Fix bug in GenericXLogFinish().

Mark the buffers dirty before writing WAL.

Discussion: https://postgr.es/m/25104133-7df8-cae3-b9a2-1c0aaa1c094a@iki.fi
Reviewed-by: Heikki Linnakangas
Backpatch-through: 11

Author: jeff-davis

src/backend/access/transam/generic_xlog.c

@@ -347,6 +347,10 @@ GenericXLogFinish(GenericXLogState *state)
 
 		START_CRIT_SECTION();
 
+		/*
+		 * Compute deltas if necessary, write changes to buffers, mark
+		 * buffers dirty, and register changes.
+		 */
 		for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
 		{
 			PageData   *pageData = &state->pages[i];
@@ -359,41 +363,34 @@ GenericXLogFinish(GenericXLogState *state)
 			page = BufferGetPage(pageData->buffer);
 			pageHeader = (PageHeader) pageData->image;
 
+			/*
+			 * Compute delta while we still have both the unmodified page and
+			 * the new image. Not needed if we are logging the full image.
+			 */
+			if (!(pageData->flags & GENERIC_XLOG_FULL_IMAGE))
+				computeDelta(pageData, page, (Page) pageData->image);
+
+			/*
+			 * Apply the image, being careful to zero the "hole" between
+			 * pd_lower and pd_upper in order to avoid divergence between
+			 * actual page state and what replay would produce.
+			 */
+			memcpy(page, pageData->image, pageHeader->pd_lower);
+			memset(page + pageHeader->pd_lower, 0,
+				   pageHeader->pd_upper - pageHeader->pd_lower);
+			memcpy(page + pageHeader->pd_upper,
+				   pageData->image + pageHeader->pd_upper,
+				   BLCKSZ - pageHeader->pd_upper);
+
+			MarkBufferDirty(pageData->buffer);
+
 			if (pageData->flags & GENERIC_XLOG_FULL_IMAGE)
 			{
-				/*
-				 * A full-page image does not require us to supply any xlog
-				 * data.  Just apply the image, being careful to zero the
-				 * "hole" between pd_lower and pd_upper in order to avoid
-				 * divergence between actual page state and what replay would
-				 * produce.
-				 */
-				memcpy(page, pageData->image, pageHeader->pd_lower);
-				memset(page + pageHeader->pd_lower, 0,
-					   pageHeader->pd_upper - pageHeader->pd_lower);
-				memcpy(page + pageHeader->pd_upper,
-					   pageData->image + pageHeader->pd_upper,
-					   BLCKSZ - pageHeader->pd_upper);
-
 				XLogRegisterBuffer(i, pageData->buffer,
 								   REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
 			}
 			else
 			{
-				/*
-				 * In normal mode, calculate delta and write it as xlog data
-				 * associated with this page.
-				 */
-				computeDelta(pageData, page, (Page) pageData->image);
-
-				/* Apply the image, with zeroed "hole" as above */
-				memcpy(page, pageData->image, pageHeader->pd_lower);
-				memset(page + pageHeader->pd_lower, 0,
-					   pageHeader->pd_upper - pageHeader->pd_lower);
-				memcpy(page + pageHeader->pd_upper,
-					   pageData->image + pageHeader->pd_upper,
-					   BLCKSZ - pageHeader->pd_upper);
-
 				XLogRegisterBuffer(i, pageData->buffer, REGBUF_STANDARD);
 				XLogRegisterBufData(i, pageData->delta, pageData->deltaLen);
 			}
@@ -402,15 +399,14 @@ GenericXLogFinish(GenericXLogState *state)
 		/* Insert xlog record */
 		lsn = XLogInsert(RM_GENERIC_ID, 0);
 
-		/* Set LSN and mark buffers dirty */
+		/* Set LSN */
 		for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
 		{
 			PageData   *pageData = &state->pages[i];
 
 			if (BufferIsInvalid(pageData->buffer))
 				continue;
 			PageSetLSN(BufferGetPage(pageData->buffer), lsn);
-			MarkBufferDirty(pageData->buffer);
 		}
 		END_CRIT_SECTION();
 	}