Skip preamble before first multipart boundary

[Kludex]

Summary

• skip CR/LF-prefixed preamble bytes by jumping to the next possible boundary start
• keep the existing boundary validation once a candidate boundary is reached
• avoid spending time byte-by-byte on large leading preambles before the first boundary

Parser behavior comparison

Parser	Bytes before first boundary
Django	Searches for the boundary and discards bytes before it as preamble. See BoundaryIter._find_boundary(): https://github.com/django/django/blob/main/django/http/multipartparser.py#L582-L675
Werkzeug	Uses an explicit PREAMBLE state and regex-searches for the first boundary. See MultipartDecoder: https://github.com/pallets/werkzeug/blob/main/src/werkzeug/sansio/multipart.py#L98-L151
multipart	The core parser rejects bytes before the first boundary unless the boundary is at offset 0 or preceded by CRLF, while the high-level non-strict helper can suppress parse errors. See PushMultipartParser: https://github.com/defnull/multipart/blob/master/multipart.py#L454-L487
python-multipart before	Accepted leading CR/LF before the first boundary, but processed it byte-by-byte.
python-multipart after	Skips ahead to the next possible boundary start, preserving boundary validation from that point.

Benchmark

Parser-only benchmark on macOS arm64, Python 3.14.3, comparing this PR against base commit 4addb60:

Size	Before	After
1 MB	0.087520s	0.000052s
5 MB	0.433339s	0.000109s
10 MB	0.861698s	0.000186s
25 MB	2.167683s	0.000501s
50 MB	4.356995s	0.000903s

Payload shape:

valid = (
    b"--boundary\r\n"
    b"Content-Disposition: form-data; name=\"f\"\r\n"
    b"\r\nval\r\n"
    b"--boundary--"
)
payload = b"\r\n" * (size // 2) + valid

Tests

• uv run pytest tests/test_multipart.py -q -k "newlines_before_first_boundary or bad_start_boundary"
• uv run pytest tests/test_multipart.py -q
• uv run ruff format --check --diff python_multipart multipart tests
• uv run ruff check .

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