XLogReader general code cleanup

Some minor tweaks and comment additions, for cleanliness sake and to
avoid having the upcoming timeline-following patch be polluted with
unrelated cleanup.

Extracted from a larger patch by Craig Ringer, reviewed by Andres
Freund, with some additions by myself.

1 files changed, 15 insertions, 6 deletions

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 444e2180b0c..2635d80dc0c 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -19,12 +19,11 @@
 
 #include <unistd.h>
 
-#include "miscadmin.h"
-
 #include "access/xlog.h"
 #include "access/xlog_internal.h"
 #include "access/xlogutils.h"
 #include "catalog/catalog.h"
+#include "miscadmin.h"
 #include "storage/smgr.h"
 #include "utils/guc.h"
 #include "utils/hsearch.h"
@@ -638,8 +637,17 @@ XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
 }
 
 /*
- * TODO: This is duplicate code with pg_xlogdump, similar to walsender.c, but
- * we currently don't have the infrastructure (elog!) to share it.
+ * Read 'count' bytes from WAL into 'buf', starting at location 'startptr'
+ * in timeline 'tli'.
+ *
+ * Will open, and keep open, one WAL segment stored in the static file
+ * descriptor 'sendFile'. This means if XLogRead is used once, there will
+ * always be one descriptor left open until the process ends, but never
+ * more than one.
+ *
+ * XXX This is very similar to pg_xlogdump's XLogDumpXLogRead and to XLogRead
+ * in walsender.c but for small differences (such as lack of elog() in
+ * frontend).  Probably these should be merged at some point.
  */
 static void
 XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
@@ -648,6 +656,7 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 	XLogRecPtr	recptr;
 	Size		nbytes;
 
+	/* state maintained across calls */
 	static int	sendFile = -1;
 	static XLogSegNo sendSegNo = 0;
 	static uint32 sendOff = 0;
@@ -664,11 +673,11 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 
 		startoff = recptr % XLogSegSize;
 
+		/* Do we need to switch to a different xlog segment? */
 		if (sendFile < 0 || !XLByteInSeg(recptr, sendSegNo))
 		{
 			char		path[MAXPGPATH];
 
-			/* Switch to another logfile segment */
 			if (sendFile >= 0)
 				close(sendFile);
 
@@ -745,7 +754,7 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
  * Public because it would likely be very helpful for someone writing another
  * output method outside walsender, e.g. in a bgworker.
  *
- * TODO: The walsender has it's own version of this, but it relies on the
+ * TODO: The walsender has its own version of this, but it relies on the
  * walsender's latch being set whenever WAL is flushed. No such infrastructure
  * exists for normal backends, so we have to do a check/sleep/repeat style of
  * loop for now.