Tidy up XLogSource code in xlog.c.

This commit replaces 0 used as an initial value of XLogSource variable,
with XLOG_FROM_ANY. Also this commit changes those variable so that
XLogSource instead of int is used as the type for them. These changes
are for code readability and debugger-friendliness.

Author: Kyotaro Horiguchi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/20200227.124830.2197604521555566121.horikyota.ntt@gmail.com

Author: MasaoFujii

src/backend/access/transam/xlog.c

@@ -795,7 +795,7 @@ static int	readFile = -1;
 static XLogSegNo readSegNo = 0;
 static uint32 readOff = 0;
 static uint32 readLen = 0;
-static XLogSource readSource = 0;	/* XLOG_FROM_* code */
+static XLogSource readSource = XLOG_FROM_ANY;
 
 /*
  * Keeps track of which source we're currently reading from. This is
@@ -804,7 +804,7 @@ static XLogSource readSource = 0;	/* XLOG_FROM_* code */
  * attempt to read from currentSource failed, and we should try another source
  * next.
  */
-static XLogSource currentSource = 0;	/* XLOG_FROM_* code */
+static XLogSource currentSource = XLOG_FROM_ANY;
 static bool lastSourceFailed = false;
 
 typedef struct XLogPageReadPrivate
@@ -823,7 +823,7 @@ typedef struct XLogPageReadPrivate
  * XLogReceiptSource tracks where we last successfully read some WAL.)
  */
 static TimestampTz XLogReceiptTime = 0;
-static XLogSource XLogReceiptSource = 0;	/* XLOG_FROM_* code */
+static XLogSource XLogReceiptSource = XLOG_FROM_ANY;
 
 /* State information for XLOG reading */
 static XLogRecPtr ReadRecPtr;	/* start of last record read */
@@ -886,8 +886,8 @@ static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
 								   bool find_free, XLogSegNo max_segno,
 								   bool use_lock);
 static int	XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
-						 int source, bool notfoundOk);
-static int	XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
+						 XLogSource source, bool notfoundOk);
+static int	XLogFileReadAnyTLI(XLogSegNo segno, int emode, XLogSource source);
 static int	XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
 						 int reqLen, XLogRecPtr targetRecPtr, char *readBuf);
 static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
@@ -3633,7 +3633,7 @@ XLogFileOpen(XLogSegNo segno)
  */
 static int
 XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
-			 int source, bool notfoundOk)
+			 XLogSource source, bool notfoundOk)
 {
 	char		xlogfname[MAXFNAMELEN];
 	char		activitymsg[MAXFNAMELEN + 16];
@@ -3715,7 +3715,7 @@ XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
  * This version searches for the segment with any TLI listed in expectedTLEs.
  */
 static int
-XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
+XLogFileReadAnyTLI(XLogSegNo segno, int emode, XLogSource source)
 {
 	char		path[MAXPGPATH];
 	ListCell   *cell;
@@ -4387,7 +4387,7 @@ ReadRecord(XLogReaderState *xlogreader, int emode,
 				 * so that we will check the archive next.
 				 */
 				lastSourceFailed = false;
-				currentSource = 0;
+				currentSource = XLOG_FROM_ANY;
 
 				continue;
 			}
@@ -11673,7 +11673,7 @@ XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
 
 		close(readFile);
 		readFile = -1;
-		readSource = 0;
+		readSource = XLOG_FROM_ANY;
 	}
 
 	XLByteToSeg(targetPagePtr, readSegNo, wal_segment_size);
@@ -11693,7 +11693,7 @@ XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
 				close(readFile);
 			readFile = -1;
 			readLen = 0;
-			readSource = 0;
+			readSource = XLOG_FROM_ANY;
 
 			return -1;
 		}
@@ -11799,7 +11799,7 @@ XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
 		close(readFile);
 	readFile = -1;
 	readLen = 0;
-	readSource = 0;
+	readSource = XLOG_FROM_ANY;
 
 	/* In standby-mode, keep trying */
 	if (StandbyMode)
@@ -11870,7 +11870,7 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 	 */
 	if (!InArchiveRecovery)
 		currentSource = XLOG_FROM_PG_WAL;
-	else if (currentSource == 0 ||
+	else if (currentSource == XLOG_FROM_ANY ||
 			 (!StandbyMode && currentSource == XLOG_FROM_STREAM))
 	{
 		lastSourceFailed = false;
@@ -11879,7 +11879,7 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 
 	for (;;)
 	{
-		int			oldSource = currentSource;
+		XLogSource	oldSource = currentSource;
 
 		/*
 		 * First check if we failed to read from the current source, and