Bridge core: Pass a ref with the swap channel when joining a bridge.

When code imparts a channel into a bridge to swap with another channel, a
ref needs to be held on the swap channel to ensure that it cannot
dissapear before finding it in the bridge.

* The ast_bridge_join() swap channel parameter now always steals a ref for
the swap channel.  This is the only change to the bridge framework's
public API semantics.

* bridge_channel_internal_join() now requires the bridge_channel->swap
channel to pass in a ref.

ASTERISK-24649
Reported by: John Bigelow

Review: https://reviewboard.asterisk.org/r/4354/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430975 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/bridge.h

@@ -444,15 +444,19 @@ enum ast_bridge_join_flags {
 };
 
 /*!
- * \brief Join (blocking) a channel to a bridge
+ * \brief Join a channel to a bridge (blocking)
  *
  * \param bridge Bridge to join
  * \param chan Channel to join
- * \param swap Channel to swap out if swapping
+ * \param swap Channel to swap out if swapping (A channel reference is stolen.)
  * \param features Bridge features structure
  * \param tech_args Optional Bridging tech optimization parameters for this channel.
  * \param flags defined by enum ast_bridge_join_flags.
  *
+ * \note The passed in swap channel is always unreffed on return.  It is not a
+ * good idea to access the swap channel on return or for the caller to keep a
+ * reference to it.
+ *
  * \note Absolutely _NO_ locks should be held before calling
  * this function since it blocks.
  *
@@ -495,7 +499,7 @@ enum ast_bridge_impart_flags {
 };
 
 /*!
- * \brief Impart (non-blocking) a channel onto a bridge
+ * \brief Impart a channel to a bridge (non-blocking)
  *
  * \param bridge Bridge to impart on
  * \param chan Channel to impart (The channel reference is stolen if impart successful.)

include/asterisk/bridge_channel_internal.h

@@ -103,6 +103,9 @@ void bridge_channel_settle_owed_events(struct ast_bridge *orig_bridge, struct as
  *
  * \param bridge_channel Channel to push.
  *
+ * \note A ref is not held by bridge_channel->swap when calling because the
+ * push with swap happens immediately.
+ *
  * \note On entry, bridge_channel->bridge is already locked.
  *
  * \retval 0 on success.
@@ -128,16 +131,22 @@ void bridge_channel_internal_pull(struct ast_bridge_channel *bridge_channel);
 
 /*!
  * \internal
- * \brief Join the bridge_channel to the bridge
+ * \brief Join the bridge_channel to the bridge (blocking)
  *
  * \param bridge_channel The Channel in the bridge
  *
+ * \note The bridge_channel->swap holds a channel reference for the swap
+ * channel going into the bridging system.  The ref ensures that the swap
+ * pointer is valid for the bridge subclass push callbacks.  The pointer
+ * will be NULL on return if the ref was consumed.
+ *
+ * \details
+ * This API call puts the bridge_channel into the bridge and handles the
+ * bridge_channel's processing of events while it is in the bridge.  It
+ * will return when the channel has been instructed to leave the bridge.
+ *
  * \retval 0 bridge channel successfully joined the bridge
  * \retval -1 bridge channel failed to join the bridge
- *
- * \note This API call starts the bridge_channel's processing of events while
- * it is in the bridge. It will return when the channel has been instructed to
- * leave the bridge.
  */
 int bridge_channel_internal_join(struct ast_bridge_channel *bridge_channel);
 

include/asterisk/bridge_internal.h

@@ -117,6 +117,9 @@ struct ast_bridge *bridge_base_init(struct ast_bridge *self, uint32_t capabiliti
  * \param attempt_recovery TRUE if failure attempts to push channel back into original bridge.
  * \param optimized Indicates whether the move is part of an unreal channel optimization.
  *
+ * \note A ref is not held by bridge_channel->swap when calling because the
+ * move with swap happens immediately.
+ *
  * \note The dst_bridge and bridge_channel->bridge are assumed already locked.
  *
  * \retval 0 on success.