bug#28264: Accessing source directory through symlink produces false warnings

[Paul Eggert <eggert@cs.ucla.edu>]

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Thanks for the bug report. I reproduced the problem and installed the attached 
patch to fix it. It strikes me, though, that the code still won't work in other 
cases (e.g., symlinks containing ":"), and that Emacs probably has other 
instances of confusion between how it treats symlinks and how the OS treats 
them. I'll CC: this to Michael to give him a heads-up about this particular 
issue, as I'm not really up to speed on magic file names.

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From c1854b1d31e1b0a3a9e91ef41110a5fa77bedb31 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 28 Aug 2017 21:50:09 -0700
Subject: [PATCH] Silence false alarms for symlinks to sources

Problem reported by Glenn Morris (Bug#28264).
* lisp/files.el (files--splice-dirname-file): New function.
(file-truename, file-chase-links): Use it.
---
 lisp/files.el | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index b3eab29..5f55aa7 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -1153,6 +1153,21 @@ files--name-absolute-system-p
   (and (file-name-absolute-p file)
        (not (eq (aref file 0) ?~))))
 
+(defun files--splice-dirname-file (dirname file)
+  "Splice DIRNAME to FILE like the operating system would.
+If FILENAME is relative, return DIRNAME concatenated to FILE.
+Otherwise return FILE, quoted with `/:' if DIRNAME and FILE have
+different handlers; although this quoting is dubious if DIRNAME
+is remote, it is not clear what would be better.  This function
+differs from `expand-file-name' in that DIRNAME must be a
+directory name and leading `~' and `/:' are not special in FILE."
+  (if (files--name-absolute-system-p file)
+      (if (eq (find-file-name-handler dirname 'file-symlink-p)
+	      (find-file-name-handler file 'file-symlink-p))
+	  file
+	(concat "/:" file))
+    (concat dirname file)))
+
 (defun file-truename (filename &optional counter prev-dirs)
   "Return the truename of FILENAME.
 If FILENAME is not absolute, first expands it against `default-directory'.
@@ -1253,10 +1268,7 @@ file-truename
 		    ;; We can't safely use expand-file-name here
 		    ;; since target might look like foo/../bar where foo
 		    ;; is itself a link.  Instead, we handle . and .. above.
-		    (setq filename
-			  (concat (if (files--name-absolute-system-p target)
-				      "/:" dir)
-				  target)
+		    (setq filename (files--splice-dirname-file dir target)
 			  done nil)
 		  ;; No, we are done!
 		  (setq done t))))))))
@@ -1291,10 +1303,8 @@ file-chase-links
 		 (directory-file-name (file-name-directory newname))))
 	  ;; Now find the parent of that dir.
 	  (setq newname (file-name-directory newname)))
-	(setq newname (concat (if (files--name-absolute-system-p tem)
-				  "/:"
-				(file-name-directory newname))
-			      tem))
+	(setq newname (files--splice-dirname-file (file-name-directory newname)
+						  tem))
 	(setq count (1+ count))))
     newname))
 
-- 
2.7.4