Change some bogus PageGetLSN calls to BufferGetLSNAtomic

alvherre · alvherre · commit c6187964047f · 2018-01-09T17:07:24.000-03:00
As src/backend/access/transam/README says, PageGetLSN may only be called by processes holding either exclusive lock on buffer, or a shared lock on buffer plus buffer header lock. Therefore any place that only holds a shared buffer lock must use BufferGetLSNAtomic instead of PageGetLSN, which internally obtains buffer header lock prior to reading the LSN. A few callsites failed to comply with this rule. This was detected by running all tests under a new (not committed) assertion that verifies PageGetLSN locking contract. All but one of the callsites that failed the assertion are fixed by this patch. Remaining callsites were inspected manually and determined not to need any change. The exception (unfixed callsite) is in TestForOldSnapshot, which only has a Page argument, making it impossible to access the corresponding Buffer from it. Fixing that seems a much larger patch that will have to be done separately; and that's just as well, since it was only introduced in 9.6 and other bugs are much older. Some of these bugs are ancient; backpatch all the way back to 9.3. Authors: Jacob Champion, Asim Praveen, Ashwin Agrawal Reviewed-by: Michaël Paquier Discussion: https://postgr.es/m/CABAq_6GXgQDVu3u12mK9O5Xt5abBZWQ0V40LZCE+oUf95XyNFg@mail.gmail.com
diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
@@ -558,7 +558,8 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 		}
 
 		stack->page = (Page) BufferGetPage(stack->buffer);
-		stack->lsn = PageGetLSN(stack->page);
+		stack->lsn = xlocked ?
+			PageGetLSN(stack->page) : BufferGetLSNAtomic(stack->buffer);
 		Assert(!RelationNeedsWAL(state.r) || !XLogRecPtrIsInvalid(stack->lsn));
 
 		/*
@@ -808,7 +809,7 @@ gistFindPath(Relation r, BlockNumber child, OffsetNumber *downlinkoffnum)
 			break;
 		}
 
-		top->lsn = PageGetLSN(page);
+		top->lsn = BufferGetLSNAtomic(buffer);
 
 		/*
 		 * If F_FOLLOW_RIGHT is set, the page to the right doesn't have a
diff --git a/src/backend/access/gist/gistvacuum.c b/src/backend/access/gist/gistvacuum.c
@@ -257,7 +257,7 @@ gistbulkdelete(PG_FUNCTION_ARGS)
 
 				ptr = (GistBDItem *) palloc(sizeof(GistBDItem));
 				ptr->blkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
-				ptr->parentlsn = PageGetLSN(page);
+				ptr->parentlsn = BufferGetLSNAtomic(buffer);
 				ptr->next = stack->next;
 				stack->next = ptr;
 

Original file line number	Diff line number	Diff line change
`@@ -558,7 +558,8 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)`
`558`	`558`	`}`
`559`	`559`
`560`	`560`	`stack->page = (Page) BufferGetPage(stack->buffer);`
`561`		`- stack->lsn = PageGetLSN(stack->page);`
	`561`	`+ stack->lsn = xlocked ?`
	`562`	`+ PageGetLSN(stack->page) : BufferGetLSNAtomic(stack->buffer);`
`562`	`563`	`Assert(!RelationNeedsWAL(state.r) \|\| !XLogRecPtrIsInvalid(stack->lsn));`
`563`	`564`
`564`	`565`	`/*`
`@@ -808,7 +809,7 @@ gistFindPath(Relation r, BlockNumber child, OffsetNumber *downlinkoffnum)`
`808`	`809`	`break;`
`809`	`810`	`}`
`810`	`811`
`811`		`- top->lsn = PageGetLSN(page);`
	`812`	`+ top->lsn = BufferGetLSNAtomic(buffer);`
`812`	`813`
`813`	`814`	`/*`
`814`	`815`	`* If F_FOLLOW_RIGHT is set, the page to the right doesn't have a`