chan_dahdi.c: Lock private struct for ast_write().

There is a window of opportunity for DTMF to not go out if an audio frame
is in the process of being written to DAHDI while another thread starts
sending DTMF.  The thread sending the audio frame could be past the
currently dialing check before being preempted by another thread starting
a DTMF generation request.  When the thread sending the audio frame
resumes it will then cause DAHDI to stop the DTMF tone generation.  The
result is no DTMF goes out.

* Made dahdi_write() lock the private struct before writing to the DAHDI
file descriptor.

ASTERISK-25315
Reported by John Hardin

Change-Id: Ib4e0264cf63305ed5da701188447668e72ec9abb

channels/chan_dahdi.c

@@ -8772,37 +8772,50 @@ static int my_dahdi_write(struct dahdi_pvt *p, unsigned char *buf, int len, int
 
 static int dahdi_write(struct ast_channel *ast, struct ast_frame *frame)
 {
-	struct dahdi_pvt *p = ast_channel_tech_pvt(ast);
+	struct dahdi_pvt *p;
 	int res;
 	int idx;
+
+	/* Write a frame of (presumably voice) data */
+	if (frame->frametype != AST_FRAME_VOICE) {
+		if (frame->frametype != AST_FRAME_IMAGE) {
+			ast_log(LOG_WARNING, "Don't know what to do with frame type '%u'\n",
+				frame->frametype);
+		}
+		return 0;
+	}
+
+	/* Return if it's not valid data */
+	if (!frame->data.ptr || !frame->datalen) {
+		return 0;
+	}
+
+	p = ast_channel_tech_pvt(ast);
+	ast_mutex_lock(&p->lock);
+
 	idx = dahdi_get_index(ast, p, 0);
 	if (idx < 0) {
+		ast_mutex_unlock(&p->lock);
 		ast_log(LOG_WARNING, "%s doesn't really exist?\n", ast_channel_name(ast));
 		return -1;
 	}
 
-	/* Write a frame of (presumably voice) data */
-	if (frame->frametype != AST_FRAME_VOICE) {
-		if (frame->frametype != AST_FRAME_IMAGE)
-			ast_log(LOG_WARNING, "Don't know what to do with frame type '%u'\n", frame->frametype);
-		return 0;
-	}
 	if (p->dialing) {
+		ast_mutex_unlock(&p->lock);
 		ast_debug(5, "Dropping frame since I'm still dialing on %s...\n",ast_channel_name(ast));
 		return 0;
 	}
 	if (!p->owner) {
+		ast_mutex_unlock(&p->lock);
 		ast_debug(5, "Dropping frame since there is no active owner on %s...\n",ast_channel_name(ast));
 		return 0;
 	}
 	if (p->cidspill) {
+		ast_mutex_unlock(&p->lock);
 		ast_debug(5, "Dropping frame since I've still got a callerid spill on %s...\n",
 			ast_channel_name(ast));
 		return 0;
 	}
-	/* Return if it's not valid data */
-	if (!frame->data.ptr || !frame->datalen)
-		return 0;
 
 	if (ast_format_cmp(frame->subclass.format, ast_format_slin) == AST_FORMAT_CMP_EQUAL) {
 		if (!p->subs[idx].linear) {
@@ -8823,10 +8836,12 @@ static int dahdi_write(struct ast_channel *ast, struct ast_frame *frame)
 		}
 		res = my_dahdi_write(p, (unsigned char *)frame->data.ptr, frame->datalen, idx, 0);
 	} else {
+		ast_mutex_unlock(&p->lock);
 		ast_log(LOG_WARNING, "Cannot handle frames in %s format\n",
 			ast_format_get_name(frame->subclass.format));
 		return -1;
 	}
+	ast_mutex_unlock(&p->lock);
 	if (res < 0) {
 		ast_log(LOG_WARNING, "write failed: %s\n", strerror(errno));
 		return -1;