When xlogreader asks the callback function to read a page, make sure we

get a large enough part of the page to include the beginning of the next
record we're interested in. The XLogPageRead callback uses the requested
length to decide which timeline to stream WAL from, and if the first call
is short, and the page contains a timeline switch, we'll repeatedly try
to stream that page from the old timeline, and never get across the
timeline switch.

src/backend/access/transam/xlogreader.c

@@ -164,7 +164,7 @@ allocate_recordbuf(XLogReaderState *state, uint32 reclength)
  * If RecPtr is not NULL, try to read a record at that position.  Otherwise
  * try to read a record just after the last one previously read.
  *
- * If the page_read callback fails to read the requested data, NULL is
+ * If the read_page callback fails to read the requested data, NULL is
  * returned.  The callback is expected to have reported the error; errormsg
  * is set to NULL.
  *
@@ -217,10 +217,16 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 	}
 
 	targetPagePtr = RecPtr - (RecPtr % XLOG_BLCKSZ);
+	targetRecOff = RecPtr % XLOG_BLCKSZ;
 
-	/* Read the page containing the record into state->readBuf */
-	readOff = ReadPageInternal(state, targetPagePtr, SizeOfXLogRecord);
-
+	/*
+	 * Read the page containing the record into state->readBuf. Request
+	 * enough byte to cover the whole record header, or at least the part of
+	 * it that fits on the same page.
+	 */
+	readOff = ReadPageInternal(state,
+							   targetPagePtr,
+						  Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
 	if (readOff < 0)
 		goto err;
 
@@ -229,7 +235,6 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 	 * examine it now.
 	 */
 	pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
-	targetRecOff = RecPtr % XLOG_BLCKSZ;
 	if (targetRecOff == 0)
 	{
 		/*
@@ -256,16 +261,6 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 	/* ReadPageInternal has verified the page header */
 	Assert(pageHeaderSize <= readOff);
 
-	/*
-	 * Ensure the whole record header or at least the part on this page is
-	 * read.
-	 */
-	readOff = ReadPageInternal(state,
-							   targetPagePtr,
-						  Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
-	if (readOff < 0)
-		goto err;
-
 	/*
 	 * Read the record length.
 	 *
@@ -463,7 +458,7 @@ err:
 }
 
 /*
- * Read a single xlog page including at least [pagestart, RecPtr] of valid data
+ * Read a single xlog page including at least [pageptr, reqLen] of valid data
  * via the read_page() callback.
  *
  * Returns -1 if the required page cannot be read for some reason; errormsg_buf
@@ -502,8 +497,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	 * record is.  This is so that we can check the additional identification
 	 * info that is present in the first page's "long" header.
 	 */
-	if (targetSegNo != state->readSegNo &&
-		targetPageOff != 0)
+	if (targetSegNo != state->readSegNo && targetPageOff != 0)
 	{
 		XLogPageHeader hdr;
 		XLogRecPtr	targetSegmentPtr = pageptr - targetPageOff;