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shm_mq: Fix detach race condition.

Commit34db06e adopted a lock-freedesign for shm_mq.c, but it introduced a race condition that couldlose messages. When shm_mq_receive_bytes() detects that the other endhas detached, it must make sure that it has seen the final version ofmq_bytes_written, or it might miss a message sent before detaching.Thomas MunroDiscussion:https://postgr.es/m/CAEepm%3D2myZ4qxpt1a%3DC%2BwEv3o188K13K3UvD-44FK0SdAzHy%2Bw%40mail.gmail.com

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src/backend/storage/ipc
- shm_mq.c

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`‎src/backend/storage/ipc/shm_mq.c`

Lines changed: 12 additions & 0 deletions

Original file line number	Diff line number	Diff line change
`@@ -1058,7 +1058,19 @@ shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait,`
`1058`	`1058`	`* detached.`
`1059`	`1059`	`*/`
`1060`	`1060`	`if (mq->mq_detached)`
	`1061`	`+{`
	`1062`	`+/*`
	`1063`	`+ * If the writer advanced mq_bytes_written and then set`
	`1064`	`+ * mq_detached, we might not have read the final value of`
	`1065`	`+ * mq_bytes_written above. Insert a read barrier and then check`
	`1066`	`+ * again if mq_bytes_written has advanced.`
	`1067`	`+ */`
	`1068`	`+pg_read_barrier();`
	`1069`	`+if (written!=pg_atomic_read_u64(&mq->mq_bytes_written))`
	`1070`	`+continue;`
	`1071`	`+`
`1061`	`1072`	`returnSHM_MQ_DETACHED;`
	`1073`	`+}`
`1062`	`1074`
`1063`	`1075`	`/*`
`1064`	`1076`	`* We didn't get enough data to satisfy the request, so mark any data`

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Commit42d7074

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`‎src/backend/storage/ipc/shm_mq.c`

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