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[markdown] Avoid rescanning failed space runs in LineBreakSyntax - #2756

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markdown_line_break_rescans
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[markdown] Avoid rescanning failed space runs in LineBreakSyntax#2756
jonasfj merged 3 commits into
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markdown_line_break_rescans

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@jonasfj jonasfj commented Oct 29, 2026 •
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Override tryMatch in LineBreakSyntax to return false in O(1) when the
current character is a space immediately preceded by another unconsumed space
(pos > parser.start && parser.charAt(pos) == $space && parser.charAt(pos - 1) == $space).

Why this change is necessary

LineBreakSyntax matches hard line breaks using RegExp(r'(?:\\| +)\n'). When
a line contains a long run of k spaces that is not followed by \n (for
example 'a\n${' ' * n}*'), InlineParser advances through the spaces one
character at a time. At every space position i, LineBreakSyntax.tryMatch
ran pattern.matchAsPrefix(parser.source, i):

  • + consumed all remaining k - i spaces up to *;
  • \n failed to match *;
  • the regex engine failed after scanning k - i spaces.
    Summing k - i over all k spaces took O(k^2) time.

How this change works

If pos > parser.start and both parser.charAt(pos) and parser.charAt(pos - 1)
are $space:

  • pos - 1 is an unconsumed space from the same contiguous run of spaces, which
    means LineBreakSyntax.tryMatch was already tested at the start of this space
    run and failed because the run is not followed by \n.
  • Since a sub-suffix of the same contiguous space run ends at the exact same
    non-\n character, pattern.matchAsPrefix at pos is guaranteed to fail as
    well.

Checking pos > parser.start && parser.charAt(pos) == $space && parser.charAt(pos - 1) == $space is completely stateless and rejects every
subsequent space in a failed run in O(1) time, reducing total work across a
k-space run from O(k^2) to O(k).

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:

  • No HTML differences.
  • 4 OSS-Fuzz timeouts resolved. One 49 KB input was reported as a new timeout;
    standalone it takes 21.4 s on main and 18.3 s on this branch, so that was
    load during the run.
  • Cumulative parse time +0.2% (noise).

gemini-code-assist[bot]
gemini-code-assist Bot reviewed Oct 29, 2026
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Code Review

This pull request optimizes LineBreakSyntax by overriding tryMatch to prevent rescanning runs of spaces when a hard line break match fails. A corresponding test case has been added to verify this behavior. The review feedback suggests addressing two edge cases in the tryMatch implementation: handling custom startMatchPos values that differ from parser.pos to avoid incorrect optimizations, and adding an out-of-bounds check to prevent potential RangeError crashes.

Comment thread pkgs/markdown/lib/src/inline_syntaxes/line_break_syntax.dart
Comment on lines +14 to +23
@override
bool tryMatch(InlineParser parser, [int? startMatchPos]) {
final pos = startMatchPos ?? parser.pos;
if (pos > parser.start &&
parser.charAt(pos) == $space &&
parser.charAt(pos - 1) == $space) {
return false;
}
return super.tryMatch(parser, startMatchPos);
}

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medium

To ensure maximum robustness and correctness, we should address two potential edge cases in the overridden tryMatch method:

  1. Speculative/Custom startMatchPos: The optimization assumes that the parser has sequentially scanned up to pos and already attempted (and failed) to match at the start of the current space run. If a caller or custom extension calls tryMatch with a speculative startMatchPos that is different from parser.pos, this assumption might not hold, potentially leading to incorrect false results. We should bypass the optimization if startMatchPos is provided and differs from parser.pos.
  2. Out-of-bounds Guard: parser.charAt(pos) calls codeUnitAt under the hood, which throws a RangeError if pos is out of bounds (e.g., if startMatchPos is equal to parser.source.length). Adding a pos < parser.source.length check prevents any potential crashes.
  @override
  bool tryMatch(InlineParser parser, [int? startMatchPos]) {
    if (startMatchPos != null && startMatchPos != parser.pos) {
      return super.tryMatch(parser, startMatchPos);
    }
    final pos = parser.pos;
    if (pos < parser.source.length &&
        pos > parser.start &&
        parser.charAt(pos) == $space &&
        parser.charAt(pos - 1) == $space) {
      return false;
    }
    return super.tryMatch(parser, startMatchPos);
  }

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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 ✔️

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pkgs/markdown/lib/src/inline_syntaxes/line_break_syntax.dart 💚 100 %

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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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@jonasfj
jonasfj force-pushed the markdown_line_break_rescans branch from 63f31ad to eec460f Compare October 30, 2026 07:57
@jonasfj
[markdown] Avoid rescanning failed space runs in LineBreakSyntax
3a39a3d 
Override `tryMatch` in `LineBreakSyntax` to return `false` in O(1) when the
current character is a space immediately preceded by another unconsumed space
(`pos > parser.start && parser.charAt(pos) == $space && parser.charAt(pos - 1) ==
$space`).

----

### Why this change is necessary

`LineBreakSyntax` matches hard line breaks using `RegExp(r'(?:\\|  +)\n')`. When
a line contains a long run of k spaces that is *not* followed by `\n` (for
example `'a\n${' ' * n}*'`), `InlineParser` advances through the spaces one
character at a time. At every space position `i`, `LineBreakSyntax.tryMatch`
ran `pattern.matchAsPrefix(parser.source, i)`:
- `  +` consumed all remaining `k - i` spaces up to `*`;
- `\n` failed to match `*`;
- the regex engine failed after scanning `k - i` spaces.
Summing `k - i` over all `k` spaces took O(k^2) time.

### How this change works

If `pos > parser.start` and both `parser.charAt(pos)` and `parser.charAt(pos - 1)`
are `$space`:
- `pos - 1` is an unconsumed space from the same contiguous run of spaces, which
  means `LineBreakSyntax.tryMatch` was already tested at the start of this space
  run and failed because the run is *not* followed by `\n`.
- Since a sub-suffix of the same contiguous space run ends at the exact same
  non-`\n` character, `pattern.matchAsPrefix` at `pos` is guaranteed to fail as
  well.

Checking `pos > parser.start && parser.charAt(pos) == $space &&
parser.charAt(pos - 1) == $space` is completely stateless and rejects every
subsequent space in a failed run in O(1) time, reducing total work across a
k-space run from O(k^2) to O(k).
@jonasfj
[markdown] Explain the LineBreakSyntax previous-character check
beb412f
@jonasfj
jonasfj force-pushed the markdown_line_break_rescans branch from eec460f to beb412f Compare October 30, 2026 09:22
@jonasfj
jonasfj marked this pull request as ready for review October 30, 2026 09:44
@jonasfj
jonasfj requested a review from srawlins as a code owner October 30, 2026 09:44
@jonasfj
jonasfj requested review from szakarias and removed request for srawlins October 30, 2026 09:44
@jonasfj jonasfj changed the title [WIP] [markdown] Avoid rescanning failed space runs in LineBreakSyntax [markdown] Avoid rescanning failed space runs in LineBreakSyntax Oct 30, 2026
@jonasfj
jonasfj merged commit f425baf into main Oct 30, 2026
23 of 24 checks passed
@jonasfj
jonasfj deleted the markdown_line_break_rescans branch October 30, 2026 10:16
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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