Avoid torn copy of the control file in base backups

Base backups read the control file (pg_control) from disk without any
protection against concurrent writes by the server, making it exposed to
torn copies.  Attempting to use such a control file at startup would
cause a failure with a CRC not matching.

Base backups now read the control file while holding ControlFileLock.
c558e6fd92 has done the same for some SQL functions.  The contents of
the file are saved in memory and sent over the wire as-is.

Backpatch down to v15.  This issue is older than v15 but v14 is left out
of the picture as it is going to be EOL soon, so it does not seem worth
taking a risk for.

Author: David Steele <david@pgbackrest.org>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/d1697de1-eb7f-41bf-a4b7-bba7203df11f@pgbackrest.org
Backpatch-through: 15

src/backend/backup/basebackup.c

@@ -23,9 +23,11 @@
 #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"
+#include "common/controldata_utils.h"
 #include "common/file_perm.h"
 #include "common/file_utils.h"
 #include "lib/stringinfo.h"
@@ -42,6 +44,7 @@
 #include "storage/checksum.h"
 #include "storage/dsm_impl.h"
 #include "storage/ipc.h"
+#include "storage/lwlock.h"
 #include "storage/reinit.h"
 #include "utils/builtins.h"
 #include "utils/guc.h"
@@ -340,9 +343,11 @@ perform_base_backup(basebackup_options *opt, bbsink *sink,
 
 			if (ti->path == NULL)
 			{
-				struct stat statbuf;
 				bool		sendtblspclinks = true;
 				char	   *backup_label;
+				ControlFileData *controlData;
+				bool		crc_ok;
+				char		controlFile[PG_CONTROL_FILE_SIZE];
 
 				bbsink_begin_archive(sink, "base.tar");
 
@@ -365,14 +370,22 @@ 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)
+				LWLockAcquire(ControlFileLock, LW_SHARED);
+				controlData = get_controlfile(DataDir, &crc_ok);
+				LWLockRelease(ControlFileLock);
+
+				if (!crc_ok)
 					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);
+							(errmsg("calculated CRC checksum does not match value stored in file")));
+
+				/* Send the full file, zero-padded as it is on disk */
+				memset(controlFile, 0, PG_CONTROL_FILE_SIZE);
+				memcpy(controlFile, controlData, sizeof(ControlFileData));
+				pfree(controlData);
+
+				sendFileWithContent(sink, XLOG_CONTROL_FILE,
+									controlFile, PG_CONTROL_FILE_SIZE,
+									&manifest);
 			}
 			else
 			{