Fix dangling smgr_owner pointer when a fake relcache entry is freed.

A fake relcache entry can "own" a SmgrRelation object, like a regular
relcache entry. But when it was free'd, the owner field in SmgrRelation
was not cleared, so it was left pointing to free'd memory.

Amazingly this apparently hasn't caused crashes in practice, or we would've
heard about it earlier. Andres found this with Valgrind.

Report and fix by Andres Freund, with minor modifications by me. Backpatch
to all supported versions.

src/backend/access/transam/xlogutils.c

@@ -413,6 +413,9 @@ CreateFakeRelcacheEntry(RelFileNode rnode)
 void
 FreeFakeRelcacheEntry(Relation fakerel)
 {
+	/* make sure the fakerel is not referenced by the SmgrRelation anymore */
+	if (fakerel->rd_smgr != NULL)
+		smgrclearowner(&fakerel->rd_smgr, fakerel->rd_smgr);
 	pfree(fakerel);
 }
 

src/backend/storage/smgr/smgr.c

@@ -193,6 +193,24 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
 	*owner = reln;
 }
 
+/*
+ * smgrclearowner() -- Remove long-lived reference to an SMgrRelation object
+ *					   if one exists
+ */
+void
+smgrclearowner(SMgrRelation *owner, SMgrRelation reln)
+{
+	/* Do nothing if the SMgrRelation object is not owned by the owner */
+	if (reln->smgr_owner != owner)
+		return;
+
+	/* unset the owner's reference */
+	*owner = NULL;
+
+	/* unset our reference to the owner */
+	reln->smgr_owner = NULL;
+}
+
 /*
  *	smgrexists() -- Does the underlying file for a fork exist?
  */

src/include/storage/smgr.h

@@ -62,6 +62,7 @@ extern void smgrinit(void);
 extern SMgrRelation smgropen(RelFileNode rnode);
 extern bool smgrexists(SMgrRelation reln, ForkNumber forknum);
 extern void smgrsetowner(SMgrRelation *owner, SMgrRelation reln);
+extern void smgrclearowner(SMgrRelation *owner, SMgrRelation reln);
 extern void smgrclose(SMgrRelation reln);
 extern void smgrcloseall(void);
 extern void smgrclosenode(RelFileNode rnode);