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[markdown] Bound the domain of extended URL autolinks to DNS limits - #2759

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[markdown] Bound the domain of extended URL autolinks to DNS limits#2759
jonasfj merged 1 commit into
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markdown_autolink_extension_prefix_check

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@jonasfj jonasfj commented Oct 29, 2026 •
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Bound the domain part of the extended URL autolink pattern in
AutolinkExtensionSyntax to the DNS limits: labels of at most 63 characters and
at most 127 labels.

  • Before: (?:[-_a-z0-9]+\.)*(?:[-a-z0-9]+\.[-a-z0-9]+)
  • After: (?:[-_a-z0-9]{1,63}\.){0,125}(?:[-a-z0-9]{1,63}\.[-a-z0-9]{1,63})

Important

Land after #2757. See the last bullet under Differential testing.

Why this change is necessary

AutolinkExtensionSyntax tries its pattern at every position of a line. The
domain part of the URL alternative scans the whole run of [-_a-z0-9.]
characters after www. (or a scheme) and backtracks over every way of splitting
it into segments before it can fail, because the last two segments must not
contain _. On input such as 'www._a.' * n or 'www._' * n every www.
scans the entire rest of the run, so parsing took O(n^2) time: 56 KB took
14.6 s, and 32 KB of 'xwww._a.' * n took 4.3 s.

How this change works

With labels limited to 63 characters and 127 labels per domain, a single
attempt can inspect at most ~8 KB before it fails, independent of input length,
so the total work is linear. Nothing else changes: no new state, no new regular
expressions, the match semantics for every domain that fits the DNS limits are
identical.

Domains with a label longer than 63 characters or more than 127 labels are no
longer autolinked. Such names cannot exist in DNS; cmark-gfm does not enforce
the limit, so this is a (deliberate) deviation for input that is not a hostname.

Measured (GFM, 56 KB inputs): 'www._a.' * n 14.6 s -> 0.54 s,
'www._' * n 6.8 s -> 0.53 s, 'xwww._a.' * n 17 s -> 0.6 s.

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
  • 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:

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Code Review

This pull request updates AutolinkExtensionSyntax to check for a URL prefix and a valid preceding character before running the autolink regular expression, along with adding corresponding unit tests. The feedback suggests a performance optimization in tryMatch to use integer character codes instead of allocating a new String via String.fromCharCode when checking preceding characters.

Comment thread pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart Outdated
Comment on lines +65 to +70
const validPrecedingChars = {'\n', ' ', '*', '_', '~', '(', '>'};
final validLinkStart =
startMatchPos == 0 ||
validPrecedingChars.contains(
String.fromCharCode(parser.charAt(startMatchPos - 1)),
);

@gemini-code-assist gemini-code-assist Bot Oct 29, 2026

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medium

To avoid allocating a new String via String.fromCharCode at every single character position where tryMatch is evaluated, we can perform the check using integer character codes. Since tryMatch is called frequently during inline parsing, avoiding these allocations can significantly improve parsing performance.

We can use a const set of integer character codes:

  • \n is 10
  • is 32
  • * is 42
  • _ is 95
  • ~ is 126
  • ( is 40
  • > is 62
Suggested change
const validPrecedingChars = {'\n', ' ', '*', '_', '~', '(', '>'};
final validLinkStart =
startMatchPos == 0 ||
validPrecedingChars.contains(
String.fromCharCode(parser.charAt(startMatchPos - 1)),
);
final validLinkStart = startMatchPos == 0 ||
const {10, 32, 42, 95, 126, 40, 62}
.contains(parser.charAt(startMatchPos - 1));

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PR Health

Breaking changes ✔️
Package Change Current Version New Version Needed Version Looking good?
markdown Non-Breaking 7.3.1 7.4.1-wip 7.4.1-wip ✔️

This check can be disabled by tagging the PR with skip-breaking-check.

Coverage ✔️
File Coverage
pkgs/markdown/lib/src/inline_syntaxes/autolink_extension_syntax.dart 💚 98 %

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

License Headers ✔️ 
// 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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pkgs/html/test/differential_test.dart
pkgs/html/test/dom_parsing_test.dart
pkgs/html/test/mxss_test.dart
pkgs/html/test/noah_ark_clause_limit_test.dart

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Unused Dependencies ✔️
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markdown ✔️ All dependencies utilized correctly.

For details on how to fix these, see dependency_validator.

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@jonasfj
jonasfj force-pushed the markdown_autolink_extension_prefix_check branch 3 times, most recently from 03dbb02 to 46be2ad Compare October 30, 2026 10:27
@jonasfj jonasfj changed the title [WIP] [markdown] Check URL prefix and boundary before matching in AutolinkExtensionSyntax [WIP] [markdown] Skip rescanning failed URL autolink candidates in AutolinkExtensionSyntax Oct 30, 2026
@jonasfj jonasfj mentioned this pull request Oct 30, 2026
Draft
@jonasfj
jonasfj force-pushed the markdown_autolink_extension_prefix_check branch from 46be2ad to 5b5bc1c Compare October 30, 2026 11:42
@jonasfj jonasfj changed the title [WIP] [markdown] Skip rescanning failed URL autolink candidates in AutolinkExtensionSyntax [WIP] [markdown] Bound the domain of extended URL autolinks to DNS limits Oct 30, 2026
@jonasfj
jonasfj marked this pull request as ready for review October 30, 2026 11:47
@jonasfj
jonasfj requested a review from srawlins as a code owner October 30, 2026 11:47
@jonasfj jonasfj changed the title [WIP] [markdown] Bound the domain of extended URL autolinks to DNS limits [markdown] Bound the domain of extended URL autolinks to DNS limits Oct 30, 2026
szakarias
szakarias approved these changes Oct 30, 2026
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drop WIP

@jonasfj
jonasfj merged commit 6f38e9f into main Oct 30, 2026
31 of 33 checks passed
@jonasfj
jonasfj deleted the markdown_autolink_extension_prefix_check branch October 30, 2026 11:56
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 3, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, ecosystem, http, i18n, shelf, test, tools
c48ebb8 
Revisions updated by `dart tools/rev_sdk_deps.dart`.

core (https://github.com/dart-lang/core/compare/d7bd45c..24e09c4):
  24e09c4c  Sun Nov 1 21:12:19 2026 +0000  dependabot[bot]  Bump the github-actions group with 4 updates (dart-lang/core#1037)
  87ac1b4c  Fri Oct 30 16:02:43 2026 +0100  Michael Thomsen  [os_detect] Deprecate package:os_detect in favor of package:platform (dart-lang/core#1003)
  8debf335  Fri Oct 30 01:57:36 2026 +0530  Sahaja Pallapothula  Clarify Characters equality and Unicode normalization (dart-lang/core#1031)

ecosystem (https://github.com/dart-lang/ecosystem/compare/24b9ba8..f8faabf):
  f8faabf  Sun Nov 1 18:16:20 2026 +0000  dependabot[bot]  Bump the github-actions group with 2 updates (dart-lang/ecosystem#481)
  27c2fc8  Sat Oct 31 12:36:53 2026 +0100  Moritz  Fix publish workflow pins, add tag_prefix to health, and update docs (dart-lang/ecosystem#480)
  05641a9  Sat Oct 31 09:56:42 2026 +0100  Moritz  Restore deprecated publish.yaml workflow and firehose CLI entrypoint (dart-lang/ecosystem#479)
  78abf72  Fri Oct 30 08:24:07 2026 -0700  Kevin Moore  fix(repo_manage): use pub_semver for changelog -wip transitions (dart-lang/ecosystem#475)
  0bfa654  Fri Oct 30 09:33:57 2026 +0100  Moritz  Move publish check to health workflow and use individual package publishing workflows from `setup-dart` (dart-lang/ecosystem#454)
  adf7979  Thu Oct 29 23:46:49 2026 -0700  Kevin Moore  test(firehose): use hasLength, containsPair, and isFalse (dart-lang/ecosystem#477)

http (https://github.com/dart-lang/http/compare/c76dfb3..8c0d66a):
  8c0d66a  Fri Oct 30 09:54:41 2026 -0700  Brian Quinlan  chore(cupertino_http): check ffigen generation (dart-lang/http#2044)
  c8737d1  Fri Oct 30 09:11:46 2026 +0100  Moritz  chore(http): run Firefox tests with -headless instead of xvfb-run (dart-lang/http#2033)
  05d0b18  Fri Oct 30 07:08:19 2026 +0100  Moritz  fix(http2): make StreamException a StreamTransportException and forward ALPN errors to onError (dart-lang/http#2009)

i18n (https://github.com/dart-lang/i18n/compare/4c7b4ad..7475ce6):
  7475ce64  Sat Oct 31 02:49:28 2026 +0100  Moritz  [intl4x] Fix NumberFormat.percent ignoring digits (dart-lang/i18n#1115)

shelf (https://github.com/dart-lang/shelf/compare/62d3e4e..9946de5):
  9946de5  Sun Nov 1 12:27:59 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 3 updates (dart-lang/shelf#554)

test (https://github.com/dart-lang/test/compare/6d74720..2a9e017):
  2a9e0175  Sun Nov 1 03:47:24 2026 +0000  dependabot[bot]  Bump the github-actions group with 6 updates (dart-lang/test#2813)
  55cd5cd5  Fri Oct 30 13:43:47 2026 -0700  Nate Bosch  Migrate publishing workflows from firehose to setup-dart (dart-lang/test#2812)
  10dc78c0  Fri Oct 30 11:21:53 2026 -0700  Nate Bosch  [checks] Describe what the stream did in failures (dart-lang/test#2811)
  c11ad06d  Fri Oct 30 08:59:57 2026 -0700  Nate Bosch  [checks] Add CheckFailure.describeNested (dart-lang/test#2793)
  0137b43e  Fri Oct 30 05:40:43 2026 +0530  Shriraj Jadhav  [checks] Use the Iterable.last member for the last expectation (dart-lang/test#2807)
  50c9f454  Thu Oct 29 15:11:39 2026 -0700  Nate Bosch  [checks] Collapse clauses for described conditions (dart-lang/test#2794)

tools (https://github.com/dart-lang/tools/compare/3980dca..1579f8d):
  1579f8d69  Mon Nov 2 02:44:58 2026 +0530  Joshua  fix(coverage): reject package: and dart: URIs outside known roots in Resolver (dart-lang/tools#2764)
  f869fdca4  Sun Nov 1 10:26:34 2026 +0100  dart-ecosystem-bot[bot]  [boring] Update BoringSSL to ed9209cf (dart-lang/tools#2774)
  d10e2b87f  Sun Nov 1 10:26:11 2026 +0100  dart-ecosystem-bot[bot]  [boring] Pin the prebuilt binaries of boring-binaries-20261101-33e9e69722af (dart-lang/tools#2777)
  33e9e6972  Sun Nov 1 05:56:30 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 4 updates (dart-lang/tools#2775)
  f84d0d0d2  Sun Nov 1 05:36:12 2026 +0000  dependabot[bot]  build(deps): bump ffigen from 22.0.0 to 23.0.0 in /pkgs/boring (dart-lang/tools#2776)
  14ad5e515  Fri Oct 30 10:48:58 2026 -0700  Kevin Moore  [mime] Harden BoundMultipartStream error routing, headers, and epilogue (dart-lang/tools#2629)
  7a780d48c  Fri Oct 30 12:57:03 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed plain-text runs in InlineParser (dart-lang/tools#2757)
  3671d33a9  Fri Oct 30 12:56:48 2026 +0100  Jonas Finnemann Jensen  [markdown] Index footnote definitions and labels on Document for O(1) lookup (dart-lang/tools#2760)
  6f38e9fda  Fri Oct 30 12:56:09 2026 +0100  Jonas Finnemann Jensen  [markdown] Bound the domain of extended URL autolinks to DNS limits (dart-lang/tools#2759)
  aa0c38544  Fri Oct 30 12:20:40 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic removeRange in _combineAdjacentText (dart-lang/tools#2758)
  f425baf46  Fri Oct 30 11:16:28 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed space runs in LineBreakSyntax (dart-lang/tools#2756)
  c7eafbec3  Fri Oct 30 11:09:45 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject '<' in AutolinkSyntax (dart-lang/tools#2755)
  86ed0f902  Fri Oct 30 11:08:25 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic backtracking in headerPattern (dart-lang/tools#2754)
  8ea07517f  Fri Oct 30 07:45:42 2026 +0100  Moritz  [boring] Regenerate the x509-limbo expected failures in the refresh job (dart-lang/tools#2712)
  40235c365  Thu Oct 29 14:46:17 2026 +0100  Jonas Finnemann Jensen  [markdown] Make HtmlBlockSyntax.parseChildLines iterative (dart-lang/tools#2749)
  0ff03f643  Thu Oct 29 14:41:33 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid re-escaping nested image alt text in ImageSyntax.createNode (dart-lang/tools#2752)
  784654d01  Thu Oct 29 14:37:11 2026 +0100  Jonas Finnemann Jensen  [markdown] Add test cases covering parser and renderer edge cases (dart-lang/tools#2753)
  4f2718ad8  Thu Oct 29 14:36:52 2026 +0100  Jonas Finnemann Jensen  [markdown] Make Document._parseInlineContent iterative (dart-lang/tools#2750)
  cac1421d5  Thu Oct 29 14:17:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer (dart-lang/tools#2748)
  66880bcd3  Thu Oct 29 14:11:27 2026 +0100  Jonas Finnemann Jensen  [markdown] Check footnote label and empty map before scanning footnote keys (dart-lang/tools#2751)
  7f8696523  Thu Oct 29 14:07:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic blank-line lookahead in CodeBlockSyntax (dart-lang/tools#2747)
  9da6bb291  Thu Oct 29 14:03:04 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid exponential and quadratic backtracking in tablePattern (dart-lang/tools#2746)

dartdoc is held back at db94437, before dart-lang/dartdoc#4312, because its
mock SDK version changes conflict in the google3 3-way merge with the hotfix
in cl/1005963642.

native is held back at e0ca7c6, before dart-lang/native#3742, because
deprecating `MacOSCodeConfig(targetVersion: ...)` causes a fatal
`deprecated_member_use` diagnostic in `pkg/dartdev/lib/src/native_assets.dart`.

web is held back at 4e60e61, before dart-lang/web#596, because it changes
`package:web` signatures (e.g. `innerHTML=`, `parseFromString`, `Worker()`)
from `JSAny` to `String` in ways that break callers in google3.

Change-Id: I53d2cbde741c45b8fd33bca42742549819dd332b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/572780
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Nate Bosch <[email protected]>
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