fix(pure-magic): relax try_csv to match libmagic semantics

[tnaroska]

tl;dr: this is to better match the CSV recognition in libmagic. Specific problem I observed with CSV files is that pure-magic didn't recognize small csv files (few lines) and reported text/plain instead.

Apparently pure-magic required at least 10 lines of csv to identify the file type, whereas the check in libmagic matches csv with a minimum of two lines (1 header, and 1 data).

Summary

try_csv (pure-magic/src/lib.rs) required exactly 10 records before reporting text/csv, so typical small CSVs (configs, fixtures, short exports) were silently classified as plain text. This PR relaxes the threshold to match upstream libmagic and adds regression tests.

Reproducer (before this PR)

$ cargo build --release -p wiza
$ printf 'a,b\n1,2\n3,4\n' > /tmp/t.csv
$ printf 'a,b,c\n1,2,3\n4,5,6\n7,8,9\n10,11,12\n' > /tmp/short.csv
$ ./target/release/wiza /tmp/t.csv /tmp/short.csv
/tmp/t.csv     source:hardcoded strength:0 mime:text/plain magic:ASCII text
/tmp/short.csv source:hardcoded strength:0 mime:text/plain magic:ASCII text

$ file /tmp/t.csv /tmp/short.csv
/tmp/t.csv:     CSV text
/tmp/short.csv: CSV text

After this PR both files report mime:text/csv.

Why the old threshold was wrong

Upstream file/file/src/is_csv.c::csv_parse accepts any input where tf > 1 && nl >= 2 (≥2 lines with consistent column count, ≥2 fields per row). CSV_LINES = 10 is only an early-exit cap in the upstream parser, not a minimum. The 10-record floor in try_csv produced false negatives that diverge from file(1).

Upstream history confirms the policy: in 2023, file(1) explicitly loosened its detector with PR/463 (b4e621d1, "CSV can be also only 2 lines") — so even the conservative reference treats 10 as too strict.

What changed

• Threshold: n != 10 → n < 2. Reads records until EOF and accepts any input with ≥2 records and consistent column count, mirroring tf > 1 && nl >= 2 in is_csv.c.
• Header inference: switched from csv::Reader::from_reader(...) (which treats the first line as a header by default) to csv::ReaderBuilder::new().has_headers(false).from_reader(...). libmagic counts newlines, not data rows, so a 2-line a,b\n1,2\n must qualify; without this change, csv::Reader consumed a,b as a header and only saw one data record, leaving n = 1.
• Regression tests in the existing #[cfg(test)] mod tests block, using a tiny csv_magic() helper that reuses first_magic with a never-matching rule so only the hardcoded CSV detector is exercised:
  • test_csv_two_rows_two_cols — minimal 2-row CSV (the previously-failing case).
  • test_csv_short_consistent_rows — 5-row CSV.
  • test_csv_many_rows_still_detected — 12-row CSV (preserves the previously-passing case).
  • test_csv_single_field_rejected — tf > 1 boundary; multi-line single-column text must NOT be detected as CSV.
  • test_csv_ragged_columns_rejected — column-count consistency check.

Non-goals

• Semicolon/tab/pipe separators. Upstream file(1) only auto-detects comma; this PR keeps that behavior.

Test plan

• cargo test -p pure-magic — 139 passed (134 existing + 5 new), 9 doctests pass.
• cargo clippy -p pure-magic --no-deps — clean.
• Manual wiza parity check on 2-row, 5-row, 12-row, and ragged CSVs against system file(1).
• Maintainer review.

🤖 Generated with Claude Code

[tnaroska]

A few annotations on the change locations.

[tnaroska]

has_headers(false) is the load-bearing bit for two-line CSVs. With the default true, csv::Reader consumes a,b as a header and records() then yields only one data row from a,b\n1,2\n — n stays at 1 and the n < 2 reject fires. libmagic counts newlines, not data rows, so we need every line in the count.

[tnaroska]

Dropped .take(9) so we read until EOF / FILE_BYTES_MAX. Loop exits early on the first ragged row or parse error, so the cost is bounded by the buffer the haystack already provides.

[qjerome]

If you remove .take up to 7MB (i.e. FILE_BYTES_MAX) of CSV can be parsed for nothing, as we only care about the 2 first lines. This needs to be adjusted not to impact perfs on edge cases.

[tnaroska]

n < 2 matches upstream is_csv.c: return tf > 1 && nl >= 2. The first.len() <= 1 check just above is the tf > 1 half (≥2 fields per row), and this is the nl >= 2 half (≥2 records).

[qjerome]

I don't see the code equivalent to tf > 1

[qjerome]

This must be adjusted, because you don't parse the header anymore.

[qjerome]

I don't see the code equivalent to tf > 1

[qjerome]

If you remove .take up to 7MB (i.e. FILE_BYTES_MAX) of CSV can be parsed for nothing, as we only care about the 2 first lines. This needs to be adjusted not to impact perfs on edge cases.