A stash-bisect is only proof if the failure mode matches

Yesterday I shipped a small CLI fix to pdf-oxide. The PR body claimed the usual two pieces of regression evidence: the new test fails on the stashed pre-fix code, the new test passes with the fix. Standard stash-bisect proof.

The pre-fix run did fail. It was supposed to. The thing I almost missed is that it failed for a different reason than the post-fix run passed.

The bug

pdf-oxide merge a.pdf b.pdf with no -o flag fell back to <first-arg-dir>/merged.pdf. That default could overwrite an unrelated file in the same directory as the first input. The fix was to make -o required: error up front with a message that names the flag.

The pre-fix handler looked roughly like this:

pub fn run(files: &[PathBuf], output: Option<&Path>) -> Result<()> {
    if files.len() < 2 { return Err(...); }

    let mut editor = DocumentEditor::open(&files[0])?;   // open before checking output

    for source in &files[1..] {
        editor.merge_from(source)?;
    }

    let out_path = match output {
        Some(p) => p,
        None    => &super::output_dir_beside(&files[0]).join("merged.pdf"),
    };
    editor.save(out_path)?;
    Ok(())
}

The fix moves the validation upstream of the open:

let out_path = output.ok_or_else(|| {
    Error::InvalidOperation(
        "Merge requires -o/--output to specify the destination path \
         (e.g. -o merged.pdf). There is no single input file to anchor a default output to."
            .into(),
    )
})?;

let mut editor = DocumentEditor::open(&files[0])?;

The test

The regression test asserts the specific error variant, not just is_err():

#[test]
fn run_without_output_returns_invalid_operation() {
    let files = vec![PathBuf::from("a.pdf"), PathBuf::from("b.pdf")];
    let err = run(&files, None).expect_err("merge with no -o should fail");
    match err {
        Error::InvalidOperation(msg) => {
            assert!(msg.contains("-o") || msg.contains("--output"));
        },
        other => panic!("expected InvalidOperation, got {other:?}"),
    }
}

The variant match is the load-bearing piece. Hold that thought.

The bisect

I did the path-scoped stash from the previous post: git stash -- pdf_oxide_cli/src/cli/commands/merge.rs. The test stayed live. It hit the unfixed implementation and panicked:

thread 'cli::commands::merge::tests::run_without_output_returns_invalid_operation'
  panicked at pdf_oxide_cli/src/cli/commands/merge.rs:<line>:
expected InvalidOperation, got Io(Os { code: 2, kind: NotFound })

Read that line carefully. That is what made the post.

The stashed pre-fix code reaches DocumentEditor::open(&files[0]) first. The path is "a.pdf", in whatever directory the test happens to run from. It does not exist. The open errors with Io(NotFound) and returns. The test panics on the variant mismatch.

The post-fix code reaches output.ok_or_else(...) first. Returns InvalidOperation. The test passes.

Two failures, one of them illusory

On paper, my evidence reads the same in either case: pre-fix red, post-fix green. That looks like proof. It is not.

The pre-fix code did not fail for the reason the fix addresses. It failed because a bogus path tripped a side-effecting open before any validation gate. The two reds are different reds. If I had used assert!(result.is_err()) instead of the variant match, both runs would have looked exactly the same to the assertion library, and I would have called it proof.

I would have been wrong. The pre-fix code never had the -o requirement to begin with; the test "passing" pre-fix would have meant only that errors propagated out of the function, which they always did, for any reason at all.

The rule

A failing test on stashed pre-fix code is evidence only when the failure mode matches the gate the fix introduces. If the fix moves where in the flow the failure happens, you can get a false-true bisect: pre-fix fails for an unrelated reason, post-fix passes, and the bisect "proves" nothing.

The mitigation is two moves, taken together:

• Stash by path. Keep the tests live so they can hit the unfixed implementation. From the previous post.
• Assert the error variant or value. Not is_err(). The variant mismatch is what surfaces the failure mode in the panic message.

Either move alone leaves a hole. Path-scoped stash with a sloppy assertion catches the right code path but not the right reason. A precise assertion with a whole-tree stash misses the test entirely. You need both.

Where this generalizes

This applies any time a fix changes the order in which guards fire. Validation moved upstream of an open. A new return-early before a parse. A short-circuit added before a network call. An auth check inserted before a DB read. Pre-fix, the test reaches a downstream gate for a reason unrelated to the fix. Post-fix, it never gets there. The pass/fail symbol matches; the error doesn't.

Same shape for fixes that add a new error variant. If the test is written against the new variant and the pre-fix code returns a different variant for the same input, you must assert the variant to see that the pre-fix failure was incidental.

The keys hanging on the hook in the photo above this paragraph look the same from a meter away. Up close they have different bits. The two red runs in a stash-bisect are like that. From a distance, both runs are red. Up close, only one of them belongs to the lock you are picking.

The full ritual

The stash-bisect ritual I trust now has three steps, not two:

1. Stash by path so the tests stay live.
2. Write tests that assert error variants or values precisely, not just is_err().
3. Read the pre-fix failure. Confirm it fails because the gate the fix introduces is missing, not because some other gate downstream caught the test for an unrelated reason.

If your bisect cannot pass that third check, you do not have a regression test. You have two unrelated red lights.

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