dsync: race condition where done/finish is received after one side ha…

…s closed

We do not tell the remote we are closing if they have
already told us because they close the
connection after sending ITEM_FINISH or ITEM_DONE and will
not be ever receive anything else from us unless
it just happens to get combined into the same packet
as a previous response and is already in the buffer.

src/doveadm/dsync/dsync-ibc-stream.c

@@ -166,6 +166,8 @@ struct dsync_ibc_stream {
 	unsigned int version_received:1;
 	unsigned int handshake_received:1;
 	unsigned int has_pending_data:1;
+	unsigned int finish_received:1;
+	unsigned int done_received:1;
 	unsigned int stopped:1;
 };
 
@@ -365,10 +367,22 @@ static void dsync_ibc_stream_deinit(struct dsync_ibc *_ibc)
 	if (ibc->value_output != NULL)
 		i_stream_unref(&ibc->value_output);
 	else {
-		/* notify remote that we're closing. this is mainly to avoid
-		   "read() failed: EOF" errors on failing dsyncs */
-		o_stream_nsend_str(ibc->output,
-			t_strdup_printf("%c\n", items[ITEM_DONE].chr));
+		/* If the remote has not told us that they are closing we
+		   notify remote that we're closing. this is mainly to avoid
+		   "read() failed: EOF" errors on failing dsyncs.
+
+		   Avoid a race condition:
+		   We do not tell the remote we are closing if they have
+		   already told us because they close the
+		   connection after sending ITEM_DONE and will
+		   not be ever receive anything else from us unless
+		   it just happens to get combined into the same packet
+		   as a previous response and is already in the buffer.
+		*/
+		if (!ibc->done_received && !ibc->finish_received) {
+			o_stream_nsend_str(ibc->output,
+				t_strdup_printf("%c\n", items[ITEM_DONE].chr));
+		}
 		(void)o_stream_nfinish(ibc->output);
 	}
 
@@ -593,6 +607,7 @@ dsync_ibc_stream_input_next(struct dsync_ibc_stream *ibc, enum item_type item,
 		/* remote cleanly closed the connection, possibly because of
 		   some failure (which it should have logged). we don't want to
 		   log any stream errors anyway after this. */
+		ibc->done_received = TRUE;
 		dsync_ibc_stream_stop(ibc);
 		return DSYNC_IBC_RECV_RET_TRYAGAIN;
 
@@ -1938,6 +1953,8 @@ dsync_ibc_stream_recv_finish(struct dsync_ibc *_ibc, const char **error_r,
 		return DSYNC_IBC_RECV_RET_TRYAGAIN;
 	}
 	*mail_error_r = i;
+
+	ibc->finish_received = TRUE;
 	return DSYNC_IBC_RECV_RET_OK;
 }