From d4aeb3e17d7fb053af0ca94f5f59ef1ea0c34f98 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Mon, 21 Feb 2022 16:42:16 -0800
Subject: [PATCH v1 3/4] WIP: fix old-fd issues using global barriers
 everywhere.

---
 src/include/pg_config_manual.h      |  9 ---------
 src/include/storage/smgr.h          |  1 +
 src/backend/commands/tablespace.c   | 10 +++++-----
 src/backend/storage/buffer/bufmgr.c | 11 ++++++++---
 src/backend/storage/smgr/smgr.c     | 20 ++++++++++++++++++++
 5 files changed, 34 insertions(+), 17 deletions(-)

diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index 84ce5a4a5d7..1eb39884144 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -152,16 +152,7 @@
 #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()
diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index 8e3ef92cda1..d774fc98cf8 100644
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -79,6 +79,7 @@ typedef SMgrRelationData *SMgrRelation;
 
 extern void smgrinit(void);
 extern SMgrRelation smgropen(RelFileNode rnode, BackendId backend);
+extern SMgrRelation smgropen_cond(RelFileNode rnode, BackendId backend);
 extern bool smgrexists(SMgrRelation reln, ForkNumber forknum);
 extern void smgrsetowner(SMgrRelation *owner, SMgrRelation reln);
 extern void smgrclearowner(SMgrRelation *owner, SMgrRelation reln);
diff --git a/src/backend/commands/tablespace.c b/src/backend/commands/tablespace.c
index 40514ab550f..0c433360b63 100644
--- a/src/backend/commands/tablespace.c
+++ b/src/backend/commands/tablespace.c
@@ -1574,6 +1574,11 @@ tblspc_redo(XLogReaderState *record)
 	{
 		xl_tblspc_drop_rec *xlrec = (xl_tblspc_drop_rec *) XLogRecGetData(record);
 
+#if defined(USE_BARRIER_SMGRRELEASE)
+		/* Close all smgr fds in all backends. */
+		WaitForProcSignalBarrier(EmitProcSignalBarrier(PROCSIGNAL_BARRIER_SMGRRELEASE));
+#endif
+
 		/*
 		 * If we issued a WAL record for a drop tablespace it implies that
 		 * there were no files in it at all when the DROP was done. That means
@@ -1591,11 +1596,6 @@ 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);
 
 			/*
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index f5459c68f89..e9fd512df7c 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -4819,9 +4819,14 @@ IssuePendingWritebacks(WritebackContext *context)
 
 		i += ahead;
 
-		/* and finally tell the kernel to write the data to storage */
-		reln = smgropen(tag.rnode, InvalidBackendId);
-		smgrwriteback(reln, tag.forkNum, tag.blockNum, nblocks);
+		/*
+		 * Finally tell the kernel to write the data to storage. Don't smgr if
+		 * previously closed, otherwise we could end up evading fd-reuse
+		 * protection.
+		 */
+		reln = smgropen_cond(tag.rnode, InvalidBackendId);
+		if (reln)
+			smgrwriteback(reln, tag.forkNum, tag.blockNum, nblocks);
 	}
 
 	context->nr_pending = 0;
diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index d71a557a352..efa83ae3942 100644
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -190,6 +190,26 @@ smgropen(RelFileNode rnode, BackendId backend)
 	return reln;
 }
 
+/*
+ * Like smgropen(), but returns NULL if relation is not already open.
+ */
+SMgrRelation
+smgropen_cond(RelFileNode rnode, BackendId backend)
+{
+	RelFileNodeBackend brnode;
+
+	if (SMgrRelationHash == NULL)
+		return NULL;
+
+	/* Look up or create an entry */
+	brnode.node = rnode;
+	brnode.backend = backend;
+
+	return (SMgrRelation) hash_search(SMgrRelationHash,
+									  (void *) &brnode,
+									  HASH_FIND, NULL);
+}
+
 /*
  * smgrsetowner() -- Establish a long-lived reference to an SMgrRelation object
  *
-- 
2.34.0