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

doc/src/sgml/monitoring.sgml

@@ -1545,6 +1545,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry><literal>TwophaseFileWrite</literal></entry>
       <entry>Waiting for a write of a two phase state file.</entry>
      </row>
+     <row>
+      <entry><literal>VersionFileSync</literal></entry>
+      <entry>Waiting for the version file to reach durable storage while
+       creating a database.</entry>
+     </row>
      <row>
       <entry><literal>VersionFileWrite</literal></entry>
       <entry>Waiting for the version file to be written while creating a database.</entry>

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.c

@@ -717,6 +717,9 @@ pgstat_get_wait_io(WaitEventIO w)
 		case WAIT_EVENT_TWOPHASE_FILE_WRITE:
 			event_name = "TwophaseFileWrite";
 			break;
+		case WAIT_EVENT_VERSION_FILE_SYNC:
+			event_name = "VersionFileSync";
+			break;
 		case WAIT_EVENT_VERSION_FILE_WRITE:
 			event_name = "VersionFileWrite";
 			break;

src/include/utils/wait_event.h

@@ -234,7 +234,8 @@ typedef enum
 	WAIT_EVENT_WAL_READ,
 	WAIT_EVENT_WAL_SYNC,
 	WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN,
-	WAIT_EVENT_WAL_WRITE
+	WAIT_EVENT_WAL_WRITE,
+	WAIT_EVENT_VERSION_FILE_SYNC
 } WaitEventIO;