res_stasis_snoop: Spying on a single direction continually increases CPU

Creating a snoop channel in ARI and spying only on a single direction (in or
out) results in CPU utilization continually increasing until the CPU is fully
consumed. This occurs because frames are being put in the opposing direction's
slin factory queue, but not being removed.

Fixed the problem by always reading and disposing of frames from the opposite
queue of the direction selected.

ASTERISK-24938 #closes

Change-Id: I935bfd15f1db958f364d9d6b3b45582c0113dd60

res/res_stasis_snoop.c

@@ -177,12 +177,28 @@ static struct ast_frame *snoop_read(struct ast_channel *chan)
 	}
 
 	/* Only get audio from the spy audiohook if it is active */
-	if (snoop->spy_active) {
-		ast_audiohook_lock(&snoop->spy);
-		frame = ast_audiohook_read_frame(&snoop->spy, snoop->spy_samples, snoop->spy_direction, snoop->spy_format);
-		ast_audiohook_unlock(&snoop->spy);
+	if (!snoop->spy_active) {
+		return &ast_null_frame;
 	}
 
+	ast_audiohook_lock(&snoop->spy);
+	if (snoop->spy_direction != AST_AUDIOHOOK_DIRECTION_BOTH) {
+		/*
+		 * When a singular direction is chosen frames are still written to the
+		 * opposing direction's queue. Those frames must be read so the queue
+		 * does not continue to grow, however since they are not needed for the
+		 * selected direction they can be dropped.
+		 */
+		enum ast_audiohook_direction opposing_direction =
+			snoop->spy_direction == AST_AUDIOHOOK_DIRECTION_READ ?
+			AST_AUDIOHOOK_DIRECTION_WRITE : AST_AUDIOHOOK_DIRECTION_READ;
+		ast_frame_dtor(ast_audiohook_read_frame(&snoop->spy, snoop->spy_samples,
+							opposing_direction, snoop->spy_format));
+	}
+
+	frame = ast_audiohook_read_frame(&snoop->spy, snoop->spy_samples, snoop->spy_direction, snoop->spy_format);
+	ast_audiohook_unlock(&snoop->spy);
+
 	return frame ? frame : &ast_null_frame;
 }