Acquire ControlFileLock in relevant SQL functions.

Commit dc7d70e added functions that read the control file, but didn't
acquire ControlFileLock.  With unlucky timing, file systems that have
weak interlocking like ext4 and ntfs could expose partially overwritten
contents, and the checksum would fail.

Back-patch to all supported releases.

Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Anton A. Melnikov <aamelnikov@inbox.ru>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de

Author: macdice

src/backend/utils/misc/pg_controldata.c

@@ -24,6 +24,7 @@
 #include "common/controldata_utils.h"
 #include "funcapi.h"
 #include "miscadmin.h"
+#include "storage/lwlock.h"
 #include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/timestamp.h"
@@ -42,7 +43,9 @@ pg_control_system(PG_FUNCTION_ARGS)
 		elog(ERROR, "return type must be a row type");
 
 	/* read the control file */
+	LWLockAcquire(ControlFileLock, LW_SHARED);
 	ControlFile = get_controlfile(DataDir, &crc_ok);
+	LWLockRelease(ControlFileLock);
 	if (!crc_ok)
 		ereport(ERROR,
 				(errmsg("calculated CRC checksum does not match value stored in file")));
@@ -80,7 +83,9 @@ pg_control_checkpoint(PG_FUNCTION_ARGS)
 		elog(ERROR, "return type must be a row type");
 
 	/* Read the control file. */
+	LWLockAcquire(ControlFileLock, LW_SHARED);
 	ControlFile = get_controlfile(DataDir, &crc_ok);
+	LWLockRelease(ControlFileLock);
 	if (!crc_ok)
 		ereport(ERROR,
 				(errmsg("calculated CRC checksum does not match value stored in file")));
@@ -169,7 +174,9 @@ pg_control_recovery(PG_FUNCTION_ARGS)
 		elog(ERROR, "return type must be a row type");
 
 	/* read the control file */
+	LWLockAcquire(ControlFileLock, LW_SHARED);
 	ControlFile = get_controlfile(DataDir, &crc_ok);
+	LWLockRelease(ControlFileLock);
 	if (!crc_ok)
 		ereport(ERROR,
 				(errmsg("calculated CRC checksum does not match value stored in file")));
@@ -208,7 +215,9 @@ pg_control_init(PG_FUNCTION_ARGS)
 		elog(ERROR, "return type must be a row type");
 
 	/* read the control file */
+	LWLockAcquire(ControlFileLock, LW_SHARED);
 	ControlFile = get_controlfile(DataDir, &crc_ok);
+	LWLockRelease(ControlFileLock);
 	if (!crc_ok)
 		ereport(ERROR,
 				(errmsg("calculated CRC checksum does not match value stored in file")));