prevent vc-git.el from unnecessarily locking .git/index.lock

prevent vc-git.el from unnecessarily locking .git/index.lock

[Ami Fischman]

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Problem: starting with
15b73245<http://git.savannah.gnu.org/cgit/emacs.git/commit/lisp/vc-git.el?id=15b73245>
vc-git-state
(and thus anything that puts a file's git VC state in the modeline) causes
the git index to be locked periodically by emacs, even though it doesn't
need to.  This causes rebases (or any other index-lock-needing operation)
run by the user in the same repository to fail, when the locks collide.
Proposed solution: the attached patch makes locking the index more
selective.

Cheers,
-a

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From 75a7c4001414613c65cbd0dc19a5224f69d53a8d Mon Sep 17 00:00:00 2001
From: Dominique Quatravaux <domq@google.com>
Date: Thu, 1 Mar 2012 10:12:33 +0100
Subject: [PATCH] vc-git-state: don't git add --refresh except if no diff.

Under `global-auto-revert-mode', `vc-git-state' is called periodically to update
the mode line.  It is therefore unsafe to grab the index.lock from this function
(as git add --refresh does), as this may derail an ongoing git operation
(especially a rebase).  Only git add --refresh when it is useful, ie, when the
file contents is identical to HEAD but the timestamp isn't.
---
 lisp/vc/vc-git.el |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index dbe591a..f1afba6 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -217,12 +217,20 @@ matching the resulting Git log output, and KEYWORDS is a list of
   ;; operation.
   (if (not (vc-git-registered file))
       'unregistered
-    (vc-git--call nil "add" "--refresh" "--" (file-relative-name file))
     (let ((diff (vc-git--run-command-string
-                 file "diff-index" "-z" "HEAD" "--")))
-      (if (and diff (string-match ":[0-7]\\{6\\} [0-7]\\{6\\} [0-9a-f]\\{40\\} [0-9a-f]\\{40\\} \\([ADMUT]\\)\0[^\0]+\0"
+                 file "diff-index" "-p" "--raw" "-z" "HEAD" "--")))
+      (if (and diff (string-match ":[0-7]\\{6\\} [0-7]\\{6\\} [0-9a-f]\\{40\\} [0-9a-f]\\{40\\} \\([ADMUT]\\)\0[^\0]+\0\\(\\(?:.\\|\n\\)*\\)\\'"
 				  diff))
-	  (vc-git--state-code (match-string 1 diff))
+          (let ((diff-letter (match-string 1 diff))
+                (diff-contents (match-string 2 diff)))
+            (if (not (string-match "\n." diff-contents))
+                ;; Empty diff: file contents is the same as the HEAD
+                ;; revision, but timestamps are different (eg, file
+                ;; was "touch"ed).  Update timestamp in index:
+                (prog1 'up-to-date
+                  (vc-git--call nil "add" "--refresh" "--"
+                                (file-relative-name file)))
+              (vc-git--state-code diff-letter)))
 	(if (vc-git--empty-db-p) 'added 'up-to-date)))))
 
 (defun vc-git-working-revision (file)
-- 
1.7.7.3

Re: prevent vc-git.el from unnecessarily locking .git/index.lock

[Chong Yidong]

Ami Fischman <ami@fischman.org> writes:

> Problem: starting with 15b73245 vc-git-state (and thus anything that
> puts a file's git VC state in the modeline) causes the git index to be
> locked periodically by emacs, even though it doesn't need to.  This
> causes rebases (or any other index-lock-needing operation) run by the
> user in the same repository to fail, when the locks collide.  Proposed
> solution: the attached patch makes locking the index more selective.

Looks reasonable.  Committed; thanks.