modify segno. for pg_walfile_name() and pg_walfile_name_offset()

Previously these functions returned the previous segment number if the
LSN was on a segment boundary.  We now always return the current segment
number for an LSN.

Docs updated to reflect this change.  Regression tests added, author
Andres Freund.

Also mentioned in thread https://postgr.es/m/flat/20220204225057.GA1535307%40nathanxps13#d964275c9540d8395e138efc0a75f7e8

BACKWARD INCOMPATIBILITY

Reported-by: Kyotaro Horiguchi

Discussion: https://postgr.es/m/20190726.172120.101752680.horikyota.ntt@gmail.com

Co-authored-by: Kyotaro Horiguchi

Backpatch-through: master

doc/src/sgml/func.sgml

@@ -27075,11 +27075,6 @@ postgres=# SELECT * FROM pg_walfile_name_offset((pg_backup_stop()).lsn);
 (1 row)
 </programlisting>
     Similarly, <function>pg_walfile_name</function> extracts just the write-ahead log file name.
-    When the given write-ahead log location is exactly at a write-ahead log file boundary, both
-    these functions return the name of the preceding write-ahead log file.
-    This is usually the desired behavior for managing write-ahead log archiving
-    behavior, since the preceding file is the last one that currently
-    needs to be archived.
    </para>
 
    <para>

src/backend/access/transam/xlogfuncs.c

@@ -374,10 +374,6 @@ pg_last_wal_replay_lsn(PG_FUNCTION_ARGS)
 /*
  * Compute an xlog file name and decimal byte offset given a WAL location,
  * such as is returned by pg_backup_stop() or pg_switch_wal().
- *
- * Note that a location exactly at a segment boundary is taken to be in
- * the previous segment.  This is usually the right thing, since the
- * expected usage is to determine which xlog file(s) are ready to archive.
  */
 Datum
 pg_walfile_name_offset(PG_FUNCTION_ARGS)
@@ -414,7 +410,7 @@ pg_walfile_name_offset(PG_FUNCTION_ARGS)
 	/*
 	 * xlogfilename
 	 */
-	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size);
+	XLByteToSeg(locationpoint, xlogsegno, wal_segment_size);
 	XLogFileName(xlogfilename, GetWALInsertionTimeLine(), xlogsegno,
 				 wal_segment_size);
 
@@ -457,7 +453,7 @@ pg_walfile_name(PG_FUNCTION_ARGS)
 				 errhint("%s cannot be executed during recovery.",
 						 "pg_walfile_name()")));
 
-	XLByteToPrevSeg(locationpoint, xlogsegno, wal_segment_size);
+	XLByteToSeg(locationpoint, xlogsegno, wal_segment_size);
 	XLogFileName(xlogfilename, GetWALInsertionTimeLine(), xlogsegno,
 				 wal_segment_size);
 

src/test/regress/expected/misc_functions.out

@@ -619,7 +619,7 @@ SELECT count(*) > 0 AS ok FROM pg_control_system();
  t
 (1 row)
 
--- pg_split_walfile_name
+-- pg_split_walfile_name, pg_walfile_name & pg_walfile_name_offset
 SELECT * FROM pg_split_walfile_name(NULL);
  segment_number | timeline_id 
 ----------------+-------------
@@ -642,3 +642,31 @@ SELECT segment_number > 0 AS ok_segment_number, timeline_id
  t                 |  4294967295
 (1 row)
 
+SELECT setting::int8 AS segment_size
+FROM pg_settings
+WHERE name = 'wal_segment_size'
+\gset
+SELECT segment_number, file_offset
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size),
+     pg_split_walfile_name(file_name);
+ segment_number | file_offset 
+----------------+-------------
+              1 |           0
+(1 row)
+
+SELECT segment_number, file_offset
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size + 1),
+     pg_split_walfile_name(file_name);
+ segment_number | file_offset 
+----------------+-------------
+              1 |           1
+(1 row)
+
+SELECT segment_number, file_offset = :segment_size - 1
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size - 1),
+     pg_split_walfile_name(file_name);
+ segment_number | ?column? 
+----------------+----------
+              0 | t
+(1 row)
+

src/test/regress/sql/misc_functions.sql

@@ -230,10 +230,23 @@ SELECT count(*) > 0 AS ok FROM pg_control_init();
 SELECT count(*) > 0 AS ok FROM pg_control_recovery();
 SELECT count(*) > 0 AS ok FROM pg_control_system();
 
--- pg_split_walfile_name
+-- pg_split_walfile_name, pg_walfile_name & pg_walfile_name_offset
 SELECT * FROM pg_split_walfile_name(NULL);
 SELECT * FROM pg_split_walfile_name('invalid');
 SELECT segment_number > 0 AS ok_segment_number, timeline_id
   FROM pg_split_walfile_name('000000010000000100000000');
 SELECT segment_number > 0 AS ok_segment_number, timeline_id
   FROM pg_split_walfile_name('ffffffFF00000001000000af');
+SELECT setting::int8 AS segment_size
+FROM pg_settings
+WHERE name = 'wal_segment_size'
+\gset
+SELECT segment_number, file_offset
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size),
+     pg_split_walfile_name(file_name);
+SELECT segment_number, file_offset
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size + 1),
+     pg_split_walfile_name(file_name);
+SELECT segment_number, file_offset = :segment_size - 1
+FROM pg_walfile_name_offset('0/0'::pg_lsn + :segment_size - 1),
+     pg_split_walfile_name(file_name);