Re: foreign key locks

Alvaro Herrera wrote:
> Here's version 28 of this patch.  The only substantive change here from
> v26 is that I've made GetTupleForTrigger() use either LockTupleExclusive
> or LockTupleNoKeyExclusive, depending on whether the key columns are
> being modified by the update.  This needs to go through EvalPlanQual, so
> that function is now getting the lock mode as a parameter instead of
> hardcoded LockTupleExclusive.  (All other users of GetTupleForTrigger
> still use LockTupleExclusive, so there's no change for anybody other
> than FOR EACH ROW BEFORE UPDATE triggers).
> 
> At this point, I think I've done all I wanted to do in connection with
> this patch, and I intend to commit.  I think this is a good time to get
> it committed, so that people can play with it more extensively and
> report any breakage I might have caused before we even get into beta.

While trying this out this morning I noticed a bug in the XLog code:
trying to lock the updated version of a tuple when the page that
contains the updated version requires a backup block, would cause this:

PANIC:  invalid xlog record length 0

The reason is that there is an (unknown to me) rule that there must be
some data not associated with a buffer:

	/*
	 * NOTE: We disallow len == 0 because it provides a useful bit of extra
	 * error checking in ReadRecord.  This means that all callers of
	 * XLogInsert must supply at least some not-in-a-buffer data. [...]
	 */

This seems pretty strange to me.  And having the rule be spelled out
only in a comment within XLogInsert and not at its top, and not nearby
the XLogRecData struct definition either, seems pretty strange to me.
I wonder how come every PG hacker except me knows this.

For the curious, I attach an isolationtester spec file I used to
reproduce the problem (and verify the fix).

To fix the crash I needed to do this:

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 99fced1..9762890 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -4838,7 +4838,7 @@ l4:
 		{
 			xl_heap_lock_updated xlrec;
 			XLogRecPtr	recptr;
-			XLogRecData	rdata;
+			XLogRecData	rdata[2];
 			Page		page = BufferGetPage(buf);
 
 			xlrec.target.node = rel->rd_node;
@@ -4846,13 +4846,18 @@ l4:
 			xlrec.xmax = new_xmax;
 			xlrec.infobits_set = compute_infobits(new_infomask, new_infomask2);
 
-			rdata.data = (char *) &xlrec;
-			rdata.len = SizeOfHeapLockUpdated;
-			rdata.buffer = buf;
-			rdata.buffer_std = true;
-			rdata.next = NULL;
+			rdata[0].data = (char *) &xlrec;
+			rdata[0].len = SizeOfHeapLockUpdated;
+			rdata[0].buffer = InvalidBuffer;
+			rdata[0].next = &(rdata[1]);
+
+			rdata[1].data = NULL;
+			rdata[1].len = 0;
+			rdata[1].buffer = buf;
+			rdata[1].buffer_std = true;
+			rdata[1].next = NULL;
 
-			recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_LOCK_UPDATED, &rdata);
+			recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_LOCK_UPDATED, rdata);
 
 			PageSetLSN(page, recptr);
 			PageSetTLI(page, ThisTimeLineID);

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services