Fix DROP {DATABASE,TABLESPACE} on Windows.

Previously, it was possible for DROP DATABASE, DROP TABLESPACE and ALTER
DATABASE SET TABLESPACE to fail because other backends still had file
handles open for dropped tables.  Windows won't allow a directory
containing unlinked-but-still-open files to be unlinked.  Tackle this
problem by forcing all backends to close all smgr fds.  No change for
Unix systems, which don't suffer from the problem, but the new code path
can be tested by Unix-based developers by defining
USE_BARRIER_SMGRRELEASE explicitly.

It's possible that PROCSIGNAL_BARRIER_SMGRRELEASE will have more
bug-fixing applications soon (under discussion).  Note that this is the
first user of the ProcSignalBarrier mechanism from commit 16a4e4aec.  It
could in principle be back-patched as far as 14, but since field
complaints are rare and ProcSignalBarrier hasn't been battle-tested,
that seems like a bad idea.  Fix in master only, where these failures
have started to show up in automated testing due to new tests.

Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGLdemy2gBm80kz20GTe6hNVwoErE8KwcJk6-U56oStjtg@mail.gmail.com

1 files changed, 3 insertions, 21 deletions

diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index d158bb7a19f..f41563a0a48 100644
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -28,6 +28,7 @@
 #include "storage/latch.h"
 #include "storage/proc.h"
 #include "storage/shmem.h"
+#include "storage/smgr.h"
 #include "storage/sinval.h"
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
@@ -94,7 +95,6 @@ static ProcSignalSlot *MyProcSignalSlot = NULL;
 static bool CheckProcSignal(ProcSignalReason reason);
 static void CleanupProcSignalState(int status, Datum arg);
 static void ResetProcSignalBarrierBits(uint32 flags);
-static bool ProcessBarrierPlaceholder(void);
 static inline int GetNumProcSignalSlots(void);
 
 /*
@@ -536,8 +536,8 @@ ProcessProcSignalBarrier(void)
 				type = (ProcSignalBarrierType) pg_rightmost_one_pos32(flags);
 				switch (type)
 				{
-					case PROCSIGNAL_BARRIER_PLACEHOLDER:
-						processed = ProcessBarrierPlaceholder();
+					case PROCSIGNAL_BARRIER_SMGRRELEASE:
+						processed = ProcessBarrierSmgrRelease();
 						break;
 				}
 
@@ -603,24 +603,6 @@ ResetProcSignalBarrierBits(uint32 flags)
 	InterruptPending = true;
 }
 
-static bool
-ProcessBarrierPlaceholder(void)
-{
-	/*
-	 * XXX. This is just a placeholder until the first real user of this
-	 * machinery gets committed. Rename PROCSIGNAL_BARRIER_PLACEHOLDER to
-	 * PROCSIGNAL_BARRIER_SOMETHING_ELSE where SOMETHING_ELSE is something
-	 * appropriately descriptive. Get rid of this function and instead have
-	 * ProcessBarrierSomethingElse. Most likely, that function should live in
-	 * the file pertaining to that subsystem, rather than here.
-	 *
-	 * The return value should be 'true' if the barrier was successfully
-	 * absorbed and 'false' if not. Note that returning 'false' can lead to
-	 * very frequent retries, so try hard to make that an uncommon case.
-	 */
-	return true;
-}
-
 /*
  * CheckProcSignal - check to see if a particular reason has been
  * signaled, and clear the signal flag.  Should be called after receiving