Make pg_rewind skip files and directories that are removed during server start.

The target cluster that was rewound needs to perform recovery from
the checkpoint created at failover, which leads it to remove or recreate
some files and directories that may have been copied from the source
cluster. So pg_rewind can skip synchronizing such files and directories,
and which reduces the amount of data transferred during a rewind
without changing the usefulness of the operation.

Author: Michael Paquier
Reviewed-by: Anastasia Lubennikova, Stephen Frost and me

Discussion: https://postgr.es/m/20180205071022.GA17337@paquier.xyz

doc/src/sgml/ref/pg_rewind.sgml

@@ -231,7 +231,19 @@ PostgreSQL documentation
      <para>
       Copy all other files such as <filename>pg_xact</filename> and
       configuration files from the source cluster to the target cluster
-      (everything except the relation files).
+      (everything except the relation files). Similarly to base backups,
+      the contents of the directories <filename>pg_dynshmem/</filename>,
+      <filename>pg_notify/</filename>, <filename>pg_replslot/</filename>,
+      <filename>pg_serial/</filename>, <filename>pg_snapshots/</filename>,
+      <filename>pg_stat_tmp/</filename>, and
+      <filename>pg_subtrans/</filename> are omitted from the data copied
+      from the source cluster. Any file or directory beginning with
+      <filename>pgsql_tmp</filename> is omitted, as well as are
+      <filename>backup_label</filename>,
+      <filename>tablespace_map</filename>,
+      <filename>pg_internal.init</filename>,
+      <filename>postmaster.opts</filename> and
+      <filename>postmaster.pid</filename>.
      </para>
     </step>
     <step>

src/backend/replication/basebackup.c

@@ -103,6 +103,9 @@ static TimestampTz throttled_last;
  * The contents of these directories are removed or recreated during server
  * start so they are not included in backups.  The directories themselves are
  * kept and included as empty to preserve access permissions.
+ *
+ * Note: this list should be kept in sync with the filter lists in pg_rewind's
+ * filemap.c.
  */
 static const char *excludeDirContents[] =
 {

src/bin/pg_rewind/filemap.c

@@ -31,6 +31,83 @@ static char *datasegpath(RelFileNode rnode, ForkNumber forknum,
 static int	path_cmp(const void *a, const void *b);
 static int	final_filemap_cmp(const void *a, const void *b);
 static void filemap_list_to_array(filemap_t *map);
+static bool check_file_excluded(const char *path, const char *type);
+
+/*
+ * The contents of these directories are removed or recreated during server
+ * start so they are not included in data processed by pg_rewind.
+ *
+ * Note: those lists should be kept in sync with what basebackup.c provides.
+ * Some of the values, contrary to what basebackup.c uses, are hardcoded as
+ * they are defined in backend-only headers.  So this list is maintained
+ * with a best effort in mind.
+ */
+static const char *excludeDirContents[] =
+{
+	/*
+	 * Skip temporary statistics files. PG_STAT_TMP_DIR must be skipped even
+	 * when stats_temp_directory is set because PGSS_TEXT_FILE is always
+	 * created there.
+	 */
+	"pg_stat_tmp",	/* defined as PG_STAT_TMP_DIR */
+
+	/*
+	 * It is generally not useful to backup the contents of this directory
+	 * even if the intention is to restore to another master. See backup.sgml
+	 * for a more detailed description.
+	 */
+	"pg_replslot",
+
+	/* Contents removed on startup, see dsm_cleanup_for_mmap(). */
+	"pg_dynshmem",	/* defined as PG_DYNSHMEM_DIR */
+
+	/* Contents removed on startup, see AsyncShmemInit(). */
+	"pg_notify",
+
+	/*
+	 * Old contents are loaded for possible debugging but are not required for
+	 * normal operation, see OldSerXidInit().
+	 */
+	"pg_serial",
+
+	/* Contents removed on startup, see DeleteAllExportedSnapshotFiles(). */
+	"pg_snapshots",
+
+	/* Contents zeroed on startup, see StartupSUBTRANS(). */
+	"pg_subtrans",
+
+	/* end of list */
+	NULL
+};
+
+/*
+ * List of files excluded from filemap processing.
+ */
+static const char *excludeFiles[] =
+{
+	/* Skip auto conf temporary file. */
+	"postgresql.auto.conf.tmp", /* defined as PG_AUTOCONF_FILENAME */
+
+	/* Skip current log file temporary file */
+	"current_logfiles.tmp",		/* defined as LOG_METAINFO_DATAFILE_TMP */
+
+	/* Skip relation cache because it is rebuilt on startup */
+	"pg_internal.init",			/* defined as RELCACHE_INIT_FILENAME */
+
+	/*
+	 * If there's a backup_label or tablespace_map file, it belongs to a
+	 * backup started by the user with pg_start_backup().  It is *not* correct
+	 * for this backup.  Our backup_label is written later on separately.
+	 */
+	"backup_label",				/* defined as BACKUP_LABEL_FILE */
+	"tablespace_map",			/* defined as TABLESPACE_MAP */
+
+	"postmaster.pid",
+	"postmaster.opts",
+
+	/* end of list */
+	NULL
+};
 
 /*
  * Create a new file map (stored in the global pointer "filemap").
@@ -71,11 +148,8 @@ process_source_file(const char *path, file_type_t type, size_t newsize,
 
 	Assert(map->array == NULL);
 
-	/*
+	/* ignore any path matching the exclusion filters */
-	 * Completely ignore some special files in source and destination.
+	if (check_file_excluded(path, "source"))
-	 */
-	if (strcmp(path, "postmaster.pid") == 0 ||
-		strcmp(path, "postmaster.opts") == 0)
 		return;
 
 	/*
@@ -260,6 +334,14 @@ process_target_file(const char *path, file_type_t type, size_t oldsize,
 	filemap_t  *map = filemap;
 	file_entry_t *entry;
 
+	/*
+	 * Ignore any path matching the exclusion filters.  This is not actually
+	 * mandatory for target files, but this does not hurt and let's be
+	 * consistent with the source processing.
+	 */
+	if (check_file_excluded(path, "target"))
+		return;
+
 	snprintf(localpath, sizeof(localpath), "%s/%s", datadir_target, path);
 	if (lstat(localpath, &statbuf) < 0)
 	{
@@ -286,13 +368,6 @@ process_target_file(const char *path, file_type_t type, size_t oldsize,
 		qsort(map->array, map->narray, sizeof(file_entry_t *), path_cmp);
 	}
 
-	/*
-	 * Completely ignore some special files
-	 */
-	if (strcmp(path, "postmaster.pid") == 0 ||
-		strcmp(path, "postmaster.opts") == 0)
-		return;
-
 	/*
 	 * Like in process_source_file, pretend that xlog is always a  directory.
 	 */
@@ -412,6 +487,51 @@ process_block_change(ForkNumber forknum, RelFileNode rnode, BlockNumber blkno)
 	}
 }
 
+/*
+ * Is this the path of file that pg_rewind can skip copying?
+ */
+static bool
+check_file_excluded(const char *path, const char *type)
+{
+	char	localpath[MAXPGPATH];
+	int		excludeIdx;
+	const char	*filename;
+
+	/* check individual files... */
+	for (excludeIdx = 0; excludeFiles[excludeIdx] != NULL; excludeIdx++)
+	{
+		filename = last_dir_separator(path);
+		if (filename == NULL)
+			filename = path;
+		else
+			filename++;
+		if (strcmp(filename, excludeFiles[excludeIdx]) == 0)
+		{
+			pg_log(PG_DEBUG, "entry \"%s\" excluded from %s file list\n",
+				   path, type);
+			return true;
+		}
+	}
+
+	/*
+	 * ... And check some directories.  Note that this includes any contents
+	 * within the directories themselves.
+	 */
+	for (excludeIdx = 0; excludeDirContents[excludeIdx] != NULL; excludeIdx++)
+	{
+		snprintf(localpath, sizeof(localpath), "%s/",
+				 excludeDirContents[excludeIdx]);
+		if (strstr(path, localpath) == path)
+		{
+			pg_log(PG_DEBUG, "entry \"%s\" excluded from %s file list\n",
+				   path, type);
+			return true;
+		}
+	}
+
+	return false;
+}
+
 /*
  * Convert the linked list of entries in map->first/last to the array,
  * map->array.