Fix attach-related race condition in shm_mq_send_bytes.

Spotted by Antonin Houska.

diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index e765cea5aaa4e090d37b47b71a1df59c98cc979b..0e60dbcddc82ec14612980372af4a790b1450a73 100644 (file)
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/src/backend/storage/ipc/shm_mq.c
@@ -777,33 +777,37 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
                        return SHM_MQ_DETACHED;
                }
 
-               if (available == 0)
+               if (available == 0 && !mqh->mqh_counterparty_attached)
                {
-                       shm_mq_result res;
-
                        /*
                         * The queue is full, so if the receiver isn't yet known to be
                         * attached, we must wait for that to happen.
                         */
-                       if (!mqh->mqh_counterparty_attached)
+                       if (nowait)
                        {
-                               if (nowait)
+                               if (shm_mq_get_receiver(mq) == NULL)
                                {
-                                       if (shm_mq_get_receiver(mq) == NULL)
-                                       {
-                                               *bytes_written = sent;
-                                               return SHM_MQ_WOULD_BLOCK;
-                                       }
-                               }
-                               else if (!shm_mq_wait_internal(mq, &mq->mq_receiver,
-                                                                                          mqh->mqh_handle))
-                               {
-                                       mq->mq_detached = true;
                                        *bytes_written = sent;
-                                       return SHM_MQ_DETACHED;
+                                       return SHM_MQ_WOULD_BLOCK;
                                }
-                               mqh->mqh_counterparty_attached = true;
                        }
+                       else if (!shm_mq_wait_internal(mq, &mq->mq_receiver,
+                                                                                  mqh->mqh_handle))
+                       {
+                               mq->mq_detached = true;
+                               *bytes_written = sent;
+                               return SHM_MQ_DETACHED;
+                       }
+                       mqh->mqh_counterparty_attached = true;
+
+                       /*
+                        * The receiver may have read some data after attaching, so we
+                        * must not wait without rechecking the queue state.
+                        */
+               }
+               else if (available == 0)
+               {
+                       shm_mq_result res;
 
                        /* Let the receiver know that we need them to read some data. */
                        res = shm_mq_notify_receiver(mq);