v5-0002-Give-SMgrRelation-pointers-a-well-defined-lifetim.patch

From 21998ff7585e20b53f8c6f3306920acf22bf9d2d Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Thu, 10 Aug 2023 18:16:31 +1200
Subject: [PATCH v5 02/10] Give SMgrRelation pointers a well-defined lifetime.

XXX This is a stand-in version of this patch, see
https://www.postgresql.org/message-id/CA%2BhUKGJ8NTvqLHz6dqbQnt2c8XCki4r2QvXjBQcXpVwxTY_pvA%40mail.gmail.com
for later verisons being discussed separately.
---
 src/backend/access/transam/xlogutils.c |  2 +-
 src/backend/postmaster/bgwriter.c      |  8 +++--
 src/backend/postmaster/checkpointer.c  | 15 ++++----
 src/backend/storage/smgr/smgr.c        | 49 ++++++++++++++++----------
 src/include/storage/smgr.h             |  4 +--
 src/include/utils/rel.h                |  6 ----
 6 files changed, 46 insertions(+), 38 deletions(-)

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index aa8667abd1..8775b5789b 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -657,7 +657,7 @@ XLogDropDatabase(Oid dbid)
 	 * This is unnecessarily heavy-handed, as it will close SMgrRelation
 	 * objects for other databases as well. DROP DATABASE occurs seldom enough
 	 * that it's not worth introducing a variant of smgrclose for just this
-	 * purpose. XXX: Or should we rather leave the smgr entries dangling?
+	 * purpose.
 	 */
 	smgrcloseall();
 
diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
index d7d6cc0cd7..13e5376619 100644
--- a/src/backend/postmaster/bgwriter.c
+++ b/src/backend/postmaster/bgwriter.c
@@ -246,10 +246,12 @@ BackgroundWriterMain(void)
 		if (FirstCallSinceLastCheckpoint())
 		{
 			/*
-			 * After any checkpoint, close all smgr files.  This is so we
-			 * won't hang onto smgr references to deleted files indefinitely.
+			 * After any checkpoint, free all smgr objects.  Otherwise we
+			 * would never do so for dropped relations, as the bgwriter does
+			 * not process shared invalidation messages or call
+			 * AtEOXact_SMgr().
 			 */
-			smgrcloseall();
+			smgrdestroyall();
 		}
 
 		/*
diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index 5e949fc885..5d843b6142 100644
--- a/src/backend/postmaster/checkpointer.c
+++ b/src/backend/postmaster/checkpointer.c
@@ -469,10 +469,12 @@ CheckpointerMain(void)
 				ckpt_performed = CreateRestartPoint(flags);
 
 			/*
-			 * After any checkpoint, close all smgr files.  This is so we
-			 * won't hang onto smgr references to deleted files indefinitely.
+			 * After any checkpoint, free all smgr objects.  Otherwise we
+			 * would never do so for dropped relations, as the checkpointer
+			 * does not process shared invalidation messages or call
+			 * AtEOXact_SMgr().
 			 */
-			smgrcloseall();
+			smgrdestroyall();
 
 			/*
 			 * Indicate checkpoint completion to any waiting backends.
@@ -958,11 +960,8 @@ RequestCheckpoint(int flags)
 		 */
 		CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);
 
-		/*
-		 * After any checkpoint, close all smgr files.  This is so we won't
-		 * hang onto smgr references to deleted files indefinitely.
-		 */
-		smgrcloseall();
+		/* Free all smgr objects, as CheckpointerMain() normally would. */
+		smgrdestroyall();
 
 		return;
 	}
diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index 563a0be5c7..0d7272e796 100644
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -147,7 +147,9 @@ smgrshutdown(int code, Datum arg)
 /*
  * smgropen() -- Return an SMgrRelation object, creating it if need be.
  *
- * This does not attempt to actually open the underlying file.
+ * This does not attempt to actually open the underlying files.  The returned
+ * object remains valid at least until AtEOXact_SMgr() is called, or until
+ * smgrdestroy() is called in non-transaction backends.
  */
 SMgrRelation
 smgropen(RelFileLocator rlocator, BackendId backend)
@@ -259,10 +261,10 @@ smgrexists(SMgrRelation reln, ForkNumber forknum)
 }
 
 /*
- * smgrclose() -- Close and delete an SMgrRelation object.
+ * smgrdestroy() -- Delete an SMgrRelation object.
  */
 void
-smgrclose(SMgrRelation reln)
+smgrdestroy(SMgrRelation reln)
 {
 	SMgrRelation *owner;
 	ForkNumber	forknum;
@@ -289,12 +291,14 @@ smgrclose(SMgrRelation reln)
 }
 
 /*
- * smgrrelease() -- Release all resources used by this object.
+ * smgrclose() -- Release all resources used by this object.
  *
- * The object remains valid.
+ * The object remains valid, but is moved to the unknown list where it will
+ * be destroyed by AtEOXact_SMgr().  It may be re-owned if it is accessed by a
+ * relation before then.
  */
 void
-smgrrelease(SMgrRelation reln)
+smgrclose(SMgrRelation reln)
 {
 	for (ForkNumber forknum = 0; forknum <= MAX_FORKNUM; forknum++)
 	{
@@ -302,15 +306,20 @@ smgrrelease(SMgrRelation reln)
 		reln->smgr_cached_nblocks[forknum] = InvalidBlockNumber;
 	}
 	reln->smgr_targblock = InvalidBlockNumber;
+
+	if (reln->smgr_owner)
+	{
+		*reln->smgr_owner = NULL;
+		reln->smgr_owner = NULL;
+		dlist_push_tail(&unowned_relns, &reln->node);
+	}
 }
 
 /*
- * smgrreleaseall() -- Release resources used by all objects.
- *
- * This is called for PROCSIGNAL_BARRIER_SMGRRELEASE.
+ * smgrcloseall() -- Close all objects.
  */
 void
-smgrreleaseall(void)
+smgrcloseall(void)
 {
 	HASH_SEQ_STATUS status;
 	SMgrRelation reln;
@@ -322,14 +331,17 @@ smgrreleaseall(void)
 	hash_seq_init(&status, SMgrRelationHash);
 
 	while ((reln = (SMgrRelation) hash_seq_search(&status)) != NULL)
-		smgrrelease(reln);
+		smgrclose(reln);
 }
 
 /*
- * smgrcloseall() -- Close all existing SMgrRelation objects.
+ * smgrdestroyall() -- Destroy all SMgrRelation objects.
+ *
+ * It must be known that there are no pointers to SMgrRelations, other than
+ * those registered with smgrsetowner().
  */
 void
-smgrcloseall(void)
+smgrdestroyall(void)
 {
 	HASH_SEQ_STATUS status;
 	SMgrRelation reln;
@@ -341,7 +353,7 @@ smgrcloseall(void)
 	hash_seq_init(&status, SMgrRelationHash);
 
 	while ((reln = (SMgrRelation) hash_seq_search(&status)) != NULL)
-		smgrclose(reln);
+		smgrdestroy(reln);
 }
 
 /*
@@ -733,7 +745,8 @@ smgrimmedsync(SMgrRelation reln, ForkNumber forknum)
  * AtEOXact_SMgr
  *
  * This routine is called during transaction commit or abort (it doesn't
- * particularly care which).  All transient SMgrRelation objects are closed.
+ * particularly care which).  All transient SMgrRelation objects are
+ * destroyed.
  *
  * We do this as a compromise between wanting transient SMgrRelations to
  * live awhile (to amortize the costs of blind writes of multiple blocks)
@@ -747,7 +760,7 @@ AtEOXact_SMgr(void)
 	dlist_mutable_iter iter;
 
 	/*
-	 * Zap all unowned SMgrRelations.  We rely on smgrclose() to remove each
+	 * Zap all unowned SMgrRelations.  We rely on smgrdestroy() to remove each
 	 * one from the list.
 	 */
 	dlist_foreach_modify(iter, &unowned_relns)
@@ -757,7 +770,7 @@ AtEOXact_SMgr(void)
 
 		Assert(rel->smgr_owner == NULL);
 
-		smgrclose(rel);
+		smgrdestroy(rel);
 	}
 }
 
@@ -768,6 +781,6 @@ AtEOXact_SMgr(void)
 bool
 ProcessBarrierSmgrRelease(void)
 {
-	smgrreleaseall();
+	smgrcloseall();
 	return true;
 }
diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index 527cd2a056..d8ffe397fa 100644
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -85,8 +85,8 @@ extern void smgrclearowner(SMgrRelation *owner, SMgrRelation reln);
 extern void smgrclose(SMgrRelation reln);
 extern void smgrcloseall(void);
 extern void smgrcloserellocator(RelFileLocatorBackend rlocator);
-extern void smgrrelease(SMgrRelation reln);
-extern void smgrreleaseall(void);
+extern void smgrdestroy(SMgrRelation reln);
+extern void smgrdestroyall(void);
 extern void smgrcreate(SMgrRelation reln, ForkNumber forknum, bool isRedo);
 extern void smgrdosyncall(SMgrRelation *rels, int nrels);
 extern void smgrdounlinkall(SMgrRelation *rels, int nrels, bool isRedo);
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index a584b1ddff..6636cc82c0 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -561,12 +561,6 @@ typedef struct ViewOptions
  *
  * Very little code is authorized to touch rel->rd_smgr directly.  Instead
  * use this function to fetch its value.
- *
- * Note: since a relcache flush can cause the file handle to be closed again,
- * it's unwise to hold onto the pointer returned by this function for any
- * long period.  Recommended practice is to just re-execute RelationGetSmgr
- * each time you need to access the SMgrRelation.  It's quite cheap in
- * comparison to whatever an smgr function is going to do.
  */
 static inline SMgrRelation
 RelationGetSmgr(Relation rel)
-- 
2.39.2