Pass correct count to WALRead().

Previously, some callers requested XLOG_BLCKSZ bytes
unconditionally. While this did not cause a problem, because the extra
bytes are ignored, it's confusing and makes it harder to add safety
checks. Additionally, the comment about zero padding was incorrect.

With this commit, all callers request the number of bytes they
actually need.

Author: Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CALj2ACWBRFac2TingD3PE3w2EBHXUHY3=AEEZPJmqhpEOBGExg@mail.gmail.com

src/backend/access/transam/xlogutils.c

@@ -1007,12 +1007,7 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 		count = read_upto - targetPagePtr;
 	}
 
-	/*
-	 * Even though we just determined how much of the page can be validly read
-	 * as 'count', read the whole page anyway. It's guaranteed to be
-	 * zero-padded up to the page boundary if it's incomplete.
-	 */
-	if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ, tli,
+	if (!WALRead(state, cur_page, targetPagePtr, count, tli,
 				 &errinfo))
 		WALReadRaiseError(&errinfo);
 

src/backend/postmaster/walsummarizer.c

@@ -1318,12 +1318,7 @@ summarizer_read_local_xlog_page(XLogReaderState *state,
 		}
 	}
 
-	/*
-	 * Even though we just determined how much of the page can be validly read
-	 * as 'count', read the whole page anyway. It's guaranteed to be
-	 * zero-padded up to the page boundary if it's incomplete.
-	 */
-	if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ,
+	if (!WALRead(state, cur_page, targetPagePtr, count,
 				 private_data->tli, &errinfo))
 		WALReadRaiseError(&errinfo);
 

src/backend/replication/walsender.c

@@ -1099,7 +1099,7 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	if (!WALRead(state,
 				 cur_page,
 				 targetPagePtr,
-				 XLOG_BLCKSZ,
+				 count,
 				 currTLI,		/* Pass the current TLI because only
 								 * WalSndSegmentOpen controls whether new TLI
 								 * is needed. */