Description

When invoking dynamic completion on any fileset command line argument, the current implementation assumes that the file path that is being completed is a prefix of all the candidates found through jj file list. For instance, in the jj repo, typing CHANGELOG<TAB>, will produce CHANGELOG.md as the only matching path, and CHANGELOG.md indeed starts with CHANGELOG.

However, this assumption breaks down once the fileset argument is not normalized. For example, typing ./CHANGELOG<TAB> does not produce any completion candidates because jj file list resolves the glob to the normalized CHANGELOG.md which does not start with ./CHANGELOG and is discarded.

Worse still, when the non-normalized fileset is longer than the completed path it resolves to, it produces a panic due to the unchecked slicing in this function:

A more thorough solution would probably need to use something like the normalize-path crate. In contrast, std::fs::canonicalize() requires files to exist to resolve symlinks which precludes it from being used on revisions other than @.

Steps to Reproduce the Problem

Taking the jj repo as an example

1. jj file show ./CHANGELOG<TAB> → no completion results
2. jj file show ./CHANGELOG.md<TAB> panics

thread 'main' panicked at cli/src/complete.rs:629:9:
byte index 14 is out of bounds of `CHANGELOG.md`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

3. RUST_BACKTRACE=1 jj file show cli/../CHANG<TAB> panics

thread 'main' panicked at cli/src/complete.rs:629:9:
byte index 13 is out of bounds of `CHANGELOG.md`
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::str::slice_error_fail_rt
   3: core::str::slice_error_fail
   4: jj_cli::complete::dir_prefix_from
   5: <itertools::adaptors::coalesce::CoalesceBy<I,F,C> as core::iter::traits::iterator::Iterator>::next
   6: jj_cli::complete::all_files_from_rev
   7: <F as clap_complete::engine::custom::ValueCompleter>::complete
   8: clap_complete::engine::complete::complete_arg_value
   9: clap_complete::engine::complete::complete_arg
  10: clap_complete::engine::complete::complete
  11: <clap_complete::env::shells::Fish as clap_complete::env::EnvCompleter>::write_complete
  12: jj_cli::cli_util::CliRunner::run
  13: jj::main

Expected Behavior

Ideally, triggering completion on non-normalized paths should produce candidates which preserve the typed path, i.e. completing to ./CHANGELOG.md in the first two cases and to cli/../CHANGELOG.md in the third.

Alternatively, I guess it would be acceptable to use the path as returned by jj file list which would replace the argument with CHANGELOG.md in all three cases.

Specifications

• Platform: reproduced on macOS & Linux; probably Windows affected, too. (Possibly, Windows accepting both / and \ as path separators may introduce another failure mode, but I cannot check at the moment.)
• Version: reproduced with jj 0.30.0 and jj 0.28.2.