complete the migration from thiserror to gix-error

[Sebastian Thiel (Byron)]

Tasks

This section is for Byron only. Models continuing this PR must not add, remove, check, uncheck, rename, or reorder checkboxes here.

• refackiew

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Everything below this line was generated by Codex GPT-5.

Created by Codex on behalf of Byron. Byron will review before this is ready to merge.

Reported issue

$issue-full-auto etc/plan/gix-error.md lays out a plan to replace thiserror with gix-error. For each crate to replace thiserror in, also check if any of its variants is matched on. If so, hand-expand to the code that thiserror would produce and remove it. Otherwise, use gix-error in its place. Each commit should pass cargo check --workspace --all-targets.

Finally, gix (crate) should be able to use gix-error::Error via gix::Error exclusively and mostly use ?. Note that in gix there is also utilities to see if certain errors can be retried - this functionality should be put into gix-error, probably directly on gix-error::Error.

The gitoxide-core and gitoxide crates should keep anyhow, and that should work natively with gix-error - probably gix will have to forward the gix-error/anyhow feature to achieve that as well.

RetryableError should only be used when the error otherwise is too specific. If gix-error can inspect an error chain with well-known errors, it should do that. Keep an eye out for other standard classifications such as ValidationError; NotFound should be a well-known gix-error type. Object-kind mismatch can be a ValidationError.

Assuming all plumbing crates have already been processed so only thiserror in gix is left: avoid hand-expanded pattern-matched enums when gix::Error classification or a source-chain search for well-known plumbing errors works instead.

Refs #2351

Summary

• removes direct thiserror use from workspace crates and exposes top-level failures through gix::Error
• adds standard retry, corruption, not-found, and validation classifications to gix-error
• determines retryability from known source-chain errors, retaining the explicit retry wrapper only at dependency-specific boundaries
• preserves concrete source chains, classifications, and probable causes when errors are converted and raised again
• keeps anyhow in the binaries and forwards the gix-error/anyhow feature through gix

Validation

• cargo check --workspace --all-targets
• cargo test -p gix-error
• cargo test -p gix-error --features auto-chain-error --test auto-chain-error
• cargo test -p gix --test gix revision::spec::
• remote, clone, credential-helper, shallow-clone, and blocking/async network feature tests during the migration
• one Codex commit review per final commit hash

[Amey Pawar (ameyypawar)]

Did the comparison. Three things, plus one offer.

1. fetch::Error::Negotiate truncates the chain. Its source() returns None, and unlike the Http arm — which iterates the Exn's frames directly — Negotiate falls through to gix_error::can_retry(self), which walks source(). So anything retryable below a negotiate failure is unreachable. It's public-API surface rather than a live bug: nothing in-tree calls fetch::Error::can_retry today, and receive_pack already remaps Negotiate into a CorruptionError. Worth noting client::Error::source() returns None for Http and SshInvocation too — the frame-iterating arm compensates can_retry specifically, but anything else walking source() would hit the same wall.

2. is_not_found() matches any io::ErrorKind::NotFound anywhere in the chain, and it's the discriminator at Submodule::open(), Repository::head() and head_tree_id_or_empty(). I couldn't produce a concrete path where that misfires — unborn heads raise a marked NotFoundError, missing-object errors are unmarked and propagate correctly — but the predicate is broader than the question being asked at each site, and a marker planted deeper later would change behaviour silently. Worth a second look rather than a bug report.

3. from_error on something already a gix_error::Error flattens its chain. Exn::new degenerates every source below the top to strings. It compiles and the suite passes, because the classifiers downcast to crate::Error and recurse. Live instances exist — e.g. self.head().map_err(gix_error::Error::from_error) in gix/src/repository/index.rs, where head() already returns gix_error::Error.

I've written a guard for that: from_error returns the value unchanged when it's already an Error, plus a #[track_caller] debug_assert naming the caller. Silent in release, loud in tests, no unsafe. Happy to send it as its own small PR — it protects any future conversion.

Also: in traversal_names_do_not_escape_the_modules_directory, the three erased-API assertions (git_dir_try_old_form, open, state) went from matching ParentComponent to is_validation(). The first assertion on sm.git_dir() still checks the specific error, so the test isn't toothless — but those three no longer distinguish a traversal rejection from any other validation failure.

On the comparison: I audited every source() arm in #2716 — 541 across 168 hand-written impls — against the derives they replaced. All match. And the guard's assert never fired across cargo test -p gix (416 tests), so no double-wrap on any tested path. That's a runtime check over tested paths, not a static proof.

There's more from the sweep — a per-type verdict on all 42 types #2716 left concrete, an erasure order for the E0119 chains, and a list of dropped #[error] messages. Say the word if any of that is useful.

[Sebastian Thiel (Byron)]

Thanks Amey Pawar (@ameyypawar), while noting that I find no pleasure in reading these AI generated blobs of text.

My main gripe is that it's a bot speaking through you, so unless you say you produced this text by hand or think you could produce it, disclosure is the way to go. I recommend adding a few lines of yourself on top giving me your verdict, no matter what it is (i.e. something like "this looks reasonable to me, and I spot-checked one of these claims"), followed, by a separator to clearly mark the AI blob.

Thanks again.

[Amey Pawar (ameyypawar)]

Re-checked after your push. Negotiate and client::Error look right, and the repository/index.rs double-wrap is gone.

Four things I'd still look at. The message loss bothers me most; on the security tests I'd rather have your call than mine.

The sweep and this write-up are both AI-produced — I took help of AI tools throughout. I checked the two source arms and the index.rs wrap myself.

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Messages dropped: 53 sites, 50 distinct. Worst: clone/fetch/mod.rs and config/mod.rs+config/tree at 9 each, update_refs/update.rs at 7 (now a bare alias, nothing re-attached), repository/mod.rs at 5.

17 assertions weakened. Two matter: the three erased-API asserts in traversal_names_do_not_escape_the_modules_directory are bare is_validation() (assertion 1 still pins ParentComponent), and remote/connect.rs:14 lost ProtocolDenied { scheme: File }.

is_not_found() matches any raw io NotFound at any depth; is_validation() has no io disjunct. So the looser one guards head_tree_id_or_empty(). Correcting myself from last time — I said I found no misfire, but detached HEAD with a missing object yields the empty tree. Symbolic HEAD is fine.

Dead branches: clone/fetch/mod.rs:253 and update_refs/mod.rs:207 downcast out of err.sources(), which never matches in chain mode. gix defaults to auto-chain-error, binaries build tree mode — so library consumers lose those paths. (gitoxide-core/repository/diff.rs:130 too, but that predates this branch.)

Also: ~31 double-wraps left after the 19 you removed — a floor, counted from monomorphised instantiations rather than grep, so I can pull the list if useful. And 97b7a7cab leaves probable_cause() on the truncated node.

[Sebastian Thiel (Byron)]

Thanks Amey Pawar (@ameyypawar). This is an interesting experiment as you essentially take the role of a reviewer, while my agent double-checks and fixes. And all that without any human review, so I am already very curious on how the actual review can be done efficiently.
Meantime, agents do things through their meat-proxies 😅.