Add pg_control flag to prevent recovery without backup_label.

Harden recovery by adding a flag to pg_control to indicate that backup_label is
required. This prevents the user from deleting backup_label resulting in an
inconsistent recovery.

Another advantage is that the copy of pg_control used by pg_basebackup is
guaranteed not to be torn.

This functionality is limited to pg_basebackup (or any software comfortable
with modifying pg_control).

Control and catalog version bumps are required.

Author: dwsteele

doc/src/sgml/func/func-info.sgml

@@ -3598,6 +3598,11 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
        <entry><type>boolean</type></entry>
       </row>
 
+      <row>
+       <entry><structfield>backup_label_required</structfield></entry>
+       <entry><type>boolean</type></entry>
+      </row>
+
      </tbody>
     </tgroup>
    </table>

src/backend/access/transam/xlog.c

@@ -9459,6 +9459,29 @@ do_pg_abort_backup(int code, Datum arg)
 	}
 }
 
+/*
+ * Create a consistent copy of control data to be used for backup and update it
+ * to require a backup label for recovery. Also recalculate the CRC.
+ */
+void
+backup_control_file(uint8_t *controlFile)
+{
+	ControlFileData *controlData = ((ControlFileData *)controlFile);
+
+	memset(controlFile + sizeof(ControlFileData), 0,
+		   PG_CONTROL_FILE_SIZE - sizeof(ControlFileData));
+
+	LWLockAcquire(ControlFileLock, LW_SHARED);
+	memcpy(controlFile, ControlFile, sizeof(ControlFileData));
+	LWLockRelease(ControlFileLock);
+
+	controlData->backupLabelRequired = true;
+
+	INIT_CRC32C(controlData->crc);
+	COMP_CRC32C(controlData->crc, controlFile, offsetof(ControlFileData, crc));
+	FIN_CRC32C(controlData->crc);
+}
+
 /*
  * Register a handler that will warn about unterminated backups at end of
  * session, unless this has already been done.

src/backend/access/transam/xlogrecovery.c

@@ -710,7 +710,14 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 	}
 	else
 	{
-		/* No backup_label file has been found if we are here. */
+		/*
+		 * No backup_label file has been found if we are here. Error if the
+		 * control file requires backup_label.
+		 */
+		if (ControlFile->backupLabelRequired)
+			ereport(FATAL,
+					(errmsg("could not find backup_label required for recovery"),
+					 errhint("backup_label must be present for recovery to proceed")));
 
 		/*
 		 * If tablespace_map file is present without backup_label file, there
@@ -985,6 +992,7 @@ InitWalRecovery(ControlFileData *ControlFile, bool *wasShutdown_ptr,
 		{
 			ControlFile->backupStartPoint = checkPoint.redo;
 			ControlFile->backupEndRequired = backupEndRequired;
+			ControlFile->backupLabelRequired = false;
 
 			if (backupFromStandby)
 			{

src/backend/backup/basebackup.c

@@ -23,6 +23,7 @@
 #include "backup/basebackup_incremental.h"
 #include "backup/basebackup_sink.h"
 #include "backup/basebackup_target.h"
+#include "catalog/pg_control.h"
 #include "catalog/pg_tablespace_d.h"
 #include "commands/defrem.h"
 #include "common/compression.h"
@@ -326,9 +327,9 @@ perform_base_backup(basebackup_options *opt, bbsink *sink,
 
 			if (ti->path == NULL)
 			{
-				struct stat statbuf;
 				bool		sendtblspclinks = true;
 				char	   *backup_label;
+				uint8_t controlFile[PG_CONTROL_FILE_SIZE];
 
 				bbsink_begin_archive(sink, "base.tar");
 
@@ -351,14 +352,10 @@ perform_base_backup(basebackup_options *opt, bbsink *sink,
 						sendtblspclinks, &manifest, InvalidOid, ib);
 
 				/* ... and pg_control after everything else. */
-				if (lstat(XLOG_CONTROL_FILE, &statbuf) != 0)
-					ereport(ERROR,
-							(errcode_for_file_access(),
-							 errmsg("could not stat file \"%s\": %m",
-									XLOG_CONTROL_FILE)));
-				sendFile(sink, XLOG_CONTROL_FILE, XLOG_CONTROL_FILE, &statbuf,
-						 false, InvalidOid, InvalidOid,
-						 InvalidRelFileNumber, 0, &manifest, 0, NULL, 0);
+				backup_control_file(controlFile);
+				sendFileWithContent(sink, XLOG_CONTROL_FILE,
+									(char *)controlFile, PG_CONTROL_FILE_SIZE,
+									&manifest);
 			}
 			else
 			{

src/backend/utils/misc/pg_controldata.c

@@ -162,8 +162,8 @@ pg_control_checkpoint(PG_FUNCTION_ARGS)
 Datum
 pg_control_recovery(PG_FUNCTION_ARGS)
 {
-	Datum		values[5];
-	bool		nulls[5];
+	Datum		values[6];
+	bool		nulls[6];
 	TupleDesc	tupdesc;
 	HeapTuple	htup;
 	ControlFileData *ControlFile;
@@ -195,6 +195,9 @@ pg_control_recovery(PG_FUNCTION_ARGS)
 	values[4] = BoolGetDatum(ControlFile->backupEndRequired);
 	nulls[4] = false;
 
+	values[5] = BoolGetDatum(ControlFile->backupLabelRequired);
+	nulls[5] = false;
+
 	htup = heap_form_tuple(tupdesc, values, nulls);
 
 	PG_RETURN_DATUM(HeapTupleGetDatum(htup));

src/bin/pg_controldata/pg_controldata.c

@@ -294,6 +294,8 @@ main(int argc, char *argv[])
 		   LSN_FORMAT_ARGS(ControlFile->backupEndPoint));
 	printf(_("End-of-backup record required:        %s\n"),
 		   ControlFile->backupEndRequired ? _("yes") : _("no"));
+	printf(_("Backup label required:                %s\n"),
+		   ControlFile->backupLabelRequired ? _("yes") : _("no"));
 	printf(_("wal_level setting:                    %s\n"),
 		   wal_level_str(ControlFile->wal_level));
 	printf(_("wal_log_hints setting:                %s\n"),

src/bin/pg_resetwal/pg_resetwal.c

@@ -883,6 +883,7 @@ RewriteControlFile(void)
 	ControlFile.backupStartPoint = 0;
 	ControlFile.backupEndPoint = 0;
 	ControlFile.backupEndRequired = false;
+	ControlFile.backupLabelRequired = false;
 
 	/*
 	 * Force the defaults for max_* settings. The values don't really matter

src/bin/pg_rewind/pg_rewind.c

@@ -734,6 +734,7 @@ perform_rewind(filemap_t *filemap, rewind_source *source,
 	ControlFile_new.minRecoveryPoint = endrec;
 	ControlFile_new.minRecoveryPointTLI = endtli;
 	ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
+	ControlFile_new.backupLabelRequired = true;
 	if (!dry_run)
 		update_controlfile(datadir_target, &ControlFile_new, do_sync);
 }

src/include/access/xlog.h

@@ -296,6 +296,7 @@ extern void do_pg_backup_start(const char *backupidstr, bool fast,
 							   StringInfo tblspcmapfile);
 extern void do_pg_backup_stop(BackupState *state, bool waitforarchive);
 extern void do_pg_abort_backup(int code, Datum arg);
+extern void backup_control_file(uint8_t *controlFile);
 extern void register_persistent_abort_backup_handler(void);
 extern SessionBackupState get_backup_status(void);
 

src/include/catalog/pg_control.h

@@ -164,12 +164,16 @@ typedef struct ControlFileData
 	 * If backupEndRequired is true, we know for sure that we're restoring
 	 * from a backup, and must see a backup-end record before we can safely
 	 * start up.
+	 *
+	 * If backupLabelRequired is true, then a backup_label file must be
+	 * present in order for recovery to proceed.
 	 */
 	XLogRecPtr	minRecoveryPoint;
 	TimeLineID	minRecoveryPointTLI;
 	XLogRecPtr	backupStartPoint;
 	XLogRecPtr	backupEndPoint;
 	bool		backupEndRequired;
+	bool		backupLabelRequired;
 
 	/*
 	 * Parameter settings that determine if the WAL can be used for archival