WAL-log inplace update before revealing it to other sessions.

A buffer lock won't stop a reader having already checked tuple
visibility.  If a vac_update_datfrozenid() and then a crash happened
during inplace update of a relfrozenxid value, datfrozenxid could
overtake relfrozenxid.  That could lead to "could not access status of
transaction" errors.

Back-patch to v14 - v17.  This is a back-patch of commits:

- 8e7e672cda
  (main change, on master, before v18 branched)
- 8180136652
  (defect fix, on master, before v18 branched)

It reverses commit bc6bad8857, my revert
of the original back-patch.

In v14, this also back-patches the assertion removal from commit
7fcf2faf9c.

Discussion: https://postgr.es/m/20240620012908.92.nmisch@google.com
Backpatch-through: 14-17

src/backend/access/heap/README.tuplock

@@ -198,9 +198,7 @@ Inplace updates create an exception to the rule that tuple data won't change
 under a reader holding a pin.  A reader of a heap_fetch() result tuple may
 witness a torn read.  Current inplace-updated fields are aligned and are no
 wider than four bytes, and current readers don't need consistency across
-fields.  Hence, they get by with just fetching each field once.  XXX such a
-caller may also read a value that has not reached WAL; see
-systable_inplace_update_finish().
+fields.  Hence, they get by with just fetching each field once.
 
 During logical decoding, caches reflect an inplace update no later than the
 next XLOG_XACT_INVALIDATIONS.  That record witnesses the end of a command.

src/backend/access/heap/heapam.c

@@ -6450,6 +6450,8 @@ heap_inplace_update_and_unlock(Relation relation,
 	HeapTupleHeader htup = oldtup->t_data;
 	uint32		oldlen;
 	uint32		newlen;
+	char	   *dst;
+	char	   *src;
 
 	Assert(ItemPointerEquals(&oldtup->t_self, &tuple->t_self));
 	oldlen = oldtup->t_len - htup->t_hoff;
@@ -6457,6 +6459,9 @@ heap_inplace_update_and_unlock(Relation relation,
 	if (oldlen != newlen || htup->t_hoff != tuple->t_data->t_hoff)
 		elog(ERROR, "wrong tuple length");
 
+	dst = (char *) htup + htup->t_hoff;
+	src = (char *) tuple->t_data + tuple->t_data->t_hoff;
+
 	/*
 	 * Unlink relcache init files as needed.  If unlinking, acquire
 	 * RelCacheInitLock until after associated invalidations.  By doing this
@@ -6467,15 +6472,15 @@ heap_inplace_update_and_unlock(Relation relation,
 	 */
 	PreInplace_Inval();
 
-	/* NO EREPORT(ERROR) from here till changes are logged */
-	START_CRIT_SECTION();
-
-	memcpy((char *) htup + htup->t_hoff,
-		   (char *) tuple->t_data + tuple->t_data->t_hoff,
-		   newlen);
-
 	/*----------
-	 * XXX A crash here can allow datfrozenxid() to get ahead of relfrozenxid:
+	 * NO EREPORT(ERROR) from here till changes are complete
+	 *
+	 * Our buffer lock won't stop a reader having already pinned and checked
+	 * visibility for this tuple.  Hence, we write WAL first, then mutate the
+	 * buffer.  Like in MarkBufferDirtyHint() or RecordTransactionCommit(),
+	 * checkpoint delay makes that acceptable.  With the usual order of
+	 * changes, a crash after memcpy() and before XLogInsert() could allow
+	 * datfrozenxid to overtake relfrozenxid:
 	 *
 	 * ["D" is a VACUUM (ONLY_DATABASE_STATS)]
 	 * ["R" is a VACUUM tbl]
@@ -6485,14 +6490,36 @@ heap_inplace_update_and_unlock(Relation relation,
 	 * D: raise pg_database.datfrozenxid, XLogInsert(), finish
 	 * [crash]
 	 * [recovery restores datfrozenxid w/o relfrozenxid]
+	 *
+	 * Mimic MarkBufferDirtyHint() subroutine XLogSaveBufferForHint().
+	 * Specifically, use DELAY_CHKPT_START, and copy the buffer to the stack.
+	 * The stack copy facilitates a FPI of the post-mutation block before we
+	 * accept other sessions seeing it.  DELAY_CHKPT_START allows us to
+	 * XLogInsert() before MarkBufferDirty().  Since XLogSaveBufferForHint()
+	 * can operate under BUFFER_LOCK_SHARED, it can't avoid DELAY_CHKPT_START.
+	 * This function, however, likely could avoid it with the following order
+	 * of operations: MarkBufferDirty(), XLogInsert(), memcpy().  Opt to use
+	 * DELAY_CHKPT_START here, too, as a way to have fewer distinct code
+	 * patterns to analyze.  Inplace update isn't so frequent that it should
+	 * pursue the small optimization of skipping DELAY_CHKPT_START.
 	 */
-
-	MarkBufferDirty(buffer);
+	Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
+	START_CRIT_SECTION();
+	MyProc->delayChkptFlags |= DELAY_CHKPT_START;
 
 	/* XLOG stuff */
 	if (RelationNeedsWAL(relation))
 	{
 		xl_heap_inplace xlrec;
+		PGAlignedBlock copied_buffer;
+		char	   *origdata = (char *) BufferGetBlock(buffer);
+		Page		page = BufferGetPage(buffer);
+		uint16		lower = ((PageHeader) page)->pd_lower;
+		uint16		upper = ((PageHeader) page)->pd_upper;
+		uintptr_t	dst_offset_in_block;
+		RelFileLocator rlocator;
+		ForkNumber	forkno;
+		BlockNumber blkno;
 		XLogRecPtr	recptr;
 
 		xlrec.offnum = ItemPointerGetOffsetNumber(&tuple->t_self);
@@ -6500,16 +6527,28 @@ heap_inplace_update_and_unlock(Relation relation,
 		XLogBeginInsert();
 		XLogRegisterData((char *) &xlrec, SizeOfHeapInplace);
 
-		XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
-		XLogRegisterBufData(0, (char *) htup + htup->t_hoff, newlen);
+		/* register block matching what buffer will look like after changes */
+		memcpy(copied_buffer.data, origdata, lower);
+		memcpy(copied_buffer.data + upper, origdata + upper, BLCKSZ - upper);
+		dst_offset_in_block = dst - origdata;
+		memcpy(copied_buffer.data + dst_offset_in_block, src, newlen);
+		BufferGetTag(buffer, &rlocator, &forkno, &blkno);
+		Assert(forkno == MAIN_FORKNUM);
+		XLogRegisterBlock(0, &rlocator, forkno, blkno, copied_buffer.data,
+						  REGBUF_STANDARD);
+		XLogRegisterBufData(0, src, newlen);
 
 		/* inplace updates aren't decoded atm, don't log the origin */
 
 		recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_INPLACE);
 
-		PageSetLSN(BufferGetPage(buffer), recptr);
+		PageSetLSN(page, recptr);
 	}
 
+	memcpy(dst, src, newlen);
+
+	MarkBufferDirty(buffer);
+
 	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 
 	/*
@@ -6518,6 +6557,7 @@ heap_inplace_update_and_unlock(Relation relation,
 	 */
 	AtInplace_Inval();
 
+	MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;
 	END_CRIT_SECTION();
 	UnlockTuple(relation, &tuple->t_self, InplaceUpdateTupleLock);
 

src/include/storage/proc.h

@@ -104,10 +104,10 @@ struct XidCache
  * is inserted prior to the new redo point, the corresponding data changes will
  * also be flushed to disk before the checkpoint can complete. (In the
  * extremely common case where the data being modified is in shared buffers
- * and we acquire an exclusive content lock on the relevant buffers before
- * writing WAL, this mechanism is not needed, because phase 2 will block
- * until we release the content lock and then flush the modified data to
- * disk.)
+ * and we acquire an exclusive content lock and MarkBufferDirty() on the
+ * relevant buffers before writing WAL, this mechanism is not needed, because
+ * phase 2 will block until we release the content lock and then flush the
+ * modified data to disk.  See transam/README and SyncOneBuffer().)
  *
  * Setting DELAY_CHKPT_COMPLETE prevents the system from moving from phase 2
  * to phase 3. This is useful if we are performing a WAL-logged operation that