Fix code comments regarding logical decoding

Back in 3b02ea4f I added some comments in various places to explain
how logical decoding and other things worked.  Not all of the changes
were welcome, because they were misleading or wrong.  This changes them
a little bit to make them more accurate.

Some other comments are also changed to be more accurate.  Also, fix a
bunch of typos.

Author: Álvaro Herrera, Craig Ringer

Andres Freund reviewed some parts of this.

src/backend/replication/logical/logical.c

@@ -305,10 +305,18 @@ CreateInitDecodingContext(char *plugin,
  * Create a new decoding context, for a logical slot that has previously been
  * used already.
  *
- * start_lsn contains the LSN of the last received data or InvalidXLogRecPtr
- * output_plugin_options contains options passed to the output plugin
- * read_page, prepare_write, do_write are callbacks that have to be filled to
- *		perform the use-case dependent, actual, work.
+ * start_lsn
+ *		The LSN at which to start decoding.  If InvalidXLogRecPtr, restart
+ *		from the slot's confirmed_flush; otherwise, start from the specified
+ *		location (but move it forwards to confirmed_flush if it's older than
+ *		that, see below).
+ *
+ * output_plugin_options
+ *		contains options passed to the output plugin.
+ *
+ * read_page, prepare_write, do_write
+ *		callbacks that have to be filled to perform the use-case dependent,
+ *		actual work.
  *
  * Needs to be called while in a memory context that's at least as long lived
  * as the decoding context because further memory contexts will be created
@@ -745,7 +753,7 @@ message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
  * replication slot.
  *
  * Note that in the most cases, we won't be able to immediately use the xmin
- * to increase the xmin horizon, we need to wait till the client has confirmed
+ * to increase the xmin horizon: we need to wait till the client has confirmed
  * receiving current_lsn with LogicalConfirmReceivedLocation().
  */
 void
@@ -890,7 +898,7 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn)
 
 		MyReplicationSlot->data.confirmed_flush = lsn;
 
-		/* if were past the location required for bumping xmin, do so */
+		/* if we're past the location required for bumping xmin, do so */
 		if (MyReplicationSlot->candidate_xmin_lsn != InvalidXLogRecPtr &&
 			MyReplicationSlot->candidate_xmin_lsn <= lsn)
 		{
@@ -926,7 +934,7 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn)
 
 		SpinLockRelease(&MyReplicationSlot->mutex);
 
-		/* first write new xmin to disk, so we know whats up after a crash */
+		/* first write new xmin to disk, so we know what's up after a crash */
 		if (updated_xmin || updated_restart)
 		{
 			ReplicationSlotMarkDirty();

src/backend/replication/logical/logicalfuncs.c

@@ -238,10 +238,7 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 	PG_TRY();
 	{
-		/*
-		 * Passing InvalidXLogRecPtr here causes replay to start at the slot's
-		 * confirmed_flush.
-		 */
+		/* restart at slot's confirmed_flush */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									options,
 									logical_read_local_xlog_page,
@@ -265,13 +262,9 @@ 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 in
-		 * CreateDecodingContext we set the snapshot builder up using the
-		 * slot's confirmed_flush. This means we might read xlog we don't
-		 * actually decode rows from, but the snapshot builder might need it
-		 * to get to a consistent point. The point we start returning data to
-		 * *users* at is the confirmed_flush lsn set up in the decoding
-		 * context.
+		 * Decoding of WAL must start at restart_lsn so that the entirety of
+		 * xacts that committed after the slot's confirmed_flush can be
+		 * accumulated into reorder buffers.
 		 */
 		startptr = MyReplicationSlot->data.restart_lsn;
 

src/include/replication/slot.h

@@ -66,7 +66,12 @@ typedef struct ReplicationSlotPersistentData
 	/* oldest LSN that might be required by this replication slot */
 	XLogRecPtr	restart_lsn;
 
-	/* oldest LSN that the client has acked receipt for */
+	/*
+	 * Oldest LSN that the client has acked receipt for.  This is used as the
+	 * start_lsn point in case the client doesn't specify one, and also as a
+	 * safety measure to back off in case the client specifies a start_lsn
+	 * that's further in the future than this value.
+	 */
 	XLogRecPtr	confirmed_flush;
 
 	/* plugin name */
@@ -113,11 +118,10 @@ typedef struct ReplicationSlot
 
 	/* all the remaining data is only used for logical slots */
 
-	/* ----
+	/*
 	 * When the client has confirmed flushes >= candidate_xmin_lsn we can
-	 * advance the catalog xmin, when restart_valid has been passed,
+	 * advance the catalog xmin.  When restart_valid has been passed,
 	 * restart_lsn can be increased.
-	 * ----
 	 */
 	TransactionId candidate_catalog_xmin;
 	XLogRecPtr	candidate_xmin_lsn;