Re: [PATCH v3 4/6] diff: add long-running diff process via diff.<driver>.process

>
> Hi Michael,
>
> I stumbled about this patch when it broke CI in Git for Windows, where we
> do _not_ use `NO_PYTHON`, even though Python is unavailable in the
> build/test CI jobs. The existing tests handle this situation gracefully,
> this here patch does not:
>
> On Sun, 7 Jun 2026, Michael Montalbo via GitGitGadget wrote:
>
> > diff --git a/t/t4080-diff-process.sh b/t/t4080-diff-process.sh
> > new file mode 100755
> > index 0000000000..f159cd86d8
> > --- /dev/null
> > +++ b/t/t4080-diff-process.sh
> > @@ -0,0 +1,538 @@
> > +#!/bin/sh
> > +
> > +test_description='diff process via long-running process'
> > +
> > +. ./test-lib.sh
> > +
> > +if test_have_prereq PYTHON
> > +then
> > +     PYTHON_PATH=$(command -v python3) || PYTHON_PATH=$(command -v python)
>
> When neither `python3` nor `python` are available (which is the case in
> the minimal Git for Windows SDK used in Git's CI runs), this fails under
> `set -e`. Before even running the first test case. Resulting in an
> unexpected TAP format error.
>
> Now, we could "fix" this by imitating what `lib-p4` does (see
> https://github.com/dscho/git/commit/bd0b5570c744f678911a67a62da63f30655f20d8
> which demonstrates that it is indeed a work-around on Windows):
>
> -- snip --
> diff --git a/t/t4080-diff-process.sh b/t/t4080-diff-process.sh
> index fdf6da1c341e67..bd22c247ff3856 100755
> --- a/t/t4080-diff-process.sh
> +++ b/t/t4080-diff-process.sh
> @@ -4,9 +4,10 @@ test_description='diff process via long-running process'
>
>  . ./test-lib.sh
>
> -if test_have_prereq PYTHON
> +if ! test_have_prereq PYTHON || ! test -x "$PYTHON_PATH"
>  then
> -       PYTHON_PATH=$(command -v python3) || PYTHON_PATH=$(command -v python)
> +       skip_all='python interpreter not available'
> +       test_done
>  fi
>
>  #
> -- snap --
>
> Of course, this uncovers _another_ problem with the Python script: It uses
> Python3-only `f"..."` format strings, which cannot be handled by the
> Python2 to which the `PYTHON_PATH` variable in `linux-TEST-vars` points.
> So this requires _another follow-up (see also
> https://github.com/dscho/git/commit/c12a9f4c80e5ce8db0fe370fac46fb45be2b775f):
>
> -- snip --
> diff --git a/t/t4080-diff-process.sh b/t/t4080-diff-process.sh
> index bd22c247ff3856..ba14682a9086e4 100755
> --- a/t/t4080-diff-process.sh
> +++ b/t/t4080-diff-process.sh
> @@ -39,7 +39,8 @@ setup_backend () {
>
>         def write_pkt(line):
>             data = (line + "\n").encode()
> -           sys.stdout.buffer.write(f"{len(data)+4:04x}".encode() + data)
> +           hdr = "{:04x}".format(len(data) + 4).encode()
> +           sys.stdout.buffer.write(hdr + data)
>             sys.stdout.buffer.flush()
>
>         def write_flush():
> @@ -98,7 +99,8 @@ setup_backend () {
>             new = read_content()
>             old_first = old.split(b"\n")[0].decode(errors="replace") if old else ""
>             new_first = new.split(b"\n")[0].decode(errors="replace") if new else ""
> -           log(f"command={cmd} pathname={pathname} old={old_first} new={new_first}")
> +           log("command={} pathname={} old={} new={}".format(
> +               cmd, pathname, old_first, new_first))
>
>             if mode == "error":
>                 write_flush()
> @@ -130,7 +132,7 @@ setup_backend () {
>                 else:
>                     ol = old.count(b"\n")
>                     nl = new.count(b"\n")
> -                   write_pkt(f"hunk 1 {ol} 1 {nl}")
> +                   write_pkt("hunk 1 {} 1 {}".format(ol, nl))
>                 write_flush()
>                 write_pkt("status=success")
>                 write_flush()
> -- snap --
>
> And this is still not enough to make it work with Python2, see
> https://github.com/dscho/git/actions/runs/27091523842/job/79955895737:
>
> -- snip --
> [...]
> + git -c diff.cdiff.process=./diff-process-backend --mode=fixed-hunk diff boundary.c
>   Traceback (most recent call last):
>     File "/__w/git/git/t/trash directory.t4080-diff-process/diff-process-backend.py", line 45, in <module>
>       assert read_pkt() == "git-diff-client"
>     File "/__w/git/git/t/trash directory.t4080-diff-process/diff-process-backend.py", line 4, in read_pkt
>       hdr = sys.stdin.buffer.read(4)
>   AttributeError: 'file' object has no attribute 'buffer'
> -- snap --
>
> I have experienced similar patterns in my career, where a single decision
> required multiple follow-up fixes _just_ to avoid having to revert that
> decision. This kind of doubling down has never ended well.
>
> Therefore I would like to take a step back, and ask: Is it _really_ a good
> idea to use Python here? Are we certain that we want to _require_ Python
> to run this test and skip it if Python isn't available (as is the case in
> the Windows-related parts of Git's very own CI) even if Python has nothing
> at all to do with the feature that is being tested?
>
> I don't want to be doomed to repeat history, and we can very well learn
> e.g. from prior art in this very project, where the tests for the
> clean/smudge filters (which _also_ want to speak pkt-line over stdio)
> needlessly incurred Perl as a requirement to run the tests. It was
> Matheus's heroic work in 52917a998ef3a (t0021: implementation the
> rot13-filter.pl script in C, 2022-08-14) and 4d1d843be7a15 (tests: use the
> new C rot13-filter helper to avoid PERL prereq, 2022-08-14) that avoided
> that unnecessary prerequisite.
>
> Likewise, there is `test-tool pkt-line` intended for driving the pkt-line
> protocol via simple shell scripts.
>
> So the conscious project direction has been: fold pkt-line test backends
> into `test-tool` and drop the scripting-language prereq. Reintroducing the
> same shape in Python would walk this back.
>
> Patrick's careful effort in 27bd8ee311719 (Merge branch 'ps/fewer-perl',
> 2025-04-29) has been another clear sign that the Git project is actively
> _removing_ scripting-language dependencies from the build and test
> infrastructure, not adding new ones.
>

> The clear prior art in Git's own tests for what t4080 wants to do, as of
> today, is `t/helper/test-rot13-filter.c`, which could be imitated here
> instead of (re-)introducing a dependency on a scripting language other
> than Unix shell in Git's test suite.
>
> The `PYTHON` prereq exists in exactly five files today, all `git p4`
> related (where Python is an intrinsic prerequisite given that `git-p4.py`
> _is_ written in Python): `t/lib-git-p4.sh`, `t/t9802-git-p4-filetype.sh`,
> `t/t9810-git-p4-rcs.sh`, `t/t9835-git-p4-metadata-encoding-python2.sh`,
> and `t/t9836-git-p4-metadata-encoding-python3.sh`.
>
> After 7cdbff14d482 (remove merge-recursive-old, 2006-11-20), this here
> patch would be the first one, after almost 20 years, to re-introduce
> Python as a dependency outside `git p4`.
>
> And it would also be the first ever to embed a Python script as a heredoc:
>
> > +fi
> > +
> > +#
> > +# A single parametric diff process.
> > +# Usage: diff-process-backend --mode=<mode> [--log=<path>]
> > +#
> > +# Modes:
> > +#   whole-file  - report all lines as changed (default)
> > +#   fixed-hunk  - always report hunk 5 2 5 2
> > +#   bad-hunk    - report out-of-bounds hunk 999 1 999 1
> > +#   bad-sync    - report hunk with mismatched unchanged totals
> > +#   overlap     - report two overlapping hunks
> > +#   no-hunks   - return no hunks (files considered equivalent)
> > +#   error       - return status=error for every request
> > +#   abort       - return status=abort for every request
> > +#   crash       - read one request then exit without responding
> > +#
> > +setup_backend () {
> > +     cat >"$TRASH_DIRECTORY/diff-process-backend.py" <<-\PYEOF
> > +     import sys, os
> > +
> > +     def read_pkt():
> > +         hdr = sys.stdin.buffer.read(4)
> > +         if len(hdr) < 4: return None
> > +         length = int(hdr, 16)
> > +         if length == 0: return ""
> > +         data = sys.stdin.buffer.read(length - 4)
> > +         return data.decode().rstrip("\n")
> > +
> > +     def write_pkt(line):
> > +         data = (line + "\n").encode()
> > +         sys.stdout.buffer.write(f"{len(data)+4:04x}".encode() + data)
> > +         sys.stdout.buffer.flush()
> > +
> > +     def write_flush():
> > +         sys.stdout.buffer.write(b"0000")
> > +         sys.stdout.buffer.flush()
> > +
> > +     def read_content():
> > +         chunks = []
> > +         while True:
> > +             hdr = sys.stdin.buffer.read(4)
> > +             if len(hdr) < 4: break
> > +             length = int(hdr, 16)
> > +             if length == 0: break
> > +             chunks.append(sys.stdin.buffer.read(length - 4))
> > +         return b"".join(chunks)
> > +
> > +     mode = "whole-file"
> > +     logfile = None
> > +     for arg in sys.argv[1:]:
> > +         if arg.startswith("--mode="):
> > +             mode = arg[7:]
> > +         elif arg.startswith("--log="):
> > +             logfile = open(arg[6:], "a")
> > +
> > +     def log(msg):
> > +         if logfile:
> > +             logfile.write(msg + "\n")
> > +             logfile.flush()
> > +
> > +     # Handshake
> > +     assert read_pkt() == "git-diff-client"
> > +     assert read_pkt() == "version=1"
> > +     read_pkt()
> > +     write_pkt("git-diff-server")
> > +     write_pkt("version=1")
> > +     write_flush()
> > +     while True:
> > +         p = read_pkt()
> > +         if p == "": break
> > +     write_pkt("capability=hunks")
> > +     write_flush()
> > +
> > +     log("ready")
> > +
> > +     while True:
> > +         cmd = None
> > +         pathname = None
> > +         while True:
> > +             p = read_pkt()
> > +             if p is None: sys.exit(0)
> > +             if p == "": break
> > +             if p.startswith("command="): cmd = p.split("=",1)[1]
> > +             if p.startswith("pathname="): pathname = p.split("=",1)[1]
> > +         if cmd is None: sys.exit(0)
> > +         old = read_content()
> > +         new = read_content()
> > +         old_first = old.split(b"\n")[0].decode(errors="replace") if old else ""
> > +         new_first = new.split(b"\n")[0].decode(errors="replace") if new else ""
> > +         log(f"command={cmd} pathname={pathname} old={old_first} new={new_first}")
> > +
> > +         if mode == "error":
> > +             write_flush()
> > +             write_pkt("status=error")
> > +             write_flush()
> > +             continue
> > +
> > +         if mode == "abort":
> > +             write_flush()
> > +             write_pkt("status=abort")
> > +             write_flush()
> > +             continue
> > +
> > +         if mode == "crash":
> > +             sys.exit(1)
> > +
> > +         if cmd == "hunks":
> > +             if mode == "fixed-hunk":
> > +                 write_pkt("hunk 5 2 5 2")
> > +             elif mode == "bad-hunk":
> > +                 write_pkt("hunk 999 1 999 1")
> > +             elif mode == "bad-sync":
> > +                 write_pkt("hunk 1 2 1 1")
> > +             elif mode == "overlap":
> > +                 write_pkt("hunk 1 5 1 5")
> > +                 write_pkt("hunk 3 2 3 2")
> > +             elif mode == "no-hunks":
> > +                 pass
> > +             else:
> > +                 ol = old.count(b"\n")
> > +                 nl = new.count(b"\n")
> > +                 write_pkt(f"hunk 1 {ol} 1 {nl}")
> > +             write_flush()
> > +             write_pkt("status=success")
> > +             write_flush()
> > +         else:
> > +             write_flush()
> > +             write_pkt("status=error")
> > +             write_flush()
> > +     PYEOF
>
> The existing pattern is to provide larger scripts as fixtures in
> associated `t/tNNNN/` directories, not as heredoc, see e.g.
> `t/t1509/prepare-chroot.sh`. Writing scripts, especially lengthy ones, in
> heredoc strings makes it virtually impossible to use static code analysis
> or syntax highlighting to fend off banal errors.
>
> Given the complexity of what t4080 tries to test (error, abort, crash,
> bad-sync, no-hunks, multiple files in one session, capability
> negotiation), it would unfortunately be infeasible to use `test-tool
> pkt-line` from a shell script implementing that `diff.*.process` protocol.
>
> So I've spiked a demo how the `test-tool diff-process-backend` could look
> like (letting Opus do the menial typing, so that I can enjoy at least part
> of a sunny Sunday outside), which also passes the CI build and test:
> https://github.com/dscho/git/commit/b6e3c93381b00929476c3a00155f7cf7334a22e6
>
> That commit is of course not intended to be used as-is; Feel free to pick
> code parts of it and integrate them into your topic branch. Or write your
> own test-tool helper from scratch if that's more your jam.
>
> Ciao,
> Johannes
>
> > +     write_script diff-process-backend <<-SHEOF
> > +     exec "$PYTHON_PATH" "$TRASH_DIRECTORY/diff-process-backend.py" "\$@"
> > +     SHEOF
> > +}
> > +
> > +BACKEND="./diff-process-backend"
> > +
> > +test_expect_success PYTHON 'setup' '
> > +     setup_backend &&
> > +     echo "*.c diff=cdiff" >.gitattributes &&
> > +     git add .gitattributes &&
> > +
> > +     # boundary.c: 10 lines, changes at 5-6 and 9-10.
> > +     # Used by: hunk boundaries, error fallback, crash, bad hunks, overlap.
> > +     cat >boundary.c <<-\EOF &&
> > +     line1
> > +     line2
> > +     line3
> > +     line4
> > +     OLD5
> > +     OLD6
> > +     line7
> > +     line8
> > +     OLD9
> > +     OLD10
> > +     EOF
> > +     git add boundary.c &&
> > +
> > +     # worddiff.c: single-line function, value changes 1 -> 999.
> > +     # Used by: word-diff, --diff-algorithm, --no-ext-diff, --stat.
> > +     cat >worddiff.c <<-\EOF &&
> > +     int value(void) { return 1; }
> > +     EOF
> > +     git add worddiff.c &&
> > +
> > +     # newfile.c: single-line function, value changes 42 -> 99.
> > +     # Used by: new file, --exit-code, multiple drivers.
> > +     cat >newfile.c <<-\EOF &&
> > +     int new_func(void) { return 42; }
> > +     EOF
> > +     git add newfile.c &&
> > +
> > +     # logtest.c: single-line function for log/format-patch tests.
> > +     # Needs two commits so log -1 has a diff.
> > +     cat >logtest.c <<-\EOF &&
> > +     int logfunc(void) { return 1; }
> > +     EOF
> > +     git add logtest.c &&
> > +
> > +     # two.c/one.c: two-file pair for error/abort/startup-failure tests.
> > +     cat >one.c <<-\EOF &&
> > +     int first(void) { return 1; }
> > +     EOF
> > +     cat >two.c <<-\EOF &&
> > +     int second(void) { return 2; }
> > +     EOF
> > +     git add one.c two.c &&
> > +
> > +     git commit -m "initial" &&
> > +
> > +     # Second commit for logtest.c (so log -1 has something to show).
> > +     cat >logtest.c <<-\EOF &&
> > +     int logfunc(void) { return 2; }
> > +     EOF
> > +     git add logtest.c &&
> > +     git commit -m "change logtest.c" &&
> > +
> > +     # Working tree modifications (not committed).
> > +     cat >boundary.c <<-\EOF &&
> > +     line1
> > +     line2
> > +     line3
> > +     line4
> > +     NEW5
> > +     NEW6
> > +     line7
> > +     line8
> > +     NEW9
> > +     NEW10
> > +     EOF
> > +
> > +     cat >worddiff.c <<-\EOF &&
> > +     int value(void) { return 999; }
> > +     EOF
> > +
> > +     cat >newfile.c <<-\EOF &&
> > +     int new_func(void) { return 99; }
> > +     EOF
> > +
> > +     cat >one.c <<-\EOF &&
> > +     int first(void) { return 10; }
> > +     EOF
> > +
> > +     cat >two.c <<-\EOF
> > +     int second(void) { return 20; }
> > +     EOF
> > +'
> > +
> > +#
> > +# Core behavior: the tool controls which lines are marked as changed.
> > +#
> > +
> > +test_expect_success PYTHON 'diff process hunk boundaries affect output' '
> > +     # The file has changes at lines 5-6 and 9-10, but fixed-hunk
> > +     # only reports lines 5-6 as changed.  Lines 9-10 should not
> > +     # appear as changed in the output.
> > +     git -c diff.cdiff.process="$BACKEND --mode=fixed-hunk" \
> > +             diff boundary.c >actual &&
> > +     test_grep "^-OLD5" actual &&
> > +     test_grep "^-OLD6" actual &&
> > +     test_grep "^+NEW5" actual &&
> > +     test_grep "^+NEW6" actual &&
> > +     test_grep ! "^-OLD9" actual &&
> > +     test_grep ! "^-OLD10" actual &&
> > +     test_grep ! "^+NEW9" actual &&
> > +     test_grep ! "^+NEW10" actual
> > +'
> > +
> > +test_expect_success PYTHON 'diff process works with new file' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff -- newfile.c >actual 2>stderr &&
> > +     test_grep "return 99" actual &&
> > +     test_grep "pathname=newfile.c" backend.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process works with added file (empty old side)' '
> > +     cat >added.c <<-\EOF &&
> > +     int added(void) { return 1; }
> > +     EOF
> > +     git add added.c &&
> > +
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff --cached -- added.c >actual 2>stderr &&
> > +     test_grep "added" actual &&
> > +     test_grep "pathname=added.c" backend.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process skipped for binary files' '
> > +     printf "\\0binary" >binary.c &&
> > +     git add binary.c &&
> > +     git commit -m "add binary" &&
> > +     printf "\\0changed" >binary.c &&
> > +
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff -- binary.c >actual &&
> > +     test_grep "Binary files" actual &&
> > +     test_path_is_missing backend.log
> > +'
> > +
> > +test_expect_success PYTHON 'diff process not consulted for unmatched driver' '
> > +     echo "not tracked by cdiff" >unmatched.txt &&
> > +     git add unmatched.txt &&
> > +     git commit -m "add unmatched.txt" &&
> > +
> > +     echo "modified" >unmatched.txt &&
> > +
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff -- unmatched.txt >actual &&
> > +     test_grep "modified" actual &&
> > +     test_path_is_missing backend.log
> > +'
> > +
> > +test_expect_success PYTHON 'multiple drivers use separate processes' '
> > +     echo "*.h diff=hdiff" >>.gitattributes &&
> > +     git add .gitattributes &&
> > +
> > +     cat >multi.h <<-\EOF &&
> > +     int header(void) { return 1; }
> > +     EOF
> > +     git add multi.h &&
> > +     git commit -m "add multi.h" &&
> > +
> > +     cat >multi.h <<-\EOF &&
> > +     int header(void) { return 2; }
> > +     EOF
> > +
> > +     rm -f backend-c.log backend-h.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend-c.log" \
> > +         -c diff.hdiff.process="$BACKEND --log=backend-h.log" \
> > +             diff -- newfile.c multi.h >actual 2>stderr &&
> > +     test_grep "pathname=newfile.c" backend-c.log &&
> > +     test_grep "pathname=multi.h" backend-h.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process works alongside textconv' '
> > +     write_script uppercase-filter <<-\EOF &&
> > +     tr "a-z" "A-Z" <"$1"
> > +     EOF
> > +
> > +     cat >textconv.c <<-\EOF &&
> > +     hello world
> > +     EOF
> > +     git add textconv.c &&
> > +     git commit -m "add textconv.c" &&
> > +
> > +     cat >textconv.c <<-\EOF &&
> > +     goodbye world
> > +     EOF
> > +
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.textconv="./uppercase-filter" \
> > +         -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff -- textconv.c >actual 2>stderr &&
> > +     # The diff process receives textconv-transformed (uppercase) content.
> > +     test_grep "pathname=textconv.c" backend.log &&
> > +     test_grep "old=HELLO WORLD" backend.log &&
> > +     test_grep "new=GOODBYE WORLD" backend.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +#
> > +# Downstream features: word diff, log, equivalent files, exit code.
> > +#
> > +
> > +test_expect_success PYTHON 'diff process with --word-diff' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff --word-diff worddiff.c >actual 2>stderr &&
> > +     test_grep "\[-1;-\]" actual &&
> > +     test_grep "{+999;+}" actual &&
> > +     test_grep "pathname=worddiff.c" backend.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process works with git log -p' '
> > +     # With no-hunks mode, the tool says the files are equivalent,
> > +     # so log -p should show the commit but no diff content.
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --mode=no-hunks --log=backend.log" \
> > +             log -1 -p -- logtest.c >actual 2>stderr &&
> > +     test_grep "change logtest.c" actual &&
> > +     test_grep ! "return 2" actual &&
> > +     test_grep "command=hunks pathname=logtest.c" backend.log &&
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process no hunks suppresses diff output' '
> > +     cat >nohunks.c <<-\EOF &&
> > +     int zero(void) { return 0; }
> > +     EOF
> > +     git add nohunks.c &&
> > +     git commit -m "add nohunks.c" &&
> > +
> > +     cat >nohunks.c <<-\EOF &&
> > +     int zero(void) { return 999; }
> > +     EOF
> > +
> > +     git -c diff.cdiff.process="$BACKEND --mode=no-hunks" \
> > +             diff nohunks.c >actual &&
> > +     test_must_be_empty actual
> > +'
> > +
> > +test_expect_success PYTHON 'diff process no hunks with --exit-code returns success' '
> > +     git -c diff.cdiff.process="$BACKEND --mode=no-hunks" \
> > +             diff --exit-code nohunks.c
> > +'
> > +
> > +test_expect_success PYTHON 'diff process with --exit-code and hunks returns failure' '
> > +     test_expect_code 1 git -c diff.cdiff.process="$BACKEND" \
> > +             diff --exit-code newfile.c
> > +'
> > +
> > +#
> > +# Bypass mechanisms: flags and commands that skip the diff process.
> > +#
> > +
> > +test_expect_success PYTHON 'diff process bypassed by --diff-algorithm' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff --diff-algorithm=patience worddiff.c >actual &&
> > +     test_grep "return 999" actual &&
> > +     test_path_is_missing backend.log
> > +'
> > +
> > +test_expect_success PYTHON 'diff process not used by --stat' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --log=backend.log" \
> > +             diff --stat worddiff.c >actual &&
> > +     test_grep "worddiff.c" actual &&
> > +     test_path_is_missing backend.log
> > +'
> > +
> > +#
> > +# Error handling and fallback.
> > +#
> > +
> > +test_expect_success PYTHON 'diff process fallback on tool error status' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --mode=error --log=backend.log" \
> > +             diff boundary.c >actual 2>stderr &&
> > +     # Fallback produces the full builtin diff (both change regions).
> > +     test_grep "^-OLD5" actual &&
> > +     test_grep "^+NEW5" actual &&
> > +     test_grep "^-OLD9" actual &&
> > +     test_grep "^+NEW9" actual &&
> > +     # Tool was contacted (it replied with error, not crash).
> > +     test_grep "command=hunks pathname=boundary.c" backend.log &&
> > +     test_grep "diff process.*failed" stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process error keeps tool available for next file' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --mode=error --log=backend.log" \
> > +             diff -- one.c two.c >actual 2>stderr &&
> > +     # Unlike abort, error keeps the tool available: both files
> > +     # are sent to the tool (and both fall back).
> > +     test_grep "pathname=one.c" backend.log &&
> > +     test_grep "pathname=two.c" backend.log &&
> > +     test_grep "return 10" actual &&
> > +     test_grep "return 20" actual
> > +'
> > +
> > +test_expect_success PYTHON 'diff process abort disables for session' '
> > +     rm -f backend.log &&
> > +     git -c diff.cdiff.process="$BACKEND --mode=abort --log=backend.log" \
> > +             diff -- one.c two.c >actual &&
> > +     # Both files should still produce diff output via fallback.
> > +     test_grep "return 10" actual &&
> > +     test_grep "return 20" actual &&
> > +     # The tool aborts on the first file and git clears its
> > +     # capability.  The second file never contacts the tool.
> > +     test_grep "pathname=one.c" backend.log &&
> > +     test_grep ! "pathname=two.c" backend.log
> > +'
> > +
> > +test_expect_success PYTHON 'diff process fallback on tool crash' '
> > +     git -c diff.cdiff.process="$BACKEND --mode=crash" \
> > +             diff boundary.c >actual 2>stderr &&
> > +     test_grep "^-OLD5" actual &&
> > +     test_grep "^+NEW5" actual &&
> > +     test_grep "^-OLD9" actual &&
> > +     test_grep "^+NEW9" actual &&
> > +     # Crash is a communication failure, so a warning is emitted.
> > +     test_grep "diff process.*failed" stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process startup failure only warns once' '
> > +     git -c diff.cdiff.process="/nonexistent/tool" \
> > +             diff -- one.c two.c >actual 2>stderr &&
> > +     # Both files produce diff output via fallback.
> > +     test_grep "return 10" actual &&
> > +     test_grep "return 20" actual &&
> > +     # Sentinel prevents repeated warnings: only one, not one per file.
> > +     test_grep "diff process.*failed" stderr >warnings &&
> > +     test_line_count = 1 warnings
> > +'
> > +
> > +test_expect_success PYTHON 'diff process fallback on bad hunks' '
> > +     git -c diff.cdiff.process="$BACKEND --mode=bad-hunk" \
> > +             diff boundary.c >actual 2>stderr &&
> > +     test_grep "^-OLD5" actual &&
> > +     test_grep "^+NEW5" actual &&
> > +     test_grep "^-OLD9" actual &&
> > +     test_grep "^+NEW9" actual &&
> > +     # Invalid hunks are caught by xdiff validation, not the
> > +     # protocol layer, so no warning is emitted.
> > +     test_must_be_empty stderr
> > +'
> > +
> > +test_expect_success PYTHON 'diff process fallback on mismatched unchanged totals' '
> > +     cat >synctest.c <<-\EOF &&
> > +     line1
> > +     line2
> > +     line3
> > +     EOF
> > +     git add synctest.c &&
> > +     git commit -m "add synctest.c" &&
> > +
> > +     cat >synctest.c <<-\EOF &&
> > +     line1
> > +     changed
> > +     line3
> > +     EOF
> > +
> > +     # bad-sync reports hunk 1 2 1 1: marks 2 old lines and 1 new
> > +     # line as changed, leaving 1 unchanged old vs 2 unchanged new.
> > +     # The synchronization invariant fails and git falls back.
> > +     git -c diff.cdiff.process="$BACKEND --mode=bad-sync" \
> > +             diff synctest.c >actual 2>stderr &&
> > +     test_grep "changed" actual
> > +'
> > +
> > +test_expect_success PYTHON 'diff process fallback on overlapping hunks' '
> > +     # boundary.c has 10 lines, so both hunks are in bounds
> > +     # but they overlap at lines 3-5, triggering the ordering check.
> > +     git -c diff.cdiff.process="$BACKEND --mode=overlap" \
> > +             diff boundary.c >actual 2>stderr &&
> > +     test_grep "NEW5" actual
> > +'
> > +
> > +test_done
> > diff --git a/userdiff.h b/userdiff.h
> > index 51c26e0d41..a98eabe377 100644
> > --- a/userdiff.h
> > +++ b/userdiff.h
> > @@ -3,6 +3,7 @@
> >
> >  #include "notes-cache.h"
> >
> > +struct diff_subprocess;
> >  struct index_state;
> >  struct repository;
> >
> > @@ -33,6 +34,8 @@ struct userdiff_driver {
> >       int textconv_want_cache;
> >       const char *process;
> >       char *process_owned;
> > +     struct diff_subprocess *diff_subprocess;
> > +     unsigned diff_process_failed : 1;
> >  };
> >  enum userdiff_driver_type {
> >       USERDIFF_DRIVER_TYPE_BUILTIN = 1<<0,
> > --
> > gitgitgadget
> >
> >
> >