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 16a4e4a.  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

src/backend/commands/dbcommands.c

@@ -997,12 +997,15 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 
 	/*
 	 * Force a checkpoint to make sure the checkpointer has received the
-	 * message sent by ForgetDatabaseSyncRequests. On Windows, this also
-	 * ensures that background procs don't hold any open files, which would
-	 * cause rmdir() to fail.
+	 * message sent by ForgetDatabaseSyncRequests.
 	 */
 	RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);
 
+#if defined(USE_BARRIER_SMGRRELEASE)
+	/* Close all smgr fds in all backends. */
+	WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+#endif
+
 	/*
 	 * Remove all tablespace subdirs belonging to the database.
 	 */
@@ -1251,6 +1254,11 @@ movedb(const char *dbname, const char *tblspcname)
 	RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT
 					  | CHECKPOINT_FLUSH_ALL);
 
+#if defined(USE_BARRIER_SMGRRELEASE)
+	/* Close all smgr fds in all backends. */
+	WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+#endif
+
 	/*
 	 * Now drop all buffers holding data of the target database; they should
 	 * no longer be dirty so DropDatabaseBuffers is safe.
@@ -2258,6 +2266,11 @@ dbase_redo(XLogReaderState *record)
 		/* Clean out the xlog relcache too */
 		XLogDropDatabase(xlrec->db_id);
 
+#if defined(USE_BARRIER_SMGRRELEASE)
+		/* Close all sgmr fds in all backends. */
+		WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+#endif
+
 		for (i = 0; i < xlrec->ntablespaces; i++)
 		{
 			dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_ids[i]);

src/backend/commands/tablespace.c

@@ -536,15 +536,24 @@ DropTableSpace(DropTableSpaceStmt *stmt)
 		 * but we can't tell them apart from important data files that we
 		 * mustn't delete.  So instead, we force a checkpoint which will clean
 		 * out any lingering files, and try again.
-		 *
-		 * XXX On Windows, an unlinked file persists in the directory listing
+		 */
+		RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);
+
+		/*
+		 * On Windows, an unlinked file persists in the directory listing
 		 * until no process retains an open handle for the file.  The DDL
 		 * commands that schedule files for unlink send invalidation messages
-		 * directing other PostgreSQL processes to close the files.  DROP
-		 * TABLESPACE should not give up on the tablespace becoming empty
-		 * until all relevant invalidation processing is complete.
+		 * directing other PostgreSQL processes to close the files, but
+		 * nothing guarantees they'll be processed in time.  So, we'll also
+		 * use a global barrier to ask all backends to close all files, and
+		 * wait until they're finished.
 		 */
-		RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);
+#if defined(USE_BARRIER_SMGRRELEASE)
+		LWLockRelease(TablespaceCreateLock);
+		WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+		LWLockAcquire(TablespaceCreateLock, LW_EXCLUSIVE);
+#endif
+		/* And now try again. */
 		if (!destroy_tablespace_directories(tablespaceoid, false))
 		{
 			/* Still not empty, the files must be important then */
@@ -1582,6 +1591,11 @@ tblspc_redo(XLogReaderState *record)
 		 */
 		if (!destroy_tablespace_directories(xlrec->ts_id, true))
 		{
+#if defined(USE_BARRIER_SMGRRELEASE)
+			/* Close all smgr fds in all backends. */
+			WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+#endif
+
 			ResolveRecoveryConflictWithTablespace(xlrec->ts_id);
 
 			/*

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

src/backend/storage/smgr/md.c

@@ -549,6 +549,12 @@ mdclose(SMgrRelation reln, ForkNumber forknum)
 	}
 }
 
+void
+mdrelease(void)
+{
+	closeAllVfds();
+}
+
 /*
  *	mdprefetch() -- Initiate asynchronous read of the specified block of a relation
  */

src/backend/storage/smgr/smgr.c

@@ -41,6 +41,7 @@ typedef struct f_smgr
 {
 	void		(*smgr_init) (void);	/* may be NULL */
 	void		(*smgr_shutdown) (void);	/* may be NULL */
+	void		(*smgr_release) (void); /* may be NULL */
 	void		(*smgr_open) (SMgrRelation reln);
 	void		(*smgr_close) (SMgrRelation reln, ForkNumber forknum);
 	void		(*smgr_create) (SMgrRelation reln, ForkNumber forknum,
@@ -69,6 +70,7 @@ static const f_smgr smgrsw[] = {
 	{
 		.smgr_init = mdinit,
 		.smgr_shutdown = NULL,
+		.smgr_release = mdrelease,
 		.smgr_open = mdopen,
 		.smgr_close = mdclose,
 		.smgr_create = mdcreate,
@@ -693,3 +695,19 @@ AtEOXact_SMgr(void)
 		smgrclose(rel);
 	}
 }
+
+/*
+ * This routine is called when we are ordered to release all open files by a
+ * ProcSignalBarrier.
+ */
+bool
+ProcessBarrierSmgrRelease(void)
+{
+	for (int i = 0; i < NSmgr; i++)
+	{
+		if (smgrsw[i].smgr_release)
+			smgrsw[i].smgr_release();
+	}
+
+	return true;
+}

src/include/pg_config_manual.h

@@ -152,6 +152,17 @@
 #define EXEC_BACKEND
 #endif
 
+/*
+ * If USE_BARRIER_SMGRRELEASE is defined, certain code paths that unlink
+ * directories will ask other backends to close all smgr file descriptors.
+ * This is enabled on Windows, because otherwise unlinked but still open files
+ * can prevent rmdir(containing_directory) from succeeding.  On other
+ * platforms, it can be defined to exercise those code paths.
+ */
+#if defined(WIN32)
+#define USE_BARRIER_SMGRRELEASE
+#endif
+
 /*
  * Define this if your operating system supports link()
  */

src/include/storage/md.h

@@ -23,6 +23,7 @@
 extern void mdinit(void);
 extern void mdopen(SMgrRelation reln);
 extern void mdclose(SMgrRelation reln, ForkNumber forknum);
+extern void mdrelease(void);
 extern void mdcreate(SMgrRelation reln, ForkNumber forknum, bool isRedo);
 extern bool mdexists(SMgrRelation reln, ForkNumber forknum);
 extern void mdunlink(RelFileNodeBackend rnode, ForkNumber forknum, bool isRedo);

src/include/storage/procsignal.h

@@ -49,12 +49,7 @@ typedef enum
 
 typedef enum
 {
-	/*
-	 * XXX. PROCSIGNAL_BARRIER_PLACEHOLDER should be replaced when the first
-	 * real user of the ProcSignalBarrier mechanism is added. It's just here
-	 * for now because we can't have an empty enum.
-	 */
-	PROCSIGNAL_BARRIER_PLACEHOLDER = 0
+	PROCSIGNAL_BARRIER_SMGRRELEASE	/* ask smgr to close files */
 } ProcSignalBarrierType;
 
 /*

src/include/storage/smgr.h

@@ -104,5 +104,6 @@ extern void smgrtruncate(SMgrRelation reln, ForkNumber *forknum,
 						 int nforks, BlockNumber *nblocks);
 extern void smgrimmedsync(SMgrRelation reln, ForkNumber forknum);
 extern void AtEOXact_SMgr(void);
+extern bool ProcessBarrierSmgrRelease(void);
 
 #endif							/* SMGR_H */