Teach pg_ls_dir_files() to ignore ENOENT failures from stat().

Buildfarm experience shows that this function can fail with ENOENT
if some other process unlinks a file between when we read the directory
entry and when we try to stat() it.  The problem is old but we had
not noticed it until 085b6b66 added regression test coverage.

To fix, just ignore ENOENT failures.  There is one other case that
this might hide: a symlink that points to nowhere.  That seems okay
though, at least better than erroring.

Back-patch to v10 where this function was added, since the regression
test cases were too.

Discussion: https://postgr.es/m/20200308173103.GC1357@telsasoft.com

src/backend/utils/adt/genfile.c

@@ -597,9 +597,14 @@ pg_ls_dir_files(FunctionCallInfo fcinfo, const char *dir, bool missing_ok)
 		/* Get the file info */
 		snprintf(path, sizeof(path), "%s/%s", dir, de->d_name);
 		if (stat(path, &attrib) < 0)
+		{
+			/* Ignore concurrently-deleted files, else complain */
+			if (errno == ENOENT)
+				continue;
 			ereport(ERROR,
 					(errcode_for_file_access(),
 					 errmsg("could not stat file \"%s\": %m", path)));
+		}
 
 		/* Ignore anything but regular files */
 		if (!S_ISREG(attrib.st_mode))