Fix logic bug in dsm_attach().

The previous coding would potentially cause attaching to segment A to
fail if segment B was at the same time in the process of going away.

Andres Freund, with a comment tweak by me

diff --git a/src/backend/storage/ipc/dsm.c b/src/backend/storage/ipc/dsm.c
index 733fa5f7bd3de6f447af717aca7affafbe7aaf88..a5c008463a9de9fd2df34982e793bcfcc7241425 100644 (file)
--- a/src/backend/storage/ipc/dsm.c
+++ b/src/backend/storage/ipc/dsm.c
@@ -566,6 +566,10 @@ dsm_attach(dsm_handle h)
        if (dsm_control->item[i].refcnt == 0)
            continue;
 
+       /* If the handle doesn't match, it's not the slot we want. */
+       if (dsm_control->item[i].handle != seg->handle)
+           continue;
+
        /*
         * If the reference count is 1, the slot is still in use, but the
         * segment is in the process of going away.  Treat that as if we
@@ -574,13 +578,10 @@ dsm_attach(dsm_handle h)
        if (dsm_control->item[i].refcnt == 1)
            break;
 
-       /* Otherwise, if the descriptor matches, we've found a match. */
-       if (dsm_control->item[i].handle == seg->handle)
-       {
-           dsm_control->item[i].refcnt++;
-           seg->control_slot = i;
-           break;
-       }
+       /* Otherwise we've found a match. */
+       dsm_control->item[i].refcnt++;
+       seg->control_slot = i;
+       break;
    }
    LWLockRelease(DynamicSharedMemoryControlLock);