chan_pjsip: Fix deadlock when masquerading PJSIP channels.

Performing a directed call pickup resulted in a deadlock when PJSIP
channels were involved.

A masquerade needs to hold onto the channel locks while it swaps channel
information between the two channels involved in the masquerade.  With
PJSIP channels, the fixup routine needed to push a fixup task onto the
PJSIP channel's serializer.  Unfortunately, if the serializer was also
processing a task that needed to lock the channel, you get deadlock.

* Added a new control frame that is used to notify the channels that a
masquerade is about to start and when it has completed.

* Added the ability to query taskprocessors if the current thread is the
taskprocessor thread.

* Added the ability to suspend/unsuspend the PJSIP serializer thread so a
masquerade could fixup the PJSIP channel without using the serializer.

ASTERISK-24356 #close
Reported by: rmudgett

Review: https://reviewboard.asterisk.org/r/4034/
........

Merged revisions 424471 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@424472 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/core_unreal.c

@@ -530,6 +530,12 @@ int ast_unreal_indicate(struct ast_channel *ast, int condition, const void *data
 	ao2_ref(p, 1); /* ref for unreal_queue_frame */
 
 	switch (condition) {
+	case AST_CONTROL_MASQUERADE_NOTIFY:
+		/*
+		 * Always block this because this is the channel being
+		 * masqueraded; not anything down the chain.
+		 */
+		break;
 	case AST_CONTROL_CONNECTED_LINE:
 	case AST_CONTROL_REDIRECTING:
 		res = unreal_colp_redirect_indicate(p, ast, condition);