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[markdown] Avoid quadratic trailing-paren scan in AutolinkExtensionSyntax - #2787

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Nov 4, 2026
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[markdown] Avoid quadratic trailing-paren scan in AutolinkExtensionSyntax#2787
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markdown_autolink_trailing_parens

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@jonasfj jonasfj commented Nov 4, 2026 •
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Replace the unanchored RegExp(r'(\(.*)?(\)+)$').firstMatch(text) in
AutolinkExtensionSyntax._getConsumeLength with a single parenthesis-counting
pass and a loop that strips only unmatched trailing ) characters.

When a GFM extended URL autolink ends with ), _getConsumeLength previously
ran:

final match = RegExp(r'(\(.*)?(\)+)$').firstMatch(text)!;

and, when match[1] != null, set excludedLength = parenCount.abs():

  1. Because RegExp(r'(\(.*)?(\)+)$') is unanchored, on a URL containing many
    interior ) characters before a non-) character and a final ) (such as
    'www.a.b/' + ')' * n + 'a)'), firstMatch tried starting at every interior
    ), where (\(.*)? matched empty and (\)+)$ scanned all remaining )
    characters up to a, failed on $, and backtracked in O(n^2) time.
  2. When text contained ( and had more unmatched interior ) characters than
    trailing ) characters (such as www.example.com/a))b(c) or
    ('*www.a.b' + ')' * k) * m + '()'), setting excludedLength = parenCount.abs()
    without bounding it to trailing ) characters sliced non-) characters off
    the end of the autolink, violating the GFM extended autolink path validation
    spec and allowing excluded *www.a.b suffixes to be rescanned.

Count parenCount across text (+1 for (, -1 for )) and then increment
excludedLength while parenCount < 0 and text.codeUnitAt(text.length - 1 - excludedLength) == $rparen. This runs in O(n) time without any regular
expression and only excludes unmatched trailing ) characters.

Behavior change

Trailing ) are now stripped from an extended autolink one at a time, only
while the parentheses in the link are unbalanced, as the GFM path validation
rule and cmark-gfm do. Before, the number of unmatched ) was sliced off the
end of the link regardless of what those characters were, so
www.example.com/a))b(c) lost the c. GFM examples 624–626 render the same
before and after.

Differential testing

Rendered every input with origin/main and with this branch (GFM extension
set, 30 s per-file timeout) and compared the HTML. Sources:

  • pub.dev: 367,926 Markdown files from 93,452 packages
  • Real-world docs: 116,231 files from mdn/content, github/docs,
    kubernetes/website, freeCodeCamp, tldr, flutter/website,
    dart-lang/site-www, microsoft/vscode-docs, withastro/docs,
    rust-lang/book, GitHub READMEs, and Wikipedia
  • Repository READMEs: 491,134 unique READMEs across GitHub, PyPI, HuggingFace,
    and npm
  • Parser test suites: 6,458 cases from cmark, cmark-gfm, commonmark.js,
    pulldown-cmark, md4c, goldmark, markdown-it, comrak, marked,
    pandoc, and pathological generators
  • OSS-Fuzz: 57,716 inputs from the cmark, md4c, and markdown-it-py fuzzers

OSS-Fuzz inputs that sit at the 30 s timeout or the stack limit flip between
runs; every one reported as a new failure was re-run standalone on both trees.

Results:

  • 1 real-world difference, a pub.dev README (scratch_reveal) with a mangled
    nested badge: main cut the last character of the autolinked URL
    (…/scratch_reveal/scor), this branch keeps …/scratch_reveal/score.
  • 1 OSS-Fuzz input changed in the same way (a trailing ( was being cut).
  • No other differences; no new exceptions; 1 stack-overflow message changed
    wording. Cumulative parse time −0.2%.

@jonasfj
[markdown] Avoid quadratic trailing-paren scan in AutolinkExtensionSy…
9e8ed34 
…ntax

Replace the unanchored `RegExp(r'(\(.*)?(\)+)$').firstMatch(text)` in
`AutolinkExtensionSyntax._getConsumeLength` with a single parenthesis-counting
pass and a loop that strips only unmatched trailing `)` characters.

----

When a GFM extended URL autolink ends with `)`, `_getConsumeLength` previously
ran:
```dart
final match = RegExp(r'(\(.*)?(\)+)$').firstMatch(text)!;
```
and, when `match[1] != null`, set `excludedLength = parenCount.abs()`:
1. Because `RegExp(r'(\(.*)?(\)+)$')` is unanchored, on a URL containing many
   interior `)` characters before a non-`)` character and a final `)` (such as
   `'www.a.b/' + ')' * n + 'a)'`), `firstMatch` tried starting at every interior
   `)`, where `(\(.*)?` matched empty and `(\)+)$` scanned all remaining `)`
   characters up to `a`, failed on `$`, and backtracked in O(n^2) time.
2. When `text` contained `(` and had more unmatched interior `)` characters than
   trailing `)` characters (such as `www.example.com/a))b(c)` or
   `('*www.a.b' + ')' * k) * m + '()'`), setting `excludedLength = parenCount.abs()`
   without bounding it to trailing `)` characters sliced non-`)` characters off
   the end of the autolink, violating the GFM extended autolink path validation
   spec and allowing excluded `*www.a.b` suffixes to be rescanned.

Count `parenCount` across `text` (`+1` for `(`, `-1` for `)`) and then increment
`excludedLength` while `parenCount < 0` and `text.codeUnitAt(text.length - 1 -
excludedLength) == $rparen`. This runs in O(n) time without any regular
expression and only excludes unmatched trailing `)` characters.
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Code Review

This pull request updates the extended autolink parsing logic in autolink_extension_syntax.dart to correctly strip trailing unmatched closing parentheses one at a time, adhering to the GFM path validation rule. This replaces the previous regex-based approach and fixes issues where valid characters were incorrectly excluded. A corresponding test case has been added to autolink_extension.unit, and the CHANGELOG.md has been updated. There are no review comments to address, and no additional feedback is provided.

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package:markdown ⭐ 7.4.1-wip WIP (no publish necessary)
  • 16 already published.
  • 26 WIP (no publish necessary).

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
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@jonasfj
jonasfj marked this pull request as ready for review November 4, 2026 13:01
@jonasfj
jonasfj requested a review from srawlins as a code owner November 4, 2026 13:01
@jonasfj
jonasfj requested review from sigurdm and removed request for srawlins November 4, 2026 13:01
@jonasfj
jonasfj merged commit ab135e0 into main Nov 4, 2026
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@jonasfj
jonasfj deleted the markdown_autolink_trailing_parens branch November 4, 2026 13:30
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 5, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, dartdoc, ecosystem, http, native, protobuf, tools, web
cc100c0 
Revisions updated by `dart tools/rev_sdk_deps.dart`.

core (https://github.com/dart-lang/core/compare/24e09c4..36f57a9):
  36f57a99  Tue Nov 3 14:26:17 2026 +0100  Sigurd Meldgaard  [os_detect] Remove bin/os_detect.dart (dart-lang/core#1015)
  86bacfca  Tue Nov 3 05:15:19 2026 -0500  Justin White  [characters] Fix RangeError on an unpaired tail surrogate at range start (dart-lang/core#1033)

dartdoc (https://github.com/dart-lang/dartdoc/compare/db94437..2f6847b):
  2f6847bd  Thu Oct 29 11:47:04 2026 +0200  Sergey G. Grekhov  Add tests for invalid doc comments placement (dart-lang/dartdoc#4301)
  17f2f0a3  Thu Oct 29 10:16:36 2026 +0100  Sarah Zakarias  Compare base elements and unify properties in doc-reference-target-differs (dart-lang/dartdoc#4307)

dartdoc is held back at 2f6847b, before dart-lang/dartdoc#4312, because its
mock SDK version changes conflict in the google3 3-way merge with the hotfix
in cl/1005963642.

ecosystem (https://github.com/dart-lang/ecosystem/compare/f8faabf..54e1673):
  54e1673  Wed Nov 4 09:46:54 2026 +0100  Sigurd Meldgaard  fix(firehose): resolve dartExecutable via package:cli_util (dart-lang/ecosystem#469)
  d754065  Tue Nov 3 10:09:45 2026 -0800  Kevin Moore  refactor(corpus): use path.join and seal closed type hierarchies (dart-lang/ecosystem#476)

http (https://github.com/dart-lang/http/compare/8c0d66a..1f869d0):
  1f869d0  Tue Nov 3 15:51:52 2026 -0800  dependabot[bot]  chore(deps): bump the github-actions group with 8 updates (dart-lang/http#2052)
  6d7f402  Tue Nov 3 14:12:04 2026 -0800  Brian Quinlan  feat(cupertino_http): support custom trust eval (dart-lang/http#2038)
  73f8c36  Tue Nov 3 10:15:16 2026 -0800  Brian Quinlan  chore(cupertino_http): ready 3.1.1 release (dart-lang/http#2056)

native (https://github.com/dart-lang/native/compare/b442038..1838637):
  183863758  Wed Nov 4 09:01:27 2026 -0300  Daco Harkes  [hooks_runner] Add `PKG_CONFIG_PATH` to environment variables allowlist (dart-lang/native#3779)
  c9e8118d4  Wed Nov 4 08:16:20 2026 +1100  Liam Appelbe  [jni] Fix release mode segfault (dart-lang/native#3777)
  5999cf7e4  Wed Nov 4 08:16:16 2026 +1100  Liam Appelbe  [objective_c] Prepare to publish (dart-lang/native#3778)
  d294d9942  Tue Nov 3 15:15:49 2026 +0100  Michael Goderbauer  [hooks_runner] Write recorded_uses.json under directory lock in link() (dart-lang/native#3770)
  33aa5ef04  Tue Nov 3 11:42:38 2026 +1100  Liam Appelbe  [objective_c] Cheaper and safer object/block validity checks (dart-lang/native#3776)

protobuf (https://github.com/dart-lang/protobuf/compare/1d52301..19191c8):
  19191c8  Wed Nov 4 17:14:33 2026 +0100  Sigurd Meldgaard  Use dart.tool.dart2wasm in json.dart so DDC uses json_web.dart (google/protobuf.dart#1108)

tools (https://github.com/dart-lang/tools/compare/1579f8d..a342744):
  a3427446f  Wed Nov 4 15:11:35 2026 +0100  Jonas Finnemann Jensen  Update CODEOWNERS for pkgs/markdown (dart-lang/tools#2795)
  08c9afdc0  Wed Nov 4 14:40:44 2026 +0100  Jonas Finnemann Jensen  [markdown] Check URL boundary before matching in AutolinkExtensionSyntax (dart-lang/tools#2786)
  0fc5edcae  Wed Nov 4 14:35:48 2026 +0100  Jonas Finnemann Jensen  [markdown] Skip unclosed inline HTML tags before matching RegExp (dart-lang/tools#2788)
  b1f6f9956  Wed Nov 4 14:31:55 2026 +0100  Jonas Finnemann Jensen  [markdown] Defer shortcut reference link label extraction in LinkSyntax.close (dart-lang/tools#2785)
  ab135e072  Wed Nov 4 14:30:37 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic trailing-paren scan in AutolinkExtensionSyntax (dart-lang/tools#2787)
  f329bf3ea  Wed Nov 4 17:09:34 2026 +0530  Ananya Kaul  [code_builder] Separate chained cascades after nullable casts (dart-lang/tools#2704)
  0a57e3446  Wed Nov 4 05:28:40 2026 +0530  Aman Maurya  fix(coverage): allow empty coverage files from skipped test suites (`#2667`) (dart-lang/tools#2668)
  e95958f6a  Tue Nov 3 21:53:57 2026 +0000  Keerti Parthasarathy  [unified_analytics] Add root_tool and leaf_tool to every event; key consent on root_tool (dart-lang/tools#2784)
  5da06a508  Tue Nov 3 13:11:04 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject unescaped '(' in parenthesized inline link titles (dart-lang/tools#2770)
  f3a5c9277  Tue Nov 3 13:09:32 2026 +0100  Jonas Finnemann Jensen  [markdown] Cap parenthesis nesting in bare inline link destinations (dart-lang/tools#2769)
  820ce5fb7  Tue Nov 3 13:04:35 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject unescaped '(' in parenthesized link reference titles (dart-lang/tools#2771)
  775c5d080  Tue Nov 3 13:03:43 2026 +0100  Jonas Finnemann Jensen  [markdown] Require balanced parentheses before bare inline link titles (dart-lang/tools#2768)
  5ebd52bc5  Tue Nov 3 13:03:08 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject unescaped '<' in bracketed inline link destinations (dart-lang/tools#2767)
  feb7ea928  Tue Nov 3 13:02:15 2026 +0100  Jonas Finnemann Jensen  [markdown] Fix 999-character limit in LinkParser.parseLabel (dart-lang/tools#2766)
  31888a5da  Tue Nov 3 13:01:43 2026 +0100  Jonas Finnemann Jensen  [markdown] Enforce 999-character limit in _parseReferenceLinkLabel (dart-lang/tools#2765)
  e016c5d70  Tue Nov 3 09:26:32 2026 +0100  dart-ecosystem-bot[bot]  [boring] Pin the prebuilt binaries of boring-binaries-20261101-f869fdca423c (dart-lang/tools#2778)
  8c7935573  Mon Nov 2 10:15:24 2026 -0800  Kevin Moore  [html] Move WPT selector fixtures to third_party/wpt-selectors (dart-lang/tools#2772)

web (https://github.com/dart-lang/web/compare/45166fd..f388bf5):
  f388bf5  Sun Nov 1 06:57:22 2026 +0000  dependabot[bot]  Bump the github-actions group with 4 updates (dart-lang/web#611)
  658ce2f  Sat Oct 31 12:47:20 2026 +0100  Moritz  Migrate to ecosystem health publish check and setup-dart publish workflow (dart-lang/web#610)

Change-Id: Ifcae02c4712b4b27a5a8ab1c57d8dcf13a6700cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/574281
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Nate Bosch <[email protected]>
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