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[markdown] Skip unclosed inline HTML tags before matching RegExp - #2788

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markdown_inline_html_closer_check
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[markdown] Skip unclosed inline HTML tags before matching RegExp#2788
jonasfj merged 1 commit into
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markdown_inline_html_closer_check

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@jonasfj jonasfj commented Nov 4, 2026 •
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Override tryMatch in InlineHtmlSyntax to check the last occurrence of the
required closing delimiter (>, ?>, -->, or ]]>) via a lazily computed
HtmlClosers helper stored on InlineParser (exposed via the package-private
InlineParserScanState extension) before invoking super.tryMatch.

InlineHtmlSyntax matches raw inline HTML tags, comments (<!--...-->),
processing instructions (<?...?>), declarations (<!...>), and CDATA sections
(<![CDATA[...]]>) using a multi-branch RegExp whose comment, PI,
declaration, and CDATA branches match arbitrary characters up to their closing
terminator (-->, ?>, >, or ]]>).

When a paragraph contains many unclosed HTML tag prefixes with no closing
terminator later in the string—such as:

  • 'a ${'<!a' * n}' (unclosed declarations)
  • 'a ${'<!--a-' * n}>' (unclosed comments followed by a single >)
  • 'a ${'<?a' * n}>' (unclosed processing instructions followed by >)
  • 'a ${'<![CDATA[a' * n}>' (unclosed CDATA sections followed by >)

InlineHtmlSyntax.tryMatch is invoked at every <. At each <, the regex
engine scans all the way to the end of parser.source looking for >, -->,
?>, or ]]>, failing at EOF every time and taking O(n^2) time.

  1. InlineParser holds a private HtmlClosers? _htmlClosers; field (exposed
    inside lib/src/ via InlineParserScanState.htmlClosers, with
    lib/markdown.dart exporting show InlineParser so no public API is added).
  2. Each late final field on HtmlClosers computes source.lastIndexOf('>'),
    source.lastIndexOf('?>'), source.lastIndexOf('-->'), and
    source.lastIndexOf(']]>') at most once per InlineParser in O(n) time.
  3. Before calling super.tryMatch(parser, pos), InlineHtmlSyntax.tryMatch
    checks in O(1) time:
    • if (closers.lastGt <= pos) return false; (every valid inline HTML
      construct ends with >);
    • if <?: if (closers.lastProcessingEnd < pos + 2) return false;;
    • if <!-: if (closers.lastCommentEnd <= pos) return false; (even a
      degenerate <!---> requires --> starting at >= pos + 2 > pos);
    • if <![: if (closers.lastCdataEnd < pos + 9) return false;.
      When no matching closing delimiter exists after pos, tryMatch returns
      false in O(1) time without running the regex.

Behavior change

None. The pre-checks only skip the regular expression when the closer it needs
cannot occur after the current position, so every construct the expression
accepted before is still accepted.

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:

  • No HTML differences in any source.
  • 2 OSS-Fuzz timeouts resolved; one 161 KB OSS-Fuzz input was reported as a
    new timeout — standalone (three runs each) it takes 14.1–16.9 s on main and 13.5–15.6 s on this branch, so that was load during the run. Cumulative parse time −0.8%.

@jonasfj
jonasfj force-pushed the markdown_inline_html_closer_check branch from fa69466 to ae4a71c Compare November 4, 2026 11:44
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PR Health

Publish ✔️
Package Version Status Publish tag (post-merge)
package:markdown ⭐ 7.4.1-wip WIP (no publish necessary)
  • 16 already published.
  • 26 WIP (no publish necessary).

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Changelog Entry ❗
Package Changed Files
package:markdown pkgs/markdown/lib/markdown.dart
pkgs/markdown/lib/src/charcode.dart
pkgs/markdown/lib/src/inline_parser.dart
pkgs/markdown/lib/src/inline_syntaxes/inline_html_syntax.dart

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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 ✔️

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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Unused Dependencies ✔️
Package Status
markdown ✔️ All dependencies utilized correctly.

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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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Coverage ⚠️
File Coverage
pkgs/markdown/lib/markdown.dart 💔 Not covered
pkgs/markdown/lib/src/charcode.dart 💔 Not covered
pkgs/markdown/lib/src/inline_parser.dart 💚 98 % ⬆️ 0 %
pkgs/markdown/lib/src/inline_syntaxes/inline_html_syntax.dart 💚 100 %

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Code Review

This pull request optimizes the inline HTML parser by lazily tracking the last indices of HTML closing sequences (such as -->, ?>, ]]>, and >) to avoid rescanning unclosed HTML constructs. The feedback recommends adding a defensive bounds check to prevent a potential RangeError in tryMatch and optimizing the index offset checks based on the minimum length of valid HTML tags and comments.

Comment thread pkgs/markdown/lib/src/inline_syntaxes/inline_html_syntax.dart Resolved
@jonasfj
jonasfj requested review from sigurdm and srawlins and removed request for srawlins November 4, 2026 13:06
@jonasfj
jonasfj marked this pull request as ready for review November 4, 2026 13:06
@jonasfj
jonasfj requested a review from srawlins as a code owner November 4, 2026 13:06
@jonasfj
[markdown] Skip unclosed inline HTML tags before matching RegExp
dd1b2fc 
Record the last index of `>`, `?>`, `-->`, and `]]>` once per `InlineParser`
(`HtmlClosers`, exposed through the package-private `InlineParserScanState`
extension) and return `false` from `InlineHtmlSyntax.tryMatch` when the closer
that the construct at `pos` needs cannot occur after it.

----

### Why this change is necessary

`InlineHtmlSyntax` runs its regular expression at every `<`. The alternatives
for processing instructions (`<\?[^]*?\?>`), declarations
(`<![a-zA-Z]+[^]*?>`), and CDATA sections (`<!\[CDATA\[[^]*?\]\]>`) scan to
the end of the paragraph when the closer is missing, so input such as
`'a ' + '<?a' * n + '>'` or `'a ' + '<!a' * n` scans O(n) characters at each of
the n candidates: O(n^2) time (23 KB: 675 ms for `<?a`, 1.7 s for `<!a`,
quadrupling per doubling). The comment and tag alternatives already fail at the
next `--`/`<` and are linear.

### How this change works

The pre-checks never reject a construct the regular expression accepts:
- any construct needs a `>` after `pos` (`lastGt <= pos`);
- `<?` needs `?>` at or after `pos + 2` (`<??>`);
- `<!-` needs `-->` after `pos` (the shortest comment `<!---->` has it at
  `pos + 4`; `<= pos` is conservative);
- `<![` needs `]]>` at or after `pos + 9` (`<![CDATA[]]>`).
Using the *last* index is sufficient: if any closer exists after `pos`, the
last one does. The four `lastIndexOf` calls run at most once per paragraph, so
the check itself is O(1) per `<`.

Measured (23 KB): `'<?a' * n` 675 ms -> 26 ms, `'<!a' * n` 1.7 s -> 22 ms;
output unchanged.
@jonasfj
jonasfj force-pushed the markdown_inline_html_closer_check branch from ae4a71c to dd1b2fc Compare November 4, 2026 13:33
@jonasfj
jonasfj merged commit 0fc5edc into main Nov 4, 2026
24 of 25 checks passed
@jonasfj
jonasfj deleted the markdown_inline_html_closer_check branch November 4, 2026 13:35
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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@sigurdm sigurdm sigurdm approved these changes
@srawlins srawlins Awaiting requested review from srawlins
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