_cel_forwarder = stasis_forward_cancel(cel_cel_forwarder);
 	ast_cli_unregister(&cli_status);
 	ao2_cleanup(cel_dialstatus_store);
 	cel_dialstatus_store = NULL;
diff --git a/main/channel_internal_api.c b/main/channel_internal_api.c
index 956816d764..de2cc9c71d 100644
--- a/main/channel_internal_api.c
+++ b/main/channel_internal_api.c
@@ -207,8 +207,7 @@ struct ast_channel {
 	char sending_dtmf_digit;			/*!< Digit this channel is currently sending out. (zero if not sending) */
 	struct timeval sending_dtmf_tv;		/*!< The time this channel started sending the current digit. (Invalid if sending_dtmf_digit is zero.) */
 	struct stasis_cp_single *topics;		/*!< Topic for all channel's events */
-	struct stasis_subscription *forwarder;		/*!< Subscription for event forwarding to all topic */
-	struct stasis_subscription *endpoint_forward;	/*!< Subscription for event forwarding to endpoint's topic */
+	struct stasis_forward *endpoint_forward;	/*!< Subscription for event forwarding to endpoint's topic */
 };
 
 /*! \brief The monotonically increasing integer counter for channel uniqueids */
@@ -1429,8 +1428,7 @@ void ast_channel_internal_cleanup(struct ast_channel *chan)
 
 	ast_string_field_free_memory(chan);
 
-	chan->forwarder = stasis_unsubscribe(chan->forwarder);
-	chan->endpoint_forward = stasis_unsubscribe(chan->endpoint_forward);
+	chan->endpoint_forward = stasis_forward_cancel(chan->endpoint_forward);
 
 	stasis_cp_single_unsubscribe(chan->topics);
 	chan->topics = NULL;
diff --git a/main/manager.c b/main/manager.c
index 00649dafaa..75e20c21d5 100644
--- a/main/manager.c
+++ b/main/manager.c
@@ -1126,7 +1126,7 @@ static struct stasis_topic *manager_topic;
 static struct stasis_message_router *stasis_router;
 
 /*! \brief The \ref stasis_subscription for forwarding the RTP topic to the AMI topic */
-static struct stasis_subscription *rtp_topic_forwarder;
+static struct stasis_forward *rtp_topic_forwarder;
 
 #define MGR_SHOW_TERMINAL_WIDTH 80
 
@@ -7759,7 +7759,7 @@ static void manager_shutdown(void)
 		stasis_message_router_unsubscribe_and_join(stasis_router);
 		stasis_router = NULL;
 	}
-	stasis_unsubscribe_and_join(rtp_topic_forwarder);
+	stasis_forward_cancel(rtp_topic_forwarder);
 	rtp_topic_forwarder = NULL;
 	ao2_cleanup(manager_topic);
 	manager_topic = NULL;
diff --git a/main/manager_bridges.c b/main/manager_bridges.c
index 77d9ff05e1..38f9af4771 100644
--- a/main/manager_bridges.c
+++ b/main/manager_bridges.c
@@ -106,7 +106,7 @@ static struct stasis_message_router *bridge_state_router;
 /*! \brief The \ref stasis subscription returned by the forwarding of the channel topic
  * to the manager topic
  */
-static struct stasis_subscription *topic_forwarder;
+static struct stasis_forward *topic_forwarder;
 
 struct ast_str *ast_manager_build_bridge_state_string_prefix(
 	const struct ast_bridge_snapshot *snapshot,
@@ -456,7 +456,7 @@ static int manager_bridge_info(struct mansession *s, const struct message *m)
 
 static void manager_bridging_cleanup(void)
 {
-	stasis_unsubscribe(topic_forwarder);
+	stasis_forward_cancel(topic_forwarder);
 	topic_forwarder = NULL;
 }
 
diff --git a/main/manager_channels.c b/main/manager_channels.c
index 485841b69f..d39687ffd0 100644
--- a/main/manager_channels.c
+++ b/main/manager_channels.c
@@ -370,7 +370,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 /*! \brief The \ref stasis subscription returned by the forwarding of the channel topic
  * to the manager topic
  */
-static struct stasis_subscription *topic_forwarder;
+static struct stasis_forward *topic_forwarder;
 
 struct ast_str *ast_manager_build_channel_state_string_prefix(
 		const struct ast_channel_snapshot *snapshot,
@@ -1100,7 +1100,7 @@ static void channel_unhold_cb(void *data, struct stasis_subscription *sub,
 
 static void manager_channels_shutdown(void)
 {
-	stasis_unsubscribe(topic_forwarder);
+	stasis_forward_cancel(topic_forwarder);
 	topic_forwarder = NULL;
 }
 
diff --git a/main/manager_mwi.c b/main/manager_mwi.c
index 12a3de3613..9847bd4a73 100644
--- a/main/manager_mwi.c
+++ b/main/manager_mwi.c
@@ -41,7 +41,7 @@ struct stasis_message_router *mwi_state_router;
 /*! \brief The \ref stasis subscription returned by the forwarding of the MWI topic
  * to the manager topic
  */
-static struct stasis_subscription *topic_forwarder;
+static struct stasis_forward *topic_forwarder;
 
 /*! \brief Callback function used by \ref mwi_app_event_cb to weed out "Event" keys */
 static int exclude_event_cb(const char *key)
@@ -149,7 +149,7 @@ static void mwi_update_cb(void *data, struct stasis_subscription *sub,
 
 static void manager_mwi_shutdown(void)
 {
-	stasis_unsubscribe(topic_forwarder);
+	stasis_forward_cancel(topic_forwarder);
 	topic_forwarder = NULL;
 }
 
diff --git a/main/manager_system.c b/main/manager_system.c
index 4fef11da4c..f4e7e9e0bb 100644
--- a/main/manager_system.c
+++ b/main/manager_system.c
@@ -34,11 +34,11 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 /*! \brief The \ref stasis subscription returned by the forwarding of the system topic
  * to the manager topic
  */
-static struct stasis_subscription *topic_forwarder;
+static struct stasis_forward *topic_forwarder;
 
 static void manager_system_shutdown(void)
 {
-	stasis_unsubscribe(topic_forwarder);
+	stasis_forward_cancel(topic_forwarder);
 	topic_forwarder = NULL;
 }
 
diff --git a/main/stasis.c b/main/stasis.c
index dade0e57fe..807ba43441 100644
--- a/main/stasis.c
+++ b/main/stasis.c
@@ -29,7 +29,7 @@
 
 #include "asterisk.h"
 
-ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
+ASTERISK_FILE_VERSION(__FILE__, "$Revision$");
 
 #include "asterisk/astobj2.h"
 #include "asterisk/stasis_internal.h"
@@ -37,6 +37,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/taskprocessor.h"
 #include "asterisk/utils.h"
 #include "asterisk/uuid.h"
+#include "asterisk/vector.h"
 
 /*!
  * \page stasis-impl Stasis Implementation Notes
@@ -139,15 +140,17 @@ STASIS_MESSAGE_TYPE_DEFN(stasis_subscription_change_type);
 struct stasis_topic {
 	char *name;
 	/*! Variable length array of the subscribers */
-	struct stasis_subscription **subscribers;
-	/*! Allocated length of the subscribers array */
-	size_t num_subscribers_max;
-	/*! Current size of the subscribers array */
-	size_t num_subscribers_current;
+	ast_vector(struct stasis_subscription *) subscribers;
+
+	/*! Topics forwarding into this topic */
+	ast_vector(struct stasis_topic *) upstream_topics;
 };
 
 /* Forward declarations for the tightly-coupled subscription object */
-static int topic_add_subscription(struct stasis_topic *topic, struct stasis_subscription *sub);
+static int topic_add_subscription(struct stasis_topic *topic,
+	struct stasis_subscription *sub);
+
+static int topic_remove_subscription(struct stasis_topic *topic, struct stasis_subscription *sub);
 
 static void topic_dtor(void *obj)
 {
@@ -155,16 +158,18 @@ static void topic_dtor(void *obj)
 
 	/* Subscribers hold a reference to topics, so they should all be
 	 * unsubscribed before we get here. */
-	ast_assert(topic->num_subscribers_current == 0);
+	ast_assert(ast_vector_size(topic->subscribers) == 0);
 	ast_free(topic->name);
 	topic->name = NULL;
-	ast_free(topic->subscribers);
-	topic->subscribers = NULL;
+
+	ast_vector_free(topic->subscribers);
+	ast_vector_free(topic->upstream_topics);
 }
 
 struct stasis_topic *stasis_topic_create(const char *name)
 {
 	RAII_VAR(struct stasis_topic *, topic, NULL, ao2_cleanup);
+	int res = 0;
 
 	topic = ao2_alloc(sizeof(*topic), topic_dtor);
 
@@ -177,9 +182,10 @@ struct stasis_topic *stasis_topic_create(const char *name)
 		return NULL;
 	}
 
-	topic->num_subscribers_max = INITIAL_SUBSCRIBERS_MAX;
-	topic->subscribers = ast_calloc(topic->num_subscribers_max, sizeof(*topic->subscribers));
-	if (!topic->subscribers) {
+	res |= ast_vector_init(topic->subscribers, INITIAL_SUBSCRIBERS_MAX);
+	res |= ast_vector_init(topic->upstream_topics, 0);
+
+	if (res != 0) {
 		return NULL;
 	}
 
@@ -264,7 +270,8 @@ static void subscription_invoke(struct stasis_subscription *sub,
 	}
 }
 
-static void send_subscription_change_message(struct stasis_topic *topic, char *uniqueid, char *description);
+static void send_subscription_subscribe(struct stasis_topic *topic, struct stasis_subscription *sub);
+static void send_subscription_unsubscribe(struct stasis_topic *topic, struct stasis_subscription *sub);
 
 struct stasis_subscription *internal_stasis_subscribe(
 	struct stasis_topic *topic,
@@ -306,7 +313,7 @@ struct stasis_subscription *internal_stasis_subscribe(
 	if (topic_add_subscription(topic, sub) != 0) {
 		return NULL;
 	}
-	send_subscription_change_message(topic, sub->uniqueid, "Subscribe");
+	send_subscription_subscribe(topic, sub);
 
 	ao2_ref(sub, +1);
 	return sub;
@@ -322,27 +329,28 @@ struct stasis_subscription *stasis_subscribe(
 
 struct stasis_subscription *stasis_unsubscribe(struct stasis_subscription *sub)
 {
-	if (sub) {
-		size_t i;
-		/* The subscription may be the last ref to this topic. Hold
-		 * the topic ref open until after the unlock. */
-		RAII_VAR(struct stasis_topic *, topic, ao2_bump(sub->topic),
-			ao2_cleanup);
-		SCOPED_AO2LOCK(lock_topic, topic);
+	/* The subscription may be the last ref to this topic. Hold
+	 * the topic ref open until after the unlock. */
+	RAII_VAR(struct stasis_topic *, topic,
+		ao2_bump(sub ? sub->topic : NULL), ao2_cleanup);
 
-		for (i = 0; i < topic->num_subscribers_current; ++i) {
-			if (topic->subscribers[i] == sub) {
-				send_subscription_change_message(topic, sub->uniqueid, "Unsubscribe");
-				/* swap [i] with last entry; remove last entry */
-				topic->subscribers[i] = topic->subscribers[--topic->num_subscribers_current];
-				/* Unsubscribing unrefs the subscription */
-				ao2_cleanup(sub);
-				return NULL;
-			}
-		}
+	if (!sub) {
+		return NULL;
+	}
 
-		ast_log(LOG_ERROR, "Internal error: subscription has invalid topic\n");
+	/* We have to remove the subscription first, to ensure the unsubscribe
+	 * is the final message */
+	if (topic_remove_subscription(sub->topic, sub) != 0) {
+		ast_log(LOG_ERROR,
+			"Internal error: subscription has invalid topic\n");
+		return NULL;
 	}
+
+	/* Now let everyone know about the unsubscribe */
+	send_subscription_unsubscribe(topic, sub);
+
+	/* Unsubscribing unrefs the subscription */
+	ao2_cleanup(sub);
 	return NULL;
 }
 
@@ -392,8 +400,8 @@ int stasis_subscription_is_subscribed(const struct stasis_subscription *sub)
 		struct stasis_topic *topic = sub->topic;
 		SCOPED_AO2LOCK(lock_topic, topic);
 
-		for (i = 0; i < topic->num_subscribers_current; ++i) {
-			if (topic->subscribers[i] == sub) {
+		for (i = 0; i < ast_vector_size(topic->subscribers); ++i) {
+			if (ast_vector_get(topic->subscribers, i) == sub) {
 				return 1;
 			}
 		}
@@ -435,29 +443,38 @@ int stasis_subscription_final_message(struct stasis_subscription *sub, struct st
  */
 static int topic_add_subscription(struct stasis_topic *topic, struct stasis_subscription *sub)
 {
-	struct stasis_subscription **subscribers;
+	size_t idx;
 	SCOPED_AO2LOCK(lock, topic);
 
-	/* Increase list size, if needed */
-	if (topic->num_subscribers_current + 1 > topic->num_subscribers_max) {
-		subscribers = realloc(topic->subscribers, 2 * topic->num_subscribers_max * sizeof(*subscribers));
-		if (!subscribers) {
-			return -1;
-		}
-		topic->subscribers = subscribers;
-		topic->num_subscribers_max *= 2;
-	}
-
 	/* The reference from the topic to the subscription is shared with
 	 * the owner of the subscription, which will explicitly unsubscribe
 	 * to release it.
 	 *
 	 * If we bumped the refcount here, the owner would have to unsubscribe
 	 * and cleanup, which is a bit awkward. */
-	topic->subscribers[topic->num_subscribers_current++] = sub;
+	ast_vector_append(topic->subscribers, sub);
+
+	for (idx = 0; idx < ast_vector_size(topic->upstream_topics); ++idx) {
+		topic_add_subscription(
+			ast_vector_get(topic->upstream_topics, idx), sub);
+	}
+
 	return 0;
 }
 
+static int topic_remove_subscription(struct stasis_topic *topic, struct stasis_subscription *sub)
+{
+	size_t idx;
+	SCOPED_AO2LOCK(lock_topic, topic);
+
+	for (idx = 0; idx < ast_vector_size(topic->upstream_topics); ++idx) {
+		topic_remove_subscription(
+			ast_vector_get(topic->upstream_topics, idx), sub);
+	}
+
+	return ast_vector_remove_elem_unordered(topic->subscribers, sub);
+}
+
 /*!
  * \internal
  * \brief Information needed to dispatch a message to a subscription
@@ -520,6 +537,30 @@ static int dispatch_exec(void *data)
 	return 0;
 }
 
+static void dispatch_message(struct stasis_subscription *sub,
+	struct stasis_topic *publisher_topic, struct stasis_message *message)
+{
+	if (sub->mailbox) {
+		struct dispatch *dispatch;
+
+		dispatch = dispatch_create(publisher_topic, message, sub);
+		if (!dispatch) {
+			ast_log(LOG_DEBUG, "Dropping dispatch\n");
+			return;
+		}
+
+		if (ast_taskprocessor_push(sub->mailbox, dispatch_exec, dispatch) != 0) {
+			/* Push failed; just delete the dispatch.
+			 */
+			ast_log(LOG_DEBUG, "Dropping dispatch\n");
+			dispatch_dtor(dispatch);
+		}
+	} else {
+		/* Dispatch directly */
+		subscription_invoke(sub, publisher_topic, message);
+	}
+}
+
 void stasis_forward_message(struct stasis_topic *_topic, struct stasis_topic *publisher_topic, struct stasis_message *message)
 {
 	size_t i;
@@ -533,30 +574,12 @@ void stasis_forward_message(struct stasis_topic *_topic, struct stasis_topic *pu
 	ast_assert(publisher_topic != NULL);
 	ast_assert(message != NULL);
 
-	for (i = 0; i < topic->num_subscribers_current; ++i) {
-		struct stasis_subscription *sub = topic->subscribers[i];
+	for (i = 0; i < ast_vector_size(topic->subscribers); ++i) {
+		struct stasis_subscription *sub = ast_vector_get(topic->subscribers, i);
 
 		ast_assert(sub != NULL);
 
-		if (sub->mailbox) {
-			struct dispatch *dispatch;
-
-			dispatch = dispatch_create(publisher_topic, message, sub);
-			if (!dispatch) {
-				ast_log(LOG_ERROR, "Dropping dispatch\n");
-				break;
-			}
-
-			if (ast_taskprocessor_push(sub->mailbox, dispatch_exec, dispatch) != 0) {
-				/* Push failed; just delete the dispatch.
-				 */
-				ast_log(LOG_ERROR, "Dropping dispatch\n");
-				dispatch_dtor(dispatch);
-			}
-		} else {
-			/* Dispatch directly */
-			subscription_invoke(sub, publisher_topic, message);
-		}
+		dispatch_message(sub, publisher_topic, message);
 	}
 }
 
@@ -565,34 +588,92 @@ void stasis_publish(struct stasis_topic *topic, struct stasis_message *message)
 	stasis_forward_message(topic, topic, message);
 }
 
-/*! \brief Forwarding subscriber */
-static void stasis_forward_cb(void *data, struct stasis_subscription *sub, struct stasis_topic *topic, struct stasis_message *message)
+/*!
+ * \brief Forwarding information
+ *
+ * Any message posted to \a from_topic is forwarded to \a to_topic.
+ *
+ * In cases where both the \a from_topic and \a to_topic need to be locked,
+ * always lock the \a to_topic first, then the \a from_topic. Lest you deadlock.
+ */
+struct stasis_forward {
+	/*! Originating topic */
+	struct stasis_topic *from_topic;
+	/*! Destination topic */
+	struct stasis_topic *to_topic;
+};
+
+static void forward_dtor(void *obj)
 {
-	struct stasis_topic *to_topic = data;
-	stasis_forward_message(to_topic, topic, message);
+	struct stasis_forward *forward = obj;
 
-	if (stasis_subscription_final_message(sub, message)) {
-		ao2_cleanup(to_topic);
+	ao2_cleanup(forward->from_topic);
+	forward->from_topic = NULL;
+	ao2_cleanup(forward->to_topic);
+	forward->to_topic = NULL;
+}
+
+struct stasis_forward *stasis_forward_cancel(struct stasis_forward *forward)
+{
+	if (forward) {
+		int idx;
+
+		struct stasis_topic *from = forward->from_topic;
+		struct stasis_topic *to = forward->to_topic;
+
+		SCOPED_AO2LOCK(to_lock, to);
+
+		ast_vector_remove_elem_unordered(to->upstream_topics, from);
+
+		ao2_lock(from);
+		for (idx = 0; idx < ast_vector_size(to->subscribers); ++idx) {
+			topic_remove_subscription(
+				from, ast_vector_get(to->subscribers, idx));
+		}
+		ao2_unlock(from);
 	}
+
+	ao2_cleanup(forward);
+
+	return NULL;
 }
 
-struct stasis_subscription *stasis_forward_all(struct stasis_topic *from_topic, struct stasis_topic *to_topic)
+struct stasis_forward *stasis_forward_all(struct stasis_topic *from_topic,
+	struct stasis_topic *to_topic)
 {
-	struct stasis_subscription *sub;
+	RAII_VAR(struct stasis_forward *, forward, NULL, ao2_cleanup);
+
 	if (!from_topic || !to_topic) {
 		return NULL;
 	}
 
-	/* Forwarding subscriptions should dispatch directly instead of having a
-	 * mailbox. Otherwise, messages forwarded to the same topic from
-	 * different topics may get reordered. Which is bad.
-	 */
-	sub = internal_stasis_subscribe(from_topic, stasis_forward_cb, to_topic, 0);
-	if (sub) {
-		/* hold a ref to to_topic for this forwarding subscription */
-		ao2_ref(to_topic, +1);
+	forward = ao2_alloc(sizeof(*forward), forward_dtor);
+	if (!forward) {
+		return NULL;
 	}
-	return sub;
+
+	forward->from_topic = ao2_bump(from_topic);
+	forward->to_topic = ao2_bump(to_topic);
+
+	{
+		SCOPED_AO2LOCK(lock, to_topic);
+		int res;
+
+		res = ast_vector_append(to_topic->upstream_topics, from_topic);
+		if (res != 0) {
+			return NULL;
+		}
+
+		{
+			SCOPED_AO2LOCK(lock, from_topic);
+			size_t idx;
+			for (idx = 0; idx < ast_vector_size(to_topic->subscribers); ++idx) {
+				topic_add_subscription(from_topic, ast_vector_get(to_topic->subscribers, idx));
+			}
+		}
+	}
+
+	return ao2_bump(forward);
 }
 
 static void subscription_change_dtor(void *obj)
@@ -602,7 +683,7 @@ static void subscription_change_dtor(void *obj)
 	ao2_cleanup(change->topic);
 }
 
-static struct stasis_subscription_change *subscription_change_alloc(struct stasis_topic *topic, char *uniqueid, char *description)
+static struct stasis_subscription_change *subscription_change_alloc(struct stasis_topic *topic, const char *uniqueid, const char *description)
 {
 	RAII_VAR(struct stasis_subscription_change *, change, NULL, ao2_cleanup);
 
@@ -620,12 +701,15 @@ static struct stasis_subscription_change *subscription_change_alloc(struct stasi
 	return change;
 }
 
-static void send_subscription_change_message(struct stasis_topic *topic, char *uniqueid, char *description)
+static void send_subscription_subscribe(struct stasis_topic *topic, struct stasis_subscription *sub)
 {
 	RAII_VAR(struct stasis_subscription_change *, change, NULL, ao2_cleanup);
 	RAII_VAR(struct stasis_message *, msg, NULL, ao2_cleanup);
 
-	change = subscription_change_alloc(topic, uniqueid, description);
+	/* This assumes that we have already unsubscribed */
+	ast_assert(stasis_subscription_is_subscribed(sub));
+
+	change = subscription_change_alloc(topic, sub->uniqueid, "Subscribe");
 
 	if (!change) {
 		return;
@@ -640,15 +724,42 @@ static void send_subscription_change_message(struct stasis_topic *topic, char *u
 	stasis_publish(topic, msg);
 }
 
+static void send_subscription_unsubscribe(struct stasis_topic *topic,
+	struct stasis_subscription *sub)
+{
+	RAII_VAR(struct stasis_subscription_change *, change, NULL, ao2_cleanup);
+	RAII_VAR(struct stasis_message *, msg, NULL, ao2_cleanup);
+
+	/* This assumes that we have already unsubscribed */
+	ast_assert(!stasis_subscription_is_subscribed(sub));
+
+	change = subscription_change_alloc(topic, sub->uniqueid, "Unsubscribe");
+
+	if (!change) {
+		return;
+	}
+
+	msg = stasis_message_create(stasis_subscription_change_type(), change);
+
+	if (!msg) {
+		return;
+	}
+
+	stasis_publish(topic, msg);
+
+	/* Now we have to dispatch to the subscription itself */
+	dispatch_message(sub, topic, msg);
+}
+
 struct topic_pool_entry {
-	struct stasis_subscription *forward;
+	struct stasis_forward *forward;
 	struct stasis_topic *topic;
 };
 
 static void topic_pool_entry_dtor(void *obj)
 {
 	struct topic_pool_entry *entry = obj;
-	entry->forward = stasis_unsubscribe(entry->forward);
+	entry->forward = stasis_forward_cancel(entry->forward);
 	ao2_cleanup(entry->topic);
 	entry->topic = NULL;
 }
diff --git a/main/stasis_cache_pattern.c b/main/stasis_cache_pattern.c
index 381fdd9898..9644028c36 100644
--- a/main/stasis_cache_pattern.c
+++ b/main/stasis_cache_pattern.c
@@ -39,15 +39,15 @@ struct stasis_cp_all {
 	struct stasis_topic *topic_cached;
 	struct stasis_cache *cache;
 
-	struct stasis_subscription *forward_all_to_cached;
+	struct stasis_forward *forward_all_to_cached;
 };
 
 struct stasis_cp_single {
 	struct stasis_topic *topic;
 	struct stasis_caching_topic *topic_cached;
 
-	struct stasis_subscription *forward_topic_to_all;
-	struct stasis_subscription *forward_cached_to_all;
+	struct stasis_forward *forward_topic_to_all;
+	struct stasis_forward *forward_cached_to_all;
 };
 
 static void all_dtor(void *obj)
@@ -60,7 +60,7 @@ static void all_dtor(void *obj)
 	all->topic_cached = NULL;
 	ao2_cleanup(all->cache);
 	all->cache = NULL;
-	stasis_unsubscribe_and_join(all->forward_all_to_cached);
+	stasis_forward_cancel(all->forward_all_to_cached);
 	all->forward_all_to_cached = NULL;
 }
 
@@ -172,9 +172,9 @@ void stasis_cp_single_unsubscribe(struct stasis_cp_single *one)
 		return;
 	}
 
-	stasis_unsubscribe(one->forward_topic_to_all);
+	stasis_forward_cancel(one->forward_topic_to_all);
 	one->forward_topic_to_all = NULL;
-	stasis_unsubscribe(one->forward_cached_to_all);
+	stasis_forward_cancel(one->forward_cached_to_all);
 	one->forward_cached_to_all = NULL;
 	stasis_caching_unsubscribe(one->topic_cached);
 	one->topic_cached = NULL;
diff --git a/res/stasis/app.c b/res/stasis/app.c
index ab46be5086..2c84f0c3de 100644
--- a/res/stasis/app.c
+++ b/res/stasis/app.c
@@ -58,9 +58,9 @@ struct app_forwards {
 	int interested;
 
 	/*! Forward for the regular topic */
-	struct stasis_subscription *topic_forward;
+	struct stasis_forward *topic_forward;
 	/*! Forward for the caching topic */
-	struct stasis_subscription *topic_cached_forward;
+	struct stasis_forward *topic_cached_forward;
 
 	/*! Unique id of the object being forwarded */
 	char id[];
@@ -78,9 +78,9 @@ static void forwards_dtor(void *obj)
 
 static void forwards_unsubscribe(struct app_forwards *forwards)
 {
-	stasis_unsubscribe(forwards->topic_forward);
+	stasis_forward_cancel(forwards->topic_forward);
 	forwards->topic_forward = NULL;
-	stasis_unsubscribe(forwards->topic_cached_forward);
+	stasis_forward_cancel(forwards->topic_cached_forward);
 	forwards->topic_cached_forward = NULL;
 }
 
@@ -129,7 +129,7 @@ static struct app_forwards *forwards_create_channel(struct app *app,
 		ast_channel_topic_cached(chan), app->topic);
 	if (!forwards->topic_cached_forward) {
 		/* Half-subscribed is a bad thing */
-		stasis_unsubscribe(forwards->topic_forward);
+		stasis_forward_cancel(forwards->topic_forward);
 		forwards->topic_forward = NULL;
 		return NULL;
 	}
@@ -163,7 +163,7 @@ static struct app_forwards *forwards_create_bridge(struct app *app,
 		ast_bridge_topic_cached(bridge), app->topic);
 	if (!forwards->topic_cached_forward) {
 		/* Half-subscribed is a bad thing */
-		stasis_unsubscribe(forwards->topic_forward);
+		stasis_forward_cancel(forwards->topic_forward);
 		forwards->topic_forward = NULL;
 		return NULL;
 	}
diff --git a/tests/test_stasis.c b/tests/test_stasis.c
index 498df94402..ac6154d88d 100644
--- a/tests/test_stasis.c
+++ b/tests/test_stasis.c
@@ -427,7 +427,7 @@ AST_TEST_DEFINE(forward)
 	RAII_VAR(struct consumer *, parent_consumer, NULL, ao2_cleanup);
 	RAII_VAR(struct consumer *, consumer, NULL, ao2_cleanup);
 
-	RAII_VAR(struct stasis_subscription *, forward_sub, NULL, stasis_unsubscribe);
+	RAII_VAR(struct stasis_forward *, forward_sub, NULL, stasis_forward_cancel);
 	RAII_VAR(struct stasis_subscription *, parent_sub, NULL, stasis_unsubscribe);
 	RAII_VAR(struct stasis_subscription *, sub, NULL, stasis_unsubscribe);
 
@@ -499,8 +499,8 @@ AST_TEST_DEFINE(interleaving)
 	RAII_VAR(struct stasis_message *, test_message2, NULL, ao2_cleanup);
 	RAII_VAR(struct stasis_message *, test_message3, NULL, ao2_cleanup);
 
-	RAII_VAR(struct stasis_subscription *, forward_sub1, NULL, stasis_unsubscribe);
-	RAII_VAR(struct stasis_subscription *, forward_sub2, NULL, stasis_unsubscribe);
+	RAII_VAR(struct stasis_forward *, forward_sub1, NULL, stasis_forward_cancel);
+	RAII_VAR(struct stasis_forward *, forward_sub2, NULL, stasis_forward_cancel);
 	RAII_VAR(struct stasis_subscription *, sub, NULL, stasis_unsubscribe);
 
 	RAII_VAR(struct consumer *, consumer, NULL, ao2_cleanup);
diff --git a/tests/test_stasis_endpoints.c b/tests/test_stasis_endpoints.c
index c0be07ca83..bc0f57572b 100644
--- a/tests/test_stasis_endpoints.c
+++ b/tests/test_stasis_endpoints.c
@@ -264,11 +264,14 @@ AST_TEST_DEFINE(channel_messages)
 	type = stasis_message_type(msg);
 	ast_test_validate(test, ast_channel_snapshot_type() == type);
 
+	/* The ordering of the cache clear and endpoint snapshot are
+	 * unspecified */
 	msg = sink->messages[3];
-	type = stasis_message_type(msg);
-	ast_test_validate(test, stasis_cache_clear_type() == type);
+	if (stasis_message_type(msg) == stasis_cache_clear_type()) {
+		/* Okay; the next message should be the endpoint snapshot */
+		msg = sink->messages[4];
+	}
 
-	msg = sink->messages[4];
 	type = stasis_message_type(msg);
 	ast_test_validate(test, ast_endpoint_snapshot_type() == type);
 	actual_snapshot = stasis_message_data(msg);