fix(core): search every file when a multi-grep pattern has no bigrams

[kevin9327]

The defect

ffmulti_grep (and FilePicker::multi_grep / fff_multi_grep) reports no
matches at all for one of its patterns when another pattern in the same call
is indexable and the first one is not.

literal_candidates ORs a candidate bitset per pattern — "a file is a candidate
when it contains the bigrams of ANY pattern", as its own doc comment says — but
a pattern the index cannot prefilter (query returns None) is silently dropped from that union:

for pattern in patterns {
    if let Some(candidates) = index.query(pattern.as_bytes()) {
        combined = Some(/* OR into the accumulator */);
    }
    // no else: the pattern contributes nothing
}

BigramFilter::query returns None for "this pattern cannot be prefiltered —
scan everything". It does so when the pattern is shorter than 2 bytes, and when
none of its bigrams has a column, which query/query_skip both gate on
(32..=126).contains(&b):

if (32..=126).contains(&prev) && (32..=126).contains(&b) {

So any needle without a printable-ASCII bigram pair — a CJK, Cyrillic or Greek
string, an emoji — returns None. Dropping it from an OR is the opposite of
conservative: the union with "every file" is every file, but the result is the
union of the other patterns only. Every file that matches only the dropped
pattern is never opened, so the search reports zero hits in it, silently, until
the index is rebuilt.

grep_search's single-pattern call site gets this right by accident — with one
pattern combined stays None and the combined? below falls through to "no
prefilter". Only the multi-pattern path has somewhere to fall back to, and it
takes the wrong one.

The fix

Propagate the None instead of skipping the pattern, matching what the single
pattern path already does. Behaviour is unchanged whenever every pattern is
indexable, and unchanged for a single pattern.

Verification (Windows, default ripgrep features)

RUSTUP_TOOLCHAIN was pinned to 1.98.0-x86_64-pc-windows-msvc because
rust-toolchain.toml's stable channel could not update on this machine.

New test multi_grep_keeps_files_for_a_pattern_without_bigrams builds a bigram
index over six files and runs multi_grep(["unicorn", "日本語"]). Only e.txt
holds the CJK needle, and it holds no ASCII needle.

Before, cargo test -p fff-search --lib -- multi_grep_keeps_files:

test grep::grep_tests::multi_grep_keeps_files_for_a_pattern_without_bigrams ... FAILED

---- grep::grep_tests::multi_grep_keeps_files_for_a_pattern_without_bigrams stdout ----

thread 'grep::grep_tests::multi_grep_keeps_files_for_a_pattern_without_bigrams' (30068) panicked at crates\fff-core\src\grep\grep_tests.rs:582:5:
assertion `left == right` failed: the CJK needle has no usable bigram, so its file must still be searched
  left: ["a.txt", "b.txt", "c.txt"]
 right: ["a.txt", "b.txt", "c.txt", "e.txt"]

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 162 filtered out

After:

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 161 filtered out

The same test pins that the prefilter still narrows, so this is not a widening —
both assertions pass before and after:

• multi_grep(["unicorn", "rainbow"]) — every pattern indexable — still returns
  exactly a/b/c + f.txt, with d.txt and e.txt filtered out.
• multi_grep(["unicorn"]) still returns exactly a/b/c.

test_multi_grep_search is unchanged and still passes.

• cargo test -p fff-search --lib — 163 passed, 0 failed (baseline on unmodified
  upstream: 162 passed, 0 failed — this change adds the one new test and no
  failures).
• cargo fmt --all -- --check — clean.
• cargo clippy -p fff-search --lib — 3 warnings, identical to unmodified
  upstream (delta 0).

CI note

Test (windows-latest) is red as cancelled, not failed — the job has no
failing step. Every other platform passes here, and the same job passed on
Windows in 10m22s on #870, which branches from the same commit.

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• crates/fff-core/src/grep/grep_tests.rs
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---

📝 Walkthrough

Walkthrough

When a pattern cannot be narrowed by the index, literal_candidates now returns None instead of combining candidates from other patterns. The grep test uses production compression settings and checks search counts for three pattern sets.

Changes

Candidate matching

Layer / File(s)	Summary
Preserve un-narrowable pattern matches crates/fff-core/src/index/candidates.rs, crates/fff-core/src/index/bigram_filter.rs, crates/fff-core/src/grep/grep_tests.rs	literal_candidates now returns None when any pattern cannot be narrowed. SKIP_INDEX_MIN_DENSITY_PCT is crate-visible and retains its value of 12. The grep test uses production compression settings and checks search counts and matched paths.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: dmtrkovalenko

Merge Risk: ⚪ Minimal · up to b9413

The change preserves matches for patterns the index cannot prefilter, with no identified issue blocking merge after normal checks.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main fix: scanning every file when a multi-pattern search includes a pattern without usable bigrams.

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🛠️ Fix failing CI checks 💡

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[greptile-apps]

Confidence Score: 4/5

The functional fix appears correct, but the repository’s explicit comment requirements must be satisfied before merging.

Candidate fallback behavior is consistent with callers and prevents the reported false negatives; remaining concerns are the rule-violating test documentation and ineffective narrowing assertions.

Files Needing Attention: crates/fff-core/src/grep/grep_tests.rs

Important Files Changed

Filename	Overview
crates/fff-core/src/index/candidates.rs	Correctly propagates an unindexable pattern as a request to bypass candidate prefiltering.
crates/fff-core/src/grep/grep_tests.rs	Adds effective coverage for the false-negative defect, but violates comment rules and includes assertions that cannot observe narrowing.

_{Reviews (1): Last reviewed commit: "fix(core): search every file when a mult..." | Re-trigger Greptile}

[greptile-apps]

Private Function Documentation

This four-line /// comment documents a private test function. The repository requires private functions to have no documentation comments and limits comments to two lines. This requirement must be satisfied before merging.

Suggested change

	/// Candidate bitsets are OR-ed across the patterns of a multi-pattern grep. A
	/// pattern the bigram index cannot prefilter (no printable-ASCII bigram, e.g. a
	/// CJK needle) matches any file, so leaving it out of the union hides every file
	/// that only it matches.

Context Used: CLAUDE.md (source)

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[kevin9327]

Removed in b9413a9.

[greptile-apps]

Narrowing Remains Untested

These assertions cannot verify that the prefilter still narrows because result.files contains only files with actual matches. They pass even if every file is scanned. The fixture also uses compress(Some(0)), which keeps the single-file rainbow bigrams that production's default compression drops. Assert candidate selection directly or add observable scan instrumentation with production compression settings so a narrowing regression fails this test.

[kevin9327]

Done in b9413a9: the index is compressed with the production settings (compress(None), and SKIP_INDEX_MIN_DENSITY_PCT for the skip index), and the test asserts total_files_searched: 6 with the CJK needle, 4 for unicorn + rainbow, 3 for unicorn alone. On the base commit it fails at the first assertion with only a.txt, b.txt and c.txt found.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@crates/fff-core/src/grep/grep_tests.rs`:
- Around line 522-525: Remove the four-line doc-comment preceding the
multi-pattern grep test; keep the private test function unchanged and do not
replace it with another comment unless a single short regular comment is
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[dmtrKovalenko]

pattern the index cannot answer

what do you mean by this?

[kevin9327]

A pattern for which BigramFilter::query returns None. That happens when the pattern is shorter than two bytes, or when none of its bigrams has a column in the index: only byte pairs in 32..=126 get columns, so a CJK, Cyrillic or emoji needle has none. For such a pattern the index cannot say which files may contain it, so the only correct prefilter result is "scan every file". literal_candidates instead skipped it while OR-ing the per-pattern candidate sets, so a multi-pattern grep only opened the other patterns' candidates and never opened a file that matches only that needle.

I have reworded the description to say "cannot prefilter" instead of "cannot answer". The test now builds the index with the production compression settings and asserts total_files_searched as well: all 6 files with the CJK needle, 4 for unicorn + rainbow, 3 for unicorn alone. On the base commit it fails with only a.txt, b.txt, c.txt found.

[kevin9327]

The red e2e (ubuntu-latest) check looks like a flake rather than something this change causes. It's programmatic_search_spec.lua:228 ("cwd switch did not surface match from the new root"), the content_search-after-cwd-switch test whose comment already notes it "flaked on linux too". The same run passed on macOS, Windows and alpine-musl.

I ran it locally on Linux (nvim 0.10.4, same build flags as CI):

• PR head b9413a9: full make test-lua passes, and programmatic_search_spec.lua alone passes 20/20.
• PR merged into current main (89c1927): full make test-lua passes (49/49).
• main (89c1927): full suite passes, and the spec passes 10/10.

The change can only widen the prefilter: when a pattern can't be narrowed, literal_candidates now returns None (search every file) instead of dropping that pattern. For a single literal like this test's marker, the behavior is the same as before. Could you rerun the e2e job when you get a chance?