On Mon, Jun 01, 2020 at 12:54:08PM -0400, Jeff King wrote:
> > What could be the reason that the problem only occurs with the `--quiet` flag
> > that I use in my prompt command, but not when using `git diff` without the flag.
>
> That is curious, and I can reproduce it here.
This turned out to be quite an interesting bug to look at. The fix is
simple, but I tried to summarize my explorations below.
I'm still curious why all of your files were stat-dirty. :) As a
workaround until you have a version of Git with the fix, you can
manually refresh the index with:
git update-index --refresh
before running git-diff.
-- >8 --
Subject: [PATCH] diff: discard blob data from stat-unmatched pairs
When performing a tree-level diff against the working tree, we may find
that our index stat information is dirty, so we queue a filepair to be
examined later. If the actual content hasn't changed, we call this a
stat-unmatch; the stat information was out of date, but there's no
actual diff. Normally diffcore_std() would detect and remove these
identical filepairs via diffcore_skip_stat_unmatch(). However, when
"--quiet" is used, we want to stop the diff as soon as we see any
changes, so we check for stat-unmatches immediately in diff_change().
That check may require us to actually load the file contents into the
pair of diff_filespecs. If we find that the pair isn't a stat-unmatch,
then no big deal; we'd likely load the contents later anyway to generate
a patch, do rename detection, etc, so we want to hold on to it. But if
it a stat-unmatch, then we have no more use for that data; the whole
point is that we're going discard the pair. However, we never free the
allocated diff_filespec data.
In most cases, keeping that data isn't a problem. We don't expect a lot
of stat-unmatch entries, and since we're using --quiet, we'd quit as
soon as we saw such a real change anyway. However, there are extreme
cases where it makes a big difference:
1. We'd generally mmap() the working tree half of the pair. And since
the OS may limit the total number of maps, we can run afoul of this
in large repositories. E.g.:
$ cd linux
$ git ls-files | wc -l
67959
$ sysctl vm.max_map_count
vm.max_map_count = 65530
$ git ls-files | xargs touch ;# everything is stat-dirty!
$ git diff --quiet
fatal: mmap failed: Cannot allocate memory
It should be unusual to have so many files stat-dirty, but it's
possible if you've just run a script like "sed -i" or similar.
After this patch, the above correctly exits with code 0.
2. Even if you don't hit mmap limits, the index half of the pair will
have been pulled from the object database into heap memory. Again
in a clone of linux.git, running:
$ git ls-files | head -n 10000 | xargs touch
$ git diff --quiet
peaks at 145MB heap before this patch, and 94MB after.
This patch solves the problem by freeing any diff_filespec data we
picked up during the "--quiet" stat-unmatch check in diff_changes.
Nobody is going to need that data later, so there's no point holding on
to it. There are a few things to note:
- we could skip queueing the pair entirely, which could in theory save
a little work. But there's not much to save, as we need a
diff_filepair to feed to diff_filespec_check_stat_unmatch() anyway.
And since we cache the result of the stat-unmatch checks, a later
call to diffcore_skip_stat_unmatch() call will quickly skip over
them. The diffcore code also counts up the number of stat-unmatched
pairs as it removes them. It's doubtful any callers would care about
that in combination with --quiet, but we'd have to reimplement the
logic here to be on the safe side. So it's not really worth the
trouble.
- I didn't write a test, because we always produce the correct output
unless we run up against system mmap limits, which are both
unportable and expensive to test against. Measuring peak heap
would be interesting, but our perf suite isn't yet capable of that.
- note that diff without "--quiet" does not suffer from the same
problem. In diffcore_skip_stat_unmatch(), we detect the stat-unmatch
entries and drop them immediately, so we're not carrying their data
around.
- you _can_ still trigger the mmap limit problem if you truly have
that many files with actual changes. But it's rather unlikely. The
stat-unmatch check avoids loading the file contents if the sizes
don't match, so you'd need a pretty trivial change in every single
file. Likewise, inexact rename detection might load the data for
many files all at once. But you'd need not just 64k changes, but
that many deletions and additions. The most likely candidate is
perhaps break-detection, which would load the data for all pairs and
keep it around for the content-level diff. But again, you'd need 64k
actually changed files in the first place.
So it's still possible to trigger this case, but it seems like "I
accidentally made all my files stat-dirty" is the most likely case
in the real world.
Reported-by: Jan Christoph Uhde <Jan@xxxxxxxxxx>
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
diff.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/diff.c b/diff.c
index d1ad6a3c4a..4d46fab5d3 100644
--- a/diff.c
+++ b/diff.c
@@ -6758,8 +6758,11 @@ void diff_change(struct diff_options *options,
return;
if (options->flags.quick && options->skip_stat_unmatch &&
- !diff_filespec_check_stat_unmatch(options->repo, p))
+ !diff_filespec_check_stat_unmatch(options->repo, p)) {
+ diff_free_filespec_data(p->one);
+ diff_free_filespec_data(p->two);
return;
+ }
options->flags.has_changes = 1;
}
--
2.27.0.rc2.689.g04a94059a7
