Draft patch for comment fix.‚Äč
From 304453f67b26219dc94e6aef9dfdc1c1debf12bb Mon Sep 17 00:00:00 2001
From: Craig Ringer <cr...@2ndquadrant.com>
Date: Wed, 6 Apr 2016 14:57:26 +0800
Subject: [PATCH] Fix incorrect comments introduced in logical decoding
 timeline following

---
 src/backend/replication/logical/logicalfuncs.c | 14 +++++++-------
 src/include/replication/slot.h                 | 18 ++++++++++++++++--
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/src/backend/replication/logical/logicalfuncs.c b/src/backend/replication/logical/logicalfuncs.c
index 5af6751..5f74941 100644
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@@ -236,8 +236,10 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 	PG_TRY();
 	{
 		/*
-		 * Passing InvalidXLogRecPtr here causes decoding to start returning
-		 * commited xacts to the client at the slot's confirmed_flush.
+		 * Passing InvalidXLogRecPtr here causes logical decoding to
+		 * start calling the output plugin for transactions that commit
+		 * at or after the slot's confirmed_flush. This filters commits
+		 * out from the client but they're still decoded.
 		 */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									options,
@@ -262,11 +264,9 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 		ctx->output_writer_private = p;
 
 		/*
-		 * We start reading xlog from the restart lsn, even though we won't
-		 * start returning decoded data to the user until the point set up in
-		 * CreateDecodingContext. The restart_lsn is far enough back that we'll
-		 * see the beginning of any xact we're expected to return to the client
-		 * so we can assemble a complete reorder buffer for it.
+		 * Reading and decoding of WAL must start at restart_lsn so that the
+		 * entirety of each of the xacts that commit after confimed_lsn can be
+		 * accumulated into reorder buffers.
 		 */
 		startptr = MyReplicationSlot->data.restart_lsn;
 
diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index efcce5f..50c8a33 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -63,10 +63,24 @@ typedef struct ReplicationSlotPersistentData
 	 */
 	TransactionId catalog_xmin;
 
-	/* oldest LSN that might be required by this replication slot */
+	/*
+	 * oldest LSN that might be required by this replication slot.
+	 *
+	 * Points to the LSN of the most recent xl_running_xacts record where
+	 * no transaction that commits after confirmed_flush is already in
+	 * progress. At this point all xacts committing after confirmed_flush
+	 * can be read entirely into reorder buffers and all visibility
+	 * information can be reconstructed.
+	 */
 	XLogRecPtr	restart_lsn;
 
-	/* oldest LSN that the client has acked receipt for */
+	/*
+	 * oldest LSN that the client has acked receipt for
+	 *
+	 * Decoding will start calling output plugin callbacks for commits
+	 * after this LSN unless a different start point is specified by
+	 * the client.
+	 */
 	XLogRecPtr	confirmed_flush;
 
 	/* plugin name */
-- 
2.1.0

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