Sync PG_VERSION file in CREATE DATABASE.

An OS crash could leave PG_VERSION empty or missing.  The same symptom
appeared in a backup by block device snapshot, taken after the next
checkpoint and before the OS flushes the PG_VERSION blocks.  Device
snapshots are not a documented backup method, however.  Back-patch to
v15, where commit 9c08aea6a3 introduced
STRATEGY=WAL_LOG and made it the default.

Discussion: https://postgr.es/m/20240130195003.0a.nmisch@google.com

src/backend/commands/dbcommands.c

@@ -508,6 +508,14 @@ CreateDirAndVersionFile(char *dbpath, Oid dbid, Oid tsid, bool isRedo)
 	}
 	pgstat_report_wait_end();
 
+	pgstat_report_wait_start(WAIT_EVENT_VERSION_FILE_SYNC);
+	if (pg_fsync(fd) != 0)
+		ereport(data_sync_elevel(ERROR),
+				(errcode_for_file_access(),
+				 errmsg("could not fsync file \"%s\": %m", versionfile)));
+	fsync_fname(dbpath, true);
+	pgstat_report_wait_end();
+
 	/* Close the version file. */
 	CloseTransientFile(fd);
 

src/backend/utils/activity/wait_event_names.txt

@@ -236,6 +236,7 @@ TIMELINE_HISTORY_WRITE	"Waiting for a write of a newly created timeline history
 TWOPHASE_FILE_READ	"Waiting for a read of a two phase state file."
 TWOPHASE_FILE_SYNC	"Waiting for a two phase state file to reach durable storage."
 TWOPHASE_FILE_WRITE	"Waiting for a write of a two phase state file."
+VERSION_FILE_SYNC	"Waiting for the version file to reach durable storage while creating a database."
 VERSION_FILE_WRITE	"Waiting for the version file to be written while creating a database."
 WALSENDER_TIMELINE_HISTORY_READ	"Waiting for a read from a timeline history file during a walsender timeline command."
 WAL_BOOTSTRAP_SYNC	"Waiting for WAL to reach durable storage during bootstrapping."