messaging3: Add comments about not touching "waiters"

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/source3/lib/messages.c b/source3/lib/messages.c
index 065782ac1b4..6a08531f2a8 100644
--- a/source3/lib/messages.c
+++ b/source3/lib/messages.c
@@ -491,6 +491,14 @@ struct tevent_req *messaging_filtered_read_send(
 	state->filter = filter;
 	state->private_data = private_data;
 
+	/*
+	 * We add ourselves to the "new_waiters" array, not the "waiters"
+	 * array. If we are called from within messaging_read_done,
+	 * messaging_dispatch_rec will be in an active for-loop on
+	 * "waiters". We must be careful not to mess with this array, because
+	 * it could mean that a single event is being delivered twice.
+	 */
+
 	new_waiters_len = talloc_array_length(msg_ctx->new_waiters);
 
 	if (new_waiters_len == msg_ctx->num_new_waiters) {
@@ -521,6 +529,14 @@ static void messaging_filtered_read_cleanup(struct tevent_req *req,
 
 	tevent_req_set_cleanup_fn(req, NULL);
 
+	/*
+	 * Just set the [new_]waiters entry to NULL, be careful not to mess
+	 * with the other "waiters" array contents. We are often called from
+	 * within "messaging_dispatch_rec", which loops over
+	 * "waiters". Messing with the "waiters" array will mess up that
+	 * for-loop.
+	 */
+
 	for (i=0; i<msg_ctx->num_waiters; i++) {
 		if (msg_ctx->waiters[i] == req) {
 			msg_ctx->waiters[i] = NULL;