Document XLOG_INCLUDE_XID a little better

I noticed that commit 0bead9af484c left this flag undocumented in
XLogSetRecordFlags, which led me to discover that the flag doesn't
actually do what the one comment on it said it does.  Improve the
situation by adding some more comments.

Backpatch to 14, where the aforementioned commit appears.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/202109212119.c3nhfp64t2ql@alvherre.pgsql

diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index e596a0470a9775ffa63d3171d8889a1a9a0ca1ed..b492c656d7a56164e426ae43cb75a8ed87f0303d 100644 (file)
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -409,6 +409,8 @@ XLogRegisterBufData(uint8 block_id, char *data, int len)
  * - XLOG_MARK_UNIMPORTANT, to signal that the record is not important for
  *  durability, which allows to avoid triggering WAL archiving and other
  *  background activity.
+ * - XLOG_INCLUDE_XID, a message-passing hack between XLogRecordAssemble
+ *  and XLogResetInsertion.
  */
 void
 XLogSetRecordFlags(uint8 flags)

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 0a8ede700defc658925d3d60383ac94fd4e25988..5e2c94a05ff15d73917cc5cc322a7c87e250be97 100644 (file)
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -212,7 +212,7 @@ extern bool XLOG_DEBUG;
  */
 #define XLOG_INCLUDE_ORIGIN        0x01    /* include the replication origin */
 #define XLOG_MARK_UNIMPORTANT  0x02    /* record not important for durability */
-#define XLOG_INCLUDE_XID       0x04    /* include XID of top-level xact */
+#define XLOG_INCLUDE_XID       0x04    /* WAL-internal message-passing hack */
 
 
 /* Checkpoint statistics */

diff --git a/src/include/access/xlogrecord.h b/src/include/access/xlogrecord.h
index e06ee92a5e54e40a7958207d0d52eee4715a0b0c..8d1305eae8317da88f7f566aee07263f775cf6ff 100644 (file)
--- a/src/include/access/xlogrecord.h
+++ b/src/include/access/xlogrecord.h
@@ -215,8 +215,9 @@ typedef struct XLogRecordDataHeaderLong
  * Block IDs used to distinguish different kinds of record fragments. Block
  * references are numbered from 0 to XLR_MAX_BLOCK_ID. A rmgr is free to use
  * any ID number in that range (although you should stick to small numbers,
- * because the WAL machinery is optimized for that case). A couple of ID
- * numbers are reserved to denote the "main" data portion of the record.
+ * because the WAL machinery is optimized for that case). A few ID
+ * numbers are reserved to denote the "main" data portion of the record,
+ * as well as replication-supporting transaction metadata.
  *
  * The maximum is currently set at 32, quite arbitrarily. Most records only
  * need a handful of block references, but there are a few exceptions that