From 846f8c9483a8f31e45bf949db1721706a2765771 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 7 Aug 2015 09:04:07 -0500
Subject: [PATCH] Fix attach-related race condition in shm_mq_send_bytes.
Spotted by Antonin Houska.
---
src/backend/storage/ipc/shm_mq.c | 38 ++++++++++++++++++--------------
1 file changed, 21 insertions(+), 17 deletions(-)
diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index e765cea5aaa..0e60dbcddc8 100644
--- 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);
--
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