Improve handling of T.38 re-INVITEs that arrive before a T.38-capable

application is executing on a channel.

This patch addresses an issue found during working with end-users
using res_fax. If an incoming call is answered in the dialplan, or
jumps to the 'fax' extension due to reception of a CNG tone (with
faxdetect enabled), and then the remote endpoint sends a T.38
re-INVITE, it is possible for the channel's T.38 state to be
'T38_STATE_NEGOTIATING' when the application starts up. Unfortunately,
even if the application wants to use T.38, it can't respond to the
peer's negotiation request, because the AST_CONTROL_T38_PARAMETERS
control frame that chan_sip sent originally has been lost, and the
application needs the content of that frame to be able to formulate a
reply.

This patch adds a new 'request' type to AST_CONTROL_T38_PARAMETERS,
AST_T38_REQUEST_PARMS. If the application sends this request, chan_sip
will re-send the original control frame (with
AST_T38_REQUEST_NEGOTIATE as the request type), and the application
can respond as normal. If this occurs within the five second timeout
in chan_sip, the automatic cancellation of the peer reinvite will be
stopped, and the application will 'own' the negotiation process from
that point onwards.

This also improves the code path in chan_sip to allow sip_indicate(),
when called for AST_CONTROL_T38_PARAMETERS, to be able to return a
non-zero response, which should have been in place before since the
control frame *can* fail to be processed properly. It also modifies
ast_indicate() to return whatever result the channel driver returned
for this control frame, rather than converting all non-zero results
into '-1'. Finally, the new request type intentionally returns a
positive value, so that an application that sends
AST_T38_REQUEST_PARMS can know for certain whether the channel driver
accepted it and will be replying with a control frame of its own, or
whether it was ignored (if the sip_indicate()/ast_indicate() path had
properly supported failure responses before, this would not be
necessary).

This patch also modifies res_fax to take advantage of the new request.

In addition, this patch makes sip_t38_abort() actually lock the
private structure before doing its work... bad programmer, no donut.

This patch also enhances chan_sip's 'faxdetect' support to allow
triggering on T.38 re-INVITEs received as well as CNG tone detection.

Review: https://reviewboard.asterisk.org/r/556/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@254450 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -3715,9 +3715,10 @@ int ast_indicate_data(struct ast_channel *chan, int _condition,
 		 * control frames, so we need to return failure, but there
 		 * is also no value in the log message below being emitted
 		 * since failure to handle these frames is not an 'error'
-		 * so just return right now.
-		 */
-		return -1;
+		 * so just return right now. in addition, we want to return
+		 * whatever value the channel driver returned, in case it
+		 * has some meaning.*/
+		return res;
 	case AST_CONTROL_RINGING:
 		ts = ast_get_indication_tone(chan->zone, "ring");
 		/* It is common practice for channel drivers to return -1 if trying