Adjust walsender usage of xlogreader, simplify APIs

* Have both physical and logical walsender share a 'xlogreader' state
  struct for tracking state.  This replaces the existing globals sendSeg
  and sendCxt.

* Change WALRead not to receive XLogReaderState->seg and ->segcxt as
  separate arguments anymore; just use the ones from 'state'.  This is
  made possible by the above change.

* have the XLogReader segment_open contract require the callbacks to
  install the file descriptor in the state struct themselves instead of
  returning it.  xlogreader was already ignoring any possible failed
  return from the callbacks, relying solely on them never returning.

  (This point is not altogether excellent, as it means the callbacks
  have to know more of XLogReaderState; but to really improve on that
  we would have to pass back error info from the callbacks to
  xlogreader.  And the complexity would not be saved but instead just
  transferred to the callers of WALRead, which would have to learn how
  to throw errors from the open_segment callback in addition of, as
  currently, from pg_pread.)

* segment_open no longer receives the 'segcxt' as a separate argument,
  since it's part of the XLogReaderState argument.

Per comments from Kyotaro Horiguchi.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200511203336.GA9913@alvherre.pgsql

2 files changed, 7 insertions, 16 deletions

diff --git a/src/include/access/xlogreader.h b/src/include/access/xlogreader.h
index 80cf62acb7c..c21b0ba9722 100644
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@@ -63,10 +63,9 @@ typedef int (*XLogPageReadCB) (XLogReaderState *xlogreader,
 							   int reqLen,
 							   XLogRecPtr targetRecPtr,
 							   char *readBuf);
-typedef int (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
-								 XLogSegNo nextSegNo,
-								 WALSegmentContext *segcxt,
-								 TimeLineID *tli_p);
+typedef void (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
+								  XLogSegNo nextSegNo,
+								  TimeLineID *tli_p);
 typedef void (*WALSegmentCloseCB) (XLogReaderState *xlogreader);
 
 typedef struct XLogReaderRoutine
@@ -94,21 +93,16 @@ typedef struct XLogReaderRoutine
 	XLogPageReadCB page_read;
 
 	/*
-	 * Callback to open the specified WAL segment for reading.  The file
-	 * descriptor of the opened segment shall be returned.  In case of
+	 * Callback to open the specified WAL segment for reading.  ->seg.ws_file
+	 * shall be set to the file descriptor of the opened segment.  In case of
 	 * failure, an error shall be raised by the callback and it shall not
 	 * return.
 	 *
 	 * "nextSegNo" is the number of the segment to be opened.
 	 *
-	 * "segcxt" is additional information about the segment.
-	 *
 	 * "tli_p" is an input/output argument. WALRead() uses it to pass the
 	 * timeline in which the new segment should be found, but the callback can
 	 * use it to return the TLI that it actually opened.
-	 *
-	 * BasicOpenFile() is the preferred way to open the segment file in
-	 * backend code, whereas open(2) should be used in frontend.
 	 */
 	WALSegmentOpenCB segment_open;
 
@@ -301,9 +295,7 @@ typedef struct WALReadError
 
 extern bool WALRead(XLogReaderState *state,
 					char *buf, XLogRecPtr startptr, Size count,
-					TimeLineID tli, WALOpenSegment *seg,
-					WALSegmentContext *segcxt,
-					WALReadError *errinfo);
+					TimeLineID tli, WALReadError *errinfo);
 
 /* Functions for decoding an XLogRecord */
 
diff --git a/src/include/access/xlogutils.h b/src/include/access/xlogutils.h
index 68ce815476c..e59b6cf3a9f 100644
--- a/src/include/access/xlogutils.h
+++ b/src/include/access/xlogutils.h
@@ -50,9 +50,8 @@ extern void FreeFakeRelcacheEntry(Relation fakerel);
 extern int	read_local_xlog_page(XLogReaderState *state,
 								 XLogRecPtr targetPagePtr, int reqLen,
 								 XLogRecPtr targetRecPtr, char *cur_page);
-extern int	wal_segment_open(XLogReaderState *state,
+extern void wal_segment_open(XLogReaderState *state,
 							 XLogSegNo nextSegNo,
-							 WALSegmentContext *segcxt,
 							 TimeLineID *tli_p);
 extern void wal_segment_close(XLogReaderState *state);