[Issue #169] improve comments in merge

Author: gsmolk

src/merge.c

@@ -427,13 +427,13 @@ merge_chain(parray *parent_chain, pgBackup *full_backup, pgBackup *dest_backup)
 	/* for fancy reporting */
 	time_t		end_time;
 	char		pretty_time[20];
-	/* in-place flags */
+	/* in-place merge flags */
 	bool		compression_match = false;
 	bool		program_version_match = false;
 	/* It's redundant to check block checksumms during merge */
 	skip_block_validation = true;
 
-	/* Handle corner cases missing destination backup */
+	/* Handle corner cases of missing destination backup */
 	if (dest_backup == NULL &&
 		full_backup->status == BACKUP_STATUS_MERGED)
 		goto merge_rename;
@@ -740,12 +740,11 @@ merge_chain(parray *parent_chain, pgBackup *full_backup, pgBackup *dest_backup)
 	}
 
 	/*
-	 * If we crash now, automatic rerun of failed merge will be impossible.
-	 * The user must have to manually change start_time of FULL backup
-	 * to start_time of destination backup:
 	 * PAGE2 DELETED
 	 * PAGE1 DELETED
 	 * FULL  MERGED
+	 * If we crash now, automatic rerun of failed merge is still possible:
+	 * The user should start merge with full backup ID as an argument to option '-i'.
 	 */
 
 merge_rename:
@@ -891,7 +890,7 @@ merge_files(void *arg)
 			 *
 			 * Case 2:
 			 * in this case in place merge is possible:
-			 * 0 PAGE; file, size BYTES_INVALID (should not be possible)
+			 * 0 PAGE; file, size 0
 			 * 1 PAGE; file, size 0
 			 * 2 FULL; file, size 100500
 			 *
@@ -901,11 +900,11 @@ merge_files(void *arg)
 			 * 1 PAGE; file, size 100501
 			 * 2 FULL; file, size 100500
 			 *
-			 * Case 4:
+			 * Case 4 (good candidate for future optimization):
 			 * in this case in place merge is impossible:
 			 * 0 PAGE; file, size BYTES_INVALID
 			 * 1 PAGE; file, size 100501
-			 * 2 FULL; file, missing
+			 * 2 FULL; file, not exists yet
 			 */
 
 			in_place = true;
@@ -1138,6 +1137,7 @@ merge_data_file(parray *parent_chain, pgBackup *full_backup,
 	if (tmp_file->write_size == 0)
 		return;
 
+	/* sync second temp file to disk */
 	if (fio_sync(to_fullpath_tmp2, FIO_BACKUP_HOST) != 0)
 		elog(ERROR, "Cannot sync merge temp file \"%s\": %s",
 			to_fullpath_tmp2, strerror(errno));
@@ -1242,7 +1242,7 @@ merge_non_data_file(parray *parent_chain, pgBackup *full_backup,
 	backup_non_data_file(tmp_file, NULL, from_fullpath,
 						 to_fullpath_tmp, BACKUP_MODE_FULL, 0, false);
 
-	/* TODO: --no-sync support */
+	/* sync temp file to disk */
 	if (fio_sync(to_fullpath_tmp, FIO_BACKUP_HOST) != 0)
 		elog(ERROR, "Cannot sync merge temp file \"%s\": %s",
 			to_fullpath_tmp, strerror(errno));