Fix race in dsm_unpin_segment() when handles are reused.

Teach dsm_unpin_segment() to skip segments that are in the process of being destroyed by another backend, when searching for a handle. Such a segment cannot possibly be the one we are looking for, even if its handle matches. Another slot might hold a recently created segment that has the same handle value by coincidence, and we need to keep searching for that one. The bug caused rare "cannot unpin a segment that is not pinned" errors on 10 and 11. Similar to commit 6c0fb94 for dsm_attach(). Back-patch to 10, where dsm_unpin_segment() landed. Author: Thomas Munro Reported-by: Justin Pryzby Tested-by: Justin Pryzby (along with other recent DSA/DSM fixes) Discussion: https://postgr.es/m/20190216023854.GF30291@telsasoft.com

Author: macdice

src/backend/storage/ipc/dsm.c

@@ -844,8 +844,8 @@ dsm_unpin_segment(dsm_handle handle)
 LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
 for (i = 0; i < dsm_control->nitems; ++i)
 {
-/* Skip unused slots. */
-if (dsm_control->item[i].refcnt == 0)
+/* Skip unused slots and segments that are concurrently going away. */
+if (dsm_control->item[i].refcnt <= 1)
 continue;
 
 /* If we've found our handle, we can stop searching. */