Refactor per-page logic common to all redo routines to a new function.

Every redo routine uses the same idiom to determine what to do to a page:
check if there's a backup block for it, and if not read, the buffer if the
block exists, and check its LSN. Refactor that into a common function,
XLogReadBufferForRedo, making all the redo routines shorter and more
readable.

This has no user-visible effect, and makes no changes to the WAL format.

Reviewed by Andres Freund, Alvaro Herrera, Michael Paquier.

src/include/access/xlogutils.h

@@ -1,7 +1,7 @@
 /*
  * xlogutils.h
  *
- * PostgreSQL transaction log manager utility routines
+ * Utilities for replaying WAL records.
  *
  * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -11,6 +11,7 @@
 #ifndef XLOG_UTILS_H
 #define XLOG_UTILS_H
 
+#include "access/xlog.h"
 #include "storage/bufmgr.h"
 
 
@@ -22,6 +23,26 @@ extern void XLogDropDatabase(Oid dbid);
 extern void XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
 					 BlockNumber nblocks);
 
+/* Result codes for XLogReadBufferForRedo[Extended] */
+typedef enum
+{
+	BLK_NEEDS_REDO,		/* changes from WAL record need to be applied */
+	BLK_DONE,			/* block is already up-to-date */
+	BLK_RESTORED,		/* block was restored from a full-page image */
+	BLK_NOTFOUND		/* block was not found (and hence does not need to be
+						 * replayed) */
+} XLogRedoAction;
+
+extern XLogRedoAction XLogReadBufferForRedo(XLogRecPtr lsn, XLogRecord *record,
+					  int block_index, RelFileNode rnode, BlockNumber blkno,
+					  Buffer *buf);
+extern XLogRedoAction XLogReadBufferForRedoExtended(XLogRecPtr lsn,
+							  XLogRecord *record, int block_index,
+							  RelFileNode rnode, ForkNumber forkno,
+							  BlockNumber blkno,
+							  ReadBufferMode mode, bool get_cleanup_lock,
+							  Buffer *buf);
+
 extern Buffer XLogReadBuffer(RelFileNode rnode, BlockNumber blkno, bool init);
 extern Buffer XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
 					   BlockNumber blkno, ReadBufferMode mode);