diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index ae9d75b1fff..2c2b8fb61bc 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3544,10 +3544,16 @@ PreallocXlogFiles(XLogRecPtr endptr)
  * existed while the server has been running, as this function always
  * succeeds if no WAL segments have been removed since startup.
  * 'tli' is only used in the error message.
+ *
+ * Note: this function guarantees to keep errno unchanged on return.
+ * This supports callers that use this to possibly deliver a better
+ * error message about a missing file, while still being able to throw
+ * a normal file-access error afterwards, if this does return.
  */
 void
 CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
 {
+	int			save_errno = errno;
 	XLogSegNo	lastRemovedSegNo;
 
 	SpinLockAcquire(&XLogCtl->info_lck);
@@ -3559,11 +3565,13 @@ CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
 		char		filename[MAXFNAMELEN];
 
 		XLogFileName(filename, tli, segno);
+		errno = save_errno;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("requested WAL segment %s has already been removed",
 						filename)));
 	}
+	errno = save_errno;
 }
 
 /*
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index bb81acbd1d0..6f35105e867 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -297,7 +297,6 @@ static int	FileAccess(File file);
 static File OpenTemporaryFileInTablespace(Oid tblspcOid, bool rejectError);
 static bool reserveAllocatedDesc(void);
 static int	FreeDesc(AllocateDesc *desc);
-static struct dirent *ReadDirExtended(DIR *dir, const char *dirname, int elevel);
 
 static void AtProcExit_Files(int code, Datum arg);
 static void CleanupTempFiles(bool isProcExit);
@@ -2084,6 +2083,10 @@ CloseTransientFile(int fd)
  * necessary to open the directory, and with closing it after an elog.
  * When done, call FreeDir rather than closedir.
  *
+ * Returns NULL, with errno set, on failure.  Note that failure detection
+ * is commonly left to the following call of ReadDir or ReadDirExtended;
+ * see the comments for ReadDir.
+ *
  * Ideally this should be the *only* direct call of opendir() in the backend.
  */
 DIR *
@@ -2146,8 +2149,8 @@ TryAgain:
  *		FreeDir(dir);
  *
  * since a NULL dir parameter is taken as indicating AllocateDir failed.
- * (Make sure errno hasn't been changed since AllocateDir if you use this
- * shortcut.)
+ * (Make sure errno isn't changed between AllocateDir and ReadDir if you
+ * use this shortcut.)
  *
  * The pathname passed to AllocateDir must be passed to this routine too,
  * but it is only used for error reporting.
@@ -2159,10 +2162,15 @@ ReadDir(DIR *dir, const char *dirname)
 }
 
 /*
- * Alternate version that allows caller to specify the elevel for any
- * error report.  If elevel < ERROR, returns NULL on any error.
+ * Alternate version of ReadDir that allows caller to specify the elevel
+ * for any error report (whether it's reporting an initial failure of
+ * AllocateDir or a subsequent directory read failure).
+ *
+ * If elevel < ERROR, returns NULL after any error.  With the normal coding
+ * pattern, this will result in falling out of the loop immediately as
+ * though the directory contained no (more) entries.
  */
-static struct dirent *
+struct dirent *
 ReadDirExtended(DIR *dir, const char *dirname, int elevel)
 {
 	struct dirent *dent;
@@ -2192,14 +2200,22 @@ ReadDirExtended(DIR *dir, const char *dirname, int elevel)
 /*
  * Close a directory opened with AllocateDir.
  *
- * Note we do not check closedir's return value --- it is up to the caller
- * to handle close errors.
+ * Returns closedir's return value (with errno set if it's not 0).
+ * Note we do not check the return value --- it is up to the caller
+ * to handle close errors if wanted.
+ *
+ * Does nothing if dir == NULL; we assume that directory open failure was
+ * already reported if desired.
  */
 int
 FreeDir(DIR *dir)
 {
 	int			i;
 
+	/* Nothing to do if AllocateDir failed */
+	if (dir == NULL)
+		return 0;
+
 	DO_DB(elog(LOG, "FreeDir: Allocated %d", numAllocatedDescs));
 
 	/* Remove dir from list of allocated dirs, if it's present */
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index 4e7c7df40f4..a56fc5bd225 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -87,6 +87,8 @@ extern int	ClosePipeStream(FILE *file);
 /* Operations to allow use of the <dirent.h> library routines */
 extern DIR *AllocateDir(const char *dirname);
 extern struct dirent *ReadDir(DIR *dir, const char *dirname);
+extern struct dirent *ReadDirExtended(DIR *dir, const char *dirname,
+				int elevel);
 extern int	FreeDir(DIR *dir);
 
 /* Operations to allow use of a plain kernel FD, with automatic cleanup */