file-truename now uses realpath for local files

This uses fewer syscalls on GNU/Linux and other GNUish platforms.
* lisp/files.el (file-truename): Try fileio--truename
if there are no handlers.
* src/fileio.c: Include stdlib.h, for realpath.
(Ffileio__truename): New function.

lisp/files.el

@@ -1307,9 +1307,14 @@ containing it, until no links are left at any level.
       (let ((handler (find-file-name-handler filename 'file-truename)))
 	;; For file name that has a special handler, call handler.
 	;; This is so that ange-ftp can save time by doing a no-op.
-	(if handler
-	    (setq filename (funcall handler 'file-truename filename)
-		  done t)
+	(or
+	 (if handler
+	     (setq filename (funcall handler 'file-truename filename)
+		   done t)
+	   (condition-case nil
+	       (setq filename (fileio--truename filename)
+		     done t)
+	     (file-missing nil)))
 	  (let ((dir (or (file-name-directory filename) default-directory))
 		target dirfile)
 	    ;; Get the truename of the directory.

src/fileio.c

@@ -20,6 +20,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <config.h>
 #include <limits.h>
 #include <fcntl.h>
+#include <stdlib.h>
 #include "sysstdio.h"
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -2693,6 +2694,46 @@ file_name_absolute_p (char const *filename)
 	      && (!filename[1] || IS_DIRECTORY_SEP (filename[1])
 		  || user_homedir (&filename[1]))));
 }
+
+DEFUN ("fileio--truename", Ffileio__truename, Sfileio__truename, 1, 1, 0,
+       doc: /* Return the true name of FILENAME, without file name handlers.
+
+The returned string is an absolute file name that does not involve
+\".\", \"..\", or symbolic links.  Signal an error if FILENAME does
+not exist or if its true name cannot be determined.  */)
+     (Lisp_Object filename)
+{
+  CHECK_STRING (filename);
+  Lisp_Object absname = Fexpand_file_name (filename, Qnil);
+  Lisp_Object encoded_absname = ENCODE_FILE (absname);
+  ptrdiff_t encoded_len = SBYTES (encoded_absname);
+  char *encoded = SSDATA (encoded_absname);
+  bool append_slash = (1 < encoded_len
+		       && IS_DIRECTORY_SEP (encoded[encoded_len - 1])
+		       && !IS_DIRECTORY_SEP (encoded[encoded_len - 2]));
+  char *truename = realpath (encoded, NULL);
+  if (!truename)
+    report_file_error ("Deriving truename", filename);
+  ptrdiff_t truename_len = strlen (truename);
+  if (truename_len == encoded_len - append_slash
+      && memcmp (truename, encoded, truename_len) == 0)
+    {
+      /* ABSNAME is already the true name.  */
+      xfree (truename);
+      return absname;
+    }
+  else
+    {
+      if (append_slash)
+	{
+	  truename = xrealloc (truename, truename_len + 2);
+	  strcpy (truename + truename_len, "/");
+	}
+      Lisp_Object unibyte_truename = build_unibyte_string (truename);
+      xfree (truename);
+      return DECODE_FILE (unibyte_truename);
+    }
+}
 
 DEFUN ("file-exists-p", Ffile_exists_p, Sfile_exists_p, 1, 1, 0,
        doc: /* Return t if file FILENAME exists (whether or not you can read it).
@@ -6428,6 +6469,7 @@ This includes interactive calls to `delete-file' and
   defsubr (&Sadd_name_to_file);
   defsubr (&Smake_symbolic_link);
   defsubr (&Sfile_name_absolute_p);
+  defsubr (&Sfileio__truename);
   defsubr (&Sfile_exists_p);
   defsubr (&Sfile_executable_p);
   defsubr (&Sfile_readable_p);