Fix correctness issue with computation of FPI size in WAL stats

XLogRecordAssemble() may be called multiple times before inserting a
record in XLogInsertRecord(), and the amount of FPIs generated inside
a record whose insertion is attempted multiple times may vary.

The logic added in f9a09aa touched directly pgWalUsage in
XLogRecordAssemble(), meaning that it could be possible for pgWalUsage
to be incremented multiple times for a single record.  This commit
changes the code to use the same logic as the number of FPIs added to a
record, where XLogRecordAssemble() returns this information and feeds it
to XLogInsertRecord(), updating pgWalUsage only when a record is
inserted.

Reported-by: Shinya Kato <[email protected]>
Discussion: https://postgr.es/m/CAOzEurSiSr+rusd0GzVy8Bt30QwLTK=ugVMnF6=5WhsSrukvvw@mail.gmail.com

Author: michaelpq

src/backend/access/transam/xlog.c

@@ -749,6 +749,7 @@ XLogInsertRecord(XLogRecData *rdata,
 				 XLogRecPtr fpw_lsn,
 				 uint8 flags,
 				 int num_fpi,
+				 uint64 fpi_bytes,
 				 bool topxid_included)
 {
 	XLogCtlInsert *Insert = &XLogCtl->Insert;
@@ -1081,6 +1082,7 @@ XLogInsertRecord(XLogRecData *rdata,
 		pgWalUsage.wal_bytes += rechdr->xl_tot_len;
 		pgWalUsage.wal_records++;
 		pgWalUsage.wal_fpi += num_fpi;
+		pgWalUsage.wal_fpi_bytes += fpi_bytes;
 
 		/* Required for the flush of pending stats WAL data */
 		pgstat_report_fixed = true;

src/backend/access/transam/xloginsert.c

@@ -139,6 +139,7 @@ static MemoryContext xloginsert_cxt;
 static XLogRecData *XLogRecordAssemble(RmgrId rmid, uint8 info,
 									   XLogRecPtr RedoRecPtr, bool doPageWrites,
 									   XLogRecPtr *fpw_lsn, int *num_fpi,
+									   uint64 *fpi_bytes,
 									   bool *topxid_included);
 static bool XLogCompressBackupBlock(const PageData *page, uint16 hole_offset,
 									uint16 hole_length, void *dest, uint16 *dlen);
@@ -512,6 +513,7 @@ XLogInsert(RmgrId rmid, uint8 info)
 		XLogRecPtr	fpw_lsn;
 		XLogRecData *rdt;
 		int			num_fpi = 0;
+		uint64		fpi_bytes = 0;
 
 		/*
 		 * Get values needed to decide whether to do full-page writes. Since
@@ -521,10 +523,11 @@ XLogInsert(RmgrId rmid, uint8 info)
 		GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
 
 		rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
-								 &fpw_lsn, &num_fpi, &topxid_included);
+								 &fpw_lsn, &num_fpi, &fpi_bytes,
+								 &topxid_included);
 
 		EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags, num_fpi,
-								  topxid_included);
+								  fpi_bytes, topxid_included);
 	} while (EndPos == InvalidXLogRecPtr);
 
 	XLogResetInsertion();
@@ -562,7 +565,8 @@ XLogSimpleInsertInt64(RmgrId rmid, uint8 info, int64 value)
 static XLogRecData *
 XLogRecordAssemble(RmgrId rmid, uint8 info,
 				   XLogRecPtr RedoRecPtr, bool doPageWrites,
-				   XLogRecPtr *fpw_lsn, int *num_fpi, bool *topxid_included)
+				   XLogRecPtr *fpw_lsn, int *num_fpi, uint64 *fpi_bytes,
+				   bool *topxid_included)
 {
 	XLogRecData *rdt;
 	uint64		total_len = 0;
@@ -800,8 +804,7 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 			total_len += bimg.length;
 
 			/* Track the WAL full page images in bytes */
-			pgWalUsage.wal_fpi_bytes += bimg.length;
-			pgstat_report_fixed = true;
+			*fpi_bytes += bimg.length;
 		}
 
 		if (needs_data)

src/include/access/xlog.h

@@ -202,6 +202,7 @@ extern XLogRecPtr XLogInsertRecord(struct XLogRecData *rdata,
 								   XLogRecPtr fpw_lsn,
 								   uint8 flags,
 								   int num_fpi,
+								   uint64 fpi_bytes,
 								   bool topxid_included);
 extern void XLogFlush(XLogRecPtr record);
 extern bool XLogBackgroundFlush(void);