From c210b8b128c17929dbb8e0b0564ee25930d44dd1 Mon Sep 17 00:00:00 2001
From: Juanma Barranquero <lekktu@gmail.com>
Date: Thu, 19 Nov 2015 21:32:43 +0100
Subject: [PATCH] Discover repository version in linked worktrees (bug#21930)

* lisp/version.el (emacs-repository--version-git-1): Do not assume
HEAD is at .git/HEAD, it can also be at .git/worktrees/<branch>/HEAD.
(emacs-repository-get-version): Grok linked worktrees when EXTERNAL
is nil too.
---
 lisp/version.el | 64 +++++++++++++++++++++++++++++++------------------
 1 file changed, 41 insertions(+), 23 deletions(-)

diff --git a/lisp/version.el b/lisp/version.el
index 43103fde131..4207cb41f21 100644
--- a/lisp/version.el
+++ b/lisp/version.el
@@ -113,17 +113,17 @@ or if we could not determine the revision.")
 		  (looking-at "[0-9a-fA-F]\\{40\\}"))
 	   (match-string 0)))))
 
-(defun emacs-repository--version-git-1 (file)
+(defun emacs-repository--version-git-1 (file dir)
   "Internal subroutine of `emacs-repository-get-version'."
   (when (file-readable-p file)
-    (erase-buffer)
-    (insert-file-contents file)
-    (cond ((looking-at "[0-9a-fA-F]\\{40\\}")
-	   (match-string 0))
-	  ((looking-at "ref: \\(.*\\)")
-	   (emacs-repository--version-git-1
-	    (expand-file-name (match-string 1)
-			      (file-name-directory file)))))))
+    (with-temp-buffer
+      (insert-file-contents file)
+      (cond ((looking-at "[0-9a-fA-F]\\{40\\}")
+             (match-string 0))
+            ((looking-at "ref: \\(.*\\)")
+             (emacs-repository--version-git-1
+              (expand-file-name (match-string 1) dir)
+              dir))))))
 
 (defun emacs-repository-get-version (&optional dir external)
   "Try to return as a string the repository revision of the Emacs sources.
@@ -138,20 +138,38 @@ Optional argument EXTERNAL non-nil means to just ask the VCS itself,
 if the sources appear to be under version control.  Otherwise only ask
 the VCS if we cannot find any information ourselves."
   (or dir (setq dir source-directory))
-  (when (file-directory-p (expand-file-name ".git" dir))
-    (if external
-	(emacs-repository-version-git dir)
-      (or (let ((files '("HEAD" "refs/heads/master"))
-		file rev)
-	    (with-temp-buffer
-	      (while (and (not rev)
-			  (setq file (car files)))
-		(setq file (expand-file-name (format ".git/%s" file) dir)
-		      files (cdr files)
-		      rev (emacs-repository--version-git-1 file))))
-	    rev)
-	  ;; AFAICS this doesn't work during dumping (bug#20799).
-	  (emacs-repository-version-git dir)))))
+  (let* ((base-dir (expand-file-name ".git" dir))
+         (in-main-worktree (file-directory-p base-dir))
+         (in-linked-worktree nil)
+         sub-dir)
+    ;; If the sources are in a linked worktree, .git is a file that points to
+    ;; the location of the main worktree and the repo's administrative files.
+    (when (and (not in-main-worktree)
+               (file-regular-p base-dir)
+               (file-readable-p base-dir))
+      (with-temp-buffer
+        (insert-file-contents base-dir)
+        (when (looking-at "gitdir: \\(.*\.git\\)\\(.*\\)$")
+          (setq base-dir (match-string 1)
+                sub-dir (concat base-dir (match-string 2))
+                in-linked-worktree t))))
+    ;; We've found a worktree, either main or linked.
+    (when (or in-main-worktree in-linked-worktree)
+      (if external
+          (emacs-repository-version-git dir)
+        (or (if in-linked-worktree
+                (emacs-repository--version-git-1
+                 (expand-file-name "HEAD" sub-dir) base-dir)
+              (let ((files '("HEAD" "refs/heads/master"))
+                    file rev)
+                (while (and (not rev)
+                            (setq file (car files)))
+                  (setq file (expand-file-name file base-dir)
+                        files (cdr files)
+                        rev (emacs-repository--version-git-1 file base-dir)))
+                rev))
+            ;; AFAICS this doesn't work during dumping (bug#20799).
+            (emacs-repository-version-git dir))))))
 
 ;; We put version info into the executable in the form that `ident' uses.
 (purecopy (concat "\n$Id: " (subst-char-in-string ?\n ?\s (emacs-version))
-- 
2.39.5