On Thu, Jan 23, 2014 at 11:45:25AM +0900, IWAMOTO Toshihiro wrote:
> I found "git-diff --quiet" returns a zero exit status even if there's
> a change.  The following sequence reproduces the bug:
> 
>   $ mkdir foo
>   $ cd foo
>   $ git init
>   $ echo a > a.txt
>   $ echo b >b.txt
>   $ git add ?.txt
>   $ git commit
>   $ echo b >> b.txt
>   $ touch a.txt
>   $ git diff --quiet; echo $?
>   
> Interestingly, if you issue "git-diff --quiet" again, it returns the
> expected exit status 1.

Because stat info in index is updated and diff_change() won't be
called again on a.txt.

> The problem is in the optimization code in run_diff_files().  The
> function finds a.txt has different stat(2) data from .git/index and
> calls diff_change(), which sets DIFF_OPT_HAS_CHANGES.  As the flag
> makes diff_can_quit_early() return 1, run_diff_files()'s loop finishes
> without calling diff_change() for b.txt.
> 
> Then, diffcore_std() examines diff_queued_diff and clears
> DIFF_OPT_HAS_CHANGES, because a.txt is unchanged.
> This is how a change in b.txt is ignored by git-diff --quiet.

Thanks for the analysis. Perhaps we could make diff_change test
whether a.txt is unchanged so it does not set HAS_CHANGES prematurely?
Maybe something like below.

By the time diffcore_skip_stat_unmatch() is called, everything is
cached, so there's not much of performance regression. We still do
memcmp() twice (in diff_filespec_is_identical), but I think that has
less impact than removing diff_can_quit_early().

-- 8< --
diff --git a/diff.c b/diff.c
index 6b4cd0e..5226fc0 100644
--- a/diff.c
+++ b/diff.c
@@ -4697,6 +4697,33 @@ static int diff_filespec_is_identical(struct 
diff_filespec *one,
        return !memcmp(one->data, two->data, one->size);
 }
 
+static int diff_filespec_check_stat_unmatch(struct diff_filepair *p)
+{
+       /*
+        * 1. Entries that come from stat info dirtiness
+        *    always have both sides (iow, not create/delete),
+        *    one side of the object name is unknown, with
+        *    the same mode and size.  Keep the ones that
+        *    do not match these criteria.  They have real
+        *    differences.
+        *
+        * 2. At this point, the file is known to be modified,
+        *    with the same mode and size, and the object
+        *    name of one side is unknown.  Need to inspect
+        *    the identical contents.
+        */
+       if (!DIFF_FILE_VALID(p->one) || /* (1) */
+           !DIFF_FILE_VALID(p->two) ||
+           (p->one->sha1_valid && p->two->sha1_valid) ||
+           (p->one->mode != p->two->mode) ||
+           diff_populate_filespec(p->one, 1) ||
+           diff_populate_filespec(p->two, 1) ||
+           (p->one->size != p->two->size) ||
+           !diff_filespec_is_identical(p->one, p->two)) /* (2) */
+               return 1;
+       return 0;
+}
+
 static void diffcore_skip_stat_unmatch(struct diff_options *diffopt)
 {
        int i;
@@ -4707,27 +4734,7 @@ static void diffcore_skip_stat_unmatch(struct 
diff_options *diffopt)
        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
 
-               /*
-                * 1. Entries that come from stat info dirtiness
-                *    always have both sides (iow, not create/delete),
-                *    one side of the object name is unknown, with
-                *    the same mode and size.  Keep the ones that
-                *    do not match these criteria.  They have real
-                *    differences.
-                *
-                * 2. At this point, the file is known to be modified,
-                *    with the same mode and size, and the object
-                *    name of one side is unknown.  Need to inspect
-                *    the identical contents.
-                */
-               if (!DIFF_FILE_VALID(p->one) || /* (1) */
-                   !DIFF_FILE_VALID(p->two) ||
-                   (p->one->sha1_valid && p->two->sha1_valid) ||
-                   (p->one->mode != p->two->mode) ||
-                   diff_populate_filespec(p->one, 1) ||
-                   diff_populate_filespec(p->two, 1) ||
-                   (p->one->size != p->two->size) ||
-                   !diff_filespec_is_identical(p->one, p->two)) /* (2) */
+               if (diff_filespec_check_stat_unmatch(p))
                        diff_q(&outq, p);
                else {
                        /*
@@ -4890,6 +4897,7 @@ void diff_change(struct diff_options *options,
                 unsigned old_dirty_submodule, unsigned new_dirty_submodule)
 {
        struct diff_filespec *one, *two;
+       struct diff_filepair *p;
 
        if (S_ISGITLINK(old_mode) && S_ISGITLINK(new_mode) &&
            is_submodule_ignored(concatpath, options))
@@ -4916,10 +4924,17 @@ void diff_change(struct diff_options *options,
        fill_filespec(two, new_sha1, new_sha1_valid, new_mode);
        one->dirty_submodule = old_dirty_submodule;
        two->dirty_submodule = new_dirty_submodule;
+       p = diff_queue(&diff_queued_diff, one, two);
 
-       diff_queue(&diff_queued_diff, one, two);
-       if (!DIFF_OPT_TST(options, DIFF_FROM_CONTENTS))
-               DIFF_OPT_SET(options, HAS_CHANGES);
+       if (DIFF_OPT_TST(options, DIFF_FROM_CONTENTS))
+               return;
+
+       if (DIFF_OPT_TST(options, QUICK) &&
+           options->skip_stat_unmatch &&
+           !diff_filespec_check_stat_unmatch(p))
+               return;
+
+       DIFF_OPT_SET(options, HAS_CHANGES);
 }
 
 struct diff_filepair *diff_unmerge(struct diff_options *options, const char 
*path)
-- 8< --

> Here's a obvious fix for this bug, but I think you can find a better
> fix. Thanks in advance.
> 
> 
> diff --git a/diff-lib.c b/diff-lib.c
> index 346cac6..0b8c58d 100644
> --- a/diff-lib.c
> +++ b/diff-lib.c
> @@ -105,9 +105,6 @@ int run_diff_files(struct rev_info *revs, unsigned int 
> option)
>               int changed;
>               unsigned dirty_submodule = 0;
>  
> -             if (diff_can_quit_early(&revs->diffopt))
> -                     break;
> -
>               if (!ce_path_match(ce, &revs->prune_data))
>                       continue;
>  
> --
> IWAMOTO Toshihiro
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