Skip unnecessary stat() calls in walkdir().

Some kernels can tell us the type of a "dirent", so we can avoid a call
to stat() or lstat() in many cases.  Define a new function
get_dirent_type() to contain that logic, for use by the backend and
frontend versions of walkdir(), and perhaps other callers in future.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Juan José Santamaría Flecha <juanjo.santamaria@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BFzxupGGN4GpUdbzZN%2Btn6FQPHo8w0Q%2BAPH5Wz8RG%2Bww%40mail.gmail.com

src/include/common/file_utils.h

@@ -1,6 +1,4 @@
 /*-------------------------------------------------------------------------
- *
- * File-processing utility routines for frontend code
  *
  * Assorted utility functions to work on files.
  *
@@ -15,10 +13,28 @@
 #ifndef FILE_UTILS_H
 #define FILE_UTILS_H
 
+#include <dirent.h>
+
+typedef enum PGFileType
+{
+	PGFILETYPE_ERROR,
+	PGFILETYPE_UNKNOWN,
+	PGFILETYPE_REG,
+	PGFILETYPE_DIR,
+	PGFILETYPE_LNK
+} PGFileType;
+
+#ifdef FRONTEND
 extern int	fsync_fname(const char *fname, bool isdir);
 extern void fsync_pgdata(const char *pg_data, int serverVersion);
 extern void fsync_dir_recurse(const char *dir);
 extern int	durable_rename(const char *oldfile, const char *newfile);
 extern int	fsync_parent_path(const char *fname);
+#endif
+
+extern PGFileType get_dirent_type(const char *path,
+								  const struct dirent *de,
+								  bool look_through_symlinks,
+								  int elevel);
 
 #endif							/* FILE_UTILS_H */