res_musiconhold: Fix MOH restarting where it left off from the last hold.

Restore code removed by https://reviewboard.asterisk.org/r/3536/ that
introduced a regression that prevents MOH from restarting were it left off
the last time.

ASTERISK-24019 #close
Reported by: Jason Richards
Patches:
      jira_asterisk_24019_v1.8.patch (license #5621) patch uploaded by rmudgett

Review: https://reviewboard.asterisk.org/r/3928/
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Merged revisions 421976 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421977 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 421978 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@421979 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_musiconhold.c

@@ -138,6 +138,8 @@ struct moh_files_state {
 	int sample_queue;
 	int pos;
 	int save_pos;
+	int save_total;
+	char name[MAX_MUSICCLASS];
 	char save_pos_filename[PATH_MAX];
 };
 
@@ -474,11 +476,9 @@ static void *moh_files_alloc(struct ast_channel *chan, void *params)
 		}
 	}
 
-	/* class is reffed, so we can safely compare it against the (possibly
-	 * recently unreffed) state->class. The unref was done after the ref
-	 * of class, so we're sure that they won't point to the same memory
-	 * by accident. */
-	if (state->class != class) {
+	/* Resume MOH from where we left off last time or start from scratch? */
+	if (state->save_total != class->total_files || strcmp(state->name, class->name) != 0) {
+		/* Start MOH from scratch. */
 		memset(state, 0, sizeof(*state));
 		if (ast_test_flag(class, MOH_RANDOMIZE) && class->total_files) {
 			state->pos = ast_random() % class->total_files;
@@ -489,6 +489,9 @@ static void *moh_files_alloc(struct ast_channel *chan, void *params)
 	/* it's possible state is not a new allocation, don't leak old refs */
 	ao2_replace(state->origwfmt, ast_channel_writeformat(chan));
 	ao2_replace(state->mohwfmt, ast_channel_writeformat(chan));
+	/* For comparison on restart of MOH (see above) */
+	ast_copy_string(state->name, class->name, sizeof(state->name));
+	state->save_total = class->total_files;
 
 	moh_post_start(chan, class->name);