Refactor pgstat_prepare_io_time() with an input argument instead of a GUC

Originally, this routine relied on track_io_timing to check if a time
interval for an I/O operation stored in pg_stat_io should be initialized
or not.  However, the addition of WAL statistics to pg_stat_io requires
that the initialization happens when track_wal_io_timing is enabled,
which is dependent on the code path where the I/O operation happens.

Author: Nazir Bilal Yavuz
Discussion: https://postgr.es/m/CAN55FZ3AiQ+ZMxUuXnBpd0Rrh1YhwJ5FudkHg=JU0P+-W8T4Vg@mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -1143,7 +1143,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		MemSet((char *) bufBlock, 0, BLCKSZ);
 	else
 	{
-		instr_time	io_start = pgstat_prepare_io_time();
+		instr_time	io_start = pgstat_prepare_io_time(track_io_timing);
 
 		smgrread(smgr, forkNum, blockNum, bufBlock);
 
@@ -2070,7 +2070,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 		}
 	}
 
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/*
 	 * Note: if smgrzeroextend fails, we will end up with buffers that are
@@ -3523,7 +3523,7 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
 	 */
 	bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum);
 
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/*
 	 * bufToWrite is either the shared buffer or a copy, as appropriate.
@@ -4181,7 +4181,7 @@ FlushRelationBuffers(Relation rel)
 
 				PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
-				io_start = pgstat_prepare_io_time();
+				io_start = pgstat_prepare_io_time(track_io_timing);
 
 				smgrwrite(RelationGetSmgr(rel),
 						  BufTagGetForkNum(&bufHdr->tag),
@@ -5614,7 +5614,7 @@ IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context)
 	sort_pending_writebacks(wb_context->pending_writebacks,
 							wb_context->nr_pending);
 
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/*
 	 * Coalesce neighbouring writes, but nothing else. For that we iterate

src/backend/storage/buffer/localbuf.c

@@ -246,7 +246,7 @@ GetLocalVictimBuffer(void)
 
 		PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
-		io_start = pgstat_prepare_io_time();
+		io_start = pgstat_prepare_io_time(track_io_timing);
 
 		/* And write... */
 		smgrwrite(oreln,
@@ -411,7 +411,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		}
 	}
 
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/* actually extend relation */
 	smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);

src/backend/storage/smgr/md.c

@@ -1189,7 +1189,7 @@ register_dirty_segment(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
 		ereport(DEBUG1,
 				(errmsg_internal("could not forward fsync request because request queue is full")));
 
-		io_start = pgstat_prepare_io_time();
+		io_start = pgstat_prepare_io_time(track_io_timing);
 
 		if (FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0)
 			ereport(data_sync_elevel(ERROR),
@@ -1586,7 +1586,7 @@ mdsyncfiletag(const FileTag *ftag, char *path)
 		need_to_close = true;
 	}
 
-	io_start = pgstat_prepare_io_time();
+	io_start = pgstat_prepare_io_time(track_io_timing);
 
 	/* Sync the file. */
 	result = FileSync(file, WAIT_EVENT_DATA_FILE_SYNC);

src/backend/utils/activity/pgstat_io.c

@@ -92,15 +92,25 @@ pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint3
 	have_iostats = true;
 }
 
+/*
+ * Initialize the internal timing for an IO operation, depending on an
+ * IO timing GUC.
+ */
 instr_time
-pgstat_prepare_io_time(void)
+pgstat_prepare_io_time(bool track_io_guc)
 {
 	instr_time	io_start;
 
-	if (track_io_timing)
+	if (track_io_guc)
 		INSTR_TIME_SET_CURRENT(io_start);
 	else
+	{
+		/*
+		 * There is no need to set io_start when an IO timing GUC is disabled,
+		 * still initialize it to zero to avoid compiler warnings.
+		 */
 		INSTR_TIME_SET_ZERO(io_start);
+	}
 
 	return io_start;
 }

src/include/pgstat.h

@@ -519,7 +519,7 @@ extern bool pgstat_bktype_io_stats_valid(PgStat_BktypeIO *backend_io,
 										 BackendType bktype);
 extern void pgstat_count_io_op(IOObject io_object, IOContext io_context, IOOp io_op);
 extern void pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint32 cnt);
-extern instr_time pgstat_prepare_io_time(void);
+extern instr_time pgstat_prepare_io_time(bool track_io_guc);
 extern void pgstat_count_io_op_time(IOObject io_object, IOContext io_context,
 									IOOp io_op, instr_time start_time, uint32 cnt);