From: Koji Nakamaru <koji.nakamaru@gree.net>
Date: Sat, 28 Feb 2026 17:02:11 GMT
Subject: Re: [PATCH] fsmonitor: fix two bugs in watchman hook retry path
Message-ID: <CAOTNsDyy2ZQDhkVyML6j5naS_UC=SjE915hjygj+QmKe4bqc=A@mail.gmail.com>
In-Reply-To: <pull.2180.git.git.1769391202338.gitgitgadget@gmail.com>

On Mon, Jan 26, 2026 at 10:33 AM Paul Tarjan via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Claude <noreply@anthropic.com>
>
> The is_work_tree_watched() function in fsmonitor-watchman.sample has
> two bugs:
>
> 1. Wrong variable in error check: After calling watchman_clock(), the
>    result is stored in $o, but the code checks $output->{error} instead
>    of $o->{error}. This means errors from the clock command are silently
>    ignored.
>
> 2. Double output violates protocol: When the retry path triggers (the
>    directory wasn't initially watched), output_result() is called with
>    the "/" flag, then launch_watchman() is called recursively which
>    calls output_result() again. This outputs two clock tokens to stdout,
>    but git's fsmonitor v2 protocol expects exactly one response.
>
> Fix #1 by checking $o->{error} after watchman_clock().
>
> Fix #2 by removing the recursive launch_watchman() call. The "/"
> "everything is dirty" flag already tells git to do a full scan, and
> git will call the hook again on the next invocation with a valid clock
> token.
>
> Apply the same fixes to the test helper scripts in t/t7519/.
>
> Signed-off-by: Paul Tarjan <github@paulisageek.com>
> ---
>     fsmonitor-watchman: fix variable reference and remove redundant code
>
>     The is_work_tree_watched() function in fsmonitor-watchman.sample has two
>     bugs:
>
>      1. Wrong variable in error check: After calling watchman_clock(), the
>         result is stored in $o, but the code checks $output->{error} instead
>         of $o->{error}. This means errors from the clock command are
>         silently ignored.
>
>      2. Double output violates protocol: When the retry path triggers (the
>         directory wasn't initially watched), output_result() is called with
>         the "/" flag, then launch_watchman() is called recursively which
>         calls output_result() again. This outputs two clock tokens to
>         stdout, but git's fsmonitor v2 protocol expects exactly one
>         response.
>
>     Fix #1 by checking $o->{error} after watchman_clock().
>
>     Fix #2 by removing the recursive launch_watchman() call. The "/"
>     "everything is dirty" flag already tells git to do a full scan, and git
>     will call the hook again on the next invocation with a valid clock
>     token.
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2180%2Fptarjan%2Fclaude%2Ffix-watchman-query-bug-sfbIw-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2180/ptarjan/claude/fix-watchman-query-bug-sfbIw-v1
> Pull-Request: https://github.com/git/git/pull/2180
>
>  t/t7519/fsmonitor-watchman                | 1 -
>  t/t7519/fsmonitor-watchman-v2             | 5 +----
>  templates/hooks/fsmonitor-watchman.sample | 5 +----
>  3 files changed, 2 insertions(+), 9 deletions(-)
>
> diff --git a/t/t7519/fsmonitor-watchman b/t/t7519/fsmonitor-watchman
> index 264b9daf83..907bd1533c 100755
> --- a/t/t7519/fsmonitor-watchman
> +++ b/t/t7519/fsmonitor-watchman
> @@ -109,7 +109,6 @@ sub launch_watchman {
>                 close $fh;
>
>                 print "/\0";
> -               eval { launch_watchman() };
>                 exit 0;
>         }
>
> diff --git a/t/t7519/fsmonitor-watchman-v2 b/t/t7519/fsmonitor-watchman-v2
> index 14ed0aa42d..2c59120c3d 100755
> --- a/t/t7519/fsmonitor-watchman-v2
> +++ b/t/t7519/fsmonitor-watchman-v2
> @@ -141,15 +141,12 @@ sub is_work_tree_watched {
>                 # Watchman query just to get it over with now so we won't pay
>                 # the cost in git to look up each individual file.
>                 my $o = watchman_clock();
> -               $error = $output->{error};
> +               $error = $o->{error};
>
>                 die "Watchman: $error.\n" .
>                 "Falling back to scanning...\n" if $error;
>
>                 output_result($o->{clock}, ("/"));
> -               $last_update_token = $o->{clock};
> -
> -               eval { launch_watchman() };
>                 return 0;
>         }
>
> diff --git a/templates/hooks/fsmonitor-watchman.sample b/templates/hooks/fsmonitor-watchman.sample
> index 23e856f5de..21c81b6804 100755
> --- a/templates/hooks/fsmonitor-watchman.sample
> +++ b/templates/hooks/fsmonitor-watchman.sample
> @@ -142,15 +142,12 @@ sub is_work_tree_watched {
>                 # Watchman query just to get it over with now so we won't pay
>                 # the cost in git to look up each individual file.
>                 my $o = watchman_clock();
> -               $error = $output->{error};
> +               $error = $o->{error};
>
>                 die "Watchman: $error.\n" .
>                 "Falling back to scanning...\n" if $error;
>
>                 output_result($o->{clock}, ("/"));
> -               $last_update_token = $o->{clock};
> -
> -               eval { launch_watchman() };
>                 return 0;
>         }
>
>
> base-commit: 68cb7f9e92a5d8e9824f5b52ac3d0a9d8f653dbe
> --
> gitgitgadget
>

Although I don't have much experience with the Watchman backend, the
fixes look correct to me.

One suggestion: it seems we can make the code even cleaner by removing
$retry and its associated logic, as they appear to be no longer
necessary after these changes.

--
Koji Nakamaru