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[markdown] Avoid rescanning failed plain-text runs in InlineParser - #2757

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markdown_plain_text_rescans
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[markdown] Avoid rescanning failed plain-text runs in InlineParser#2757
jonasfj merged 2 commits into
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markdown_plain_text_rescans

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@jonasfj jonasfj commented Oct 29, 2026 •
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Replace the two TextSyntax entries in the InlineParser constructor
(r'[A-Za-z0-9]+(?=\s|$)' and r'[ \tA-Za-z0-9]*[A-Za-z0-9](?=\s|$)') with
_PlainTextSyntax, a TextSyntax subclass that returns false in O(1) when
the preceding unconsumed character at pos - 1 already belongs to the syntax's
prefix character class.

In the InlineParser constructor, two TextSyntax fast-forward patterns are
registered when no custom syntaxes match ASCII text:

  • r'[A-Za-z0-9]+(?=\s|$)' (or r'[ \tA-Za-z0-9]*[A-Za-z0-9](?=\s|$)' when no
    custom syntax starts with whitespace).
    Their purpose is to consume safe runs of ASCII spaces, tabs, and alphanumerics
    in bulk as long as the run contains an alphanumeric character followed by
    whitespace or end-of-input (so it never consumes an alphanumeric immediately
    preceding an inline delimiter like _ or *).

However, when the input contains a long run of spaces/tabs and/or alphanumerics
terminated by a non-whitespace punctuation character such as * (for example
'${'a' * n}*', '${' ' * n}*', or '${' \t' * n}${'a' * n}*'), no character
in that run is an alphanumeric followed by \s or $:

  • At the first index of the run, pattern.matchAsPrefix greedily consumes all
    remaining [ \tA-Za-z0-9] characters up to *, then backtracks
    character-by-character looking for [A-Za-z0-9](?=\s|$) before failing.
  • InlineParser then advances pos by 1 (start remains unchanged) and
    repeats the exact same scan from pos + 1, pos + 2, ..., taking O(n^2)
    time.

In _PlainTextSyntax, every character in the prefix pattern belongs to the
character class C ([A-Za-z0-9] when _allowSpaces is false, or
[ \tA-Za-z0-9] when _allowSpaces is true). If _PlainTextSyntax.tryMatch
was tried at pos - 1 (with pos > parser.start, so pos - 1 was not consumed
by any syntax) and parser.charAt(pos - 1) is in C, then matchAsPrefix at
pos - 1 already scanned the entire contiguous run of C characters starting
at pos - 1 and found no [A-Za-z0-9](?=\s|$) anywhere in that run. Therefore,
starting at pos inside the same contiguous run of C characters cannot find
one either.

By checking pos > parser.start and whether parser.charAt(pos - 1) is in C,
_PlainTextSyntax.tryMatch skips super.tryMatch in O(1) time for all
subsequent positions in the failed run, making the scan strictly O(n) and
stateless.

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.
  • 45 OSS-Fuzz timeouts resolved; cumulative parse time −24.2%
    (8,267 s → 6,264 s).
  • Two inputs were reported as new timeouts; standalone they take 4.8 s / 16.0 s
    on main and 5.0 s / 15.9 s on this branch, so that was load during the run.

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Code Review

This pull request optimizes the InlineParser by introducing a new _PlainTextSyntax class that prevents redundant rescanning of word character runs after a match failure, resolving potential quadratic backtracking. The changes also include a new test case in paragraphs.unit to verify the behavior and an update to the changelog. I have no feedback to provide as the implementation is clean and well-tested.

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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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pkgs/markdown/lib/src/inline_parser.dart 💚 98 % ⬆️ 0 %

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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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pkgs/html/test/differential_test.dart
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@jonasfj
jonasfj force-pushed the markdown_plain_text_rescans branch from f232aff to e1451f3 Compare October 30, 2026 07:57
@jonasfj
[markdown] Avoid rescanning failed plain-text runs in InlineParser
d6f7823 
Replace the two `TextSyntax` entries in the `InlineParser` constructor
(`r'[A-Za-z0-9]+(?=\s|$)'` and `r'[ \tA-Za-z0-9]*[A-Za-z0-9](?=\s|$)'`) with
`_PlainTextSyntax`, a `TextSyntax` subclass that returns `false` in O(1) when
the preceding unconsumed character at `pos - 1` already belongs to the syntax's
prefix character class.

----

In the `InlineParser` constructor, two `TextSyntax` fast-forward patterns are
registered when no custom syntaxes match ASCII text:
- `r'[A-Za-z0-9]+(?=\s|$)'` (or `r'[ \tA-Za-z0-9]*[A-Za-z0-9](?=\s|$)'` when no
  custom syntax starts with whitespace).
Their purpose is to consume safe runs of ASCII spaces, tabs, and alphanumerics
in bulk as long as the run contains an alphanumeric character followed by
whitespace or end-of-input (so it never consumes an alphanumeric immediately
preceding an inline delimiter like `_` or `*`).

However, when the input contains a long run of spaces/tabs and/or alphanumerics
terminated by a non-whitespace punctuation character such as `*` (for example
`'${'a' * n}*'`, `'${' ' * n}*'`, or `'${' \t' * n}${'a' * n}*'`), no character
in that run is an alphanumeric followed by `\s` or `$`:
- At the first index of the run, `pattern.matchAsPrefix` greedily consumes all
  remaining `[ \tA-Za-z0-9]` characters up to `*`, then backtracks
  character-by-character looking for `[A-Za-z0-9](?=\s|$)` before failing.
- `InlineParser` then advances `pos` by 1 (`start` remains unchanged) and
  repeats the exact same scan from `pos + 1`, `pos + 2`, ..., taking O(n^2)
  time.

In `_PlainTextSyntax`, every character in the prefix pattern belongs to the
character class `C` (`[A-Za-z0-9]` when `_allowSpaces` is `false`, or
`[ \tA-Za-z0-9]` when `_allowSpaces` is `true`). If `_PlainTextSyntax.tryMatch`
was tried at `pos - 1` (with `pos > parser.start`, so `pos - 1` was not consumed
by any syntax) and `parser.charAt(pos - 1)` is in `C`, then `matchAsPrefix` at
`pos - 1` already scanned the entire contiguous run of `C` characters starting
at `pos - 1` and found no `[A-Za-z0-9](?=\s|$)` anywhere in that run. Therefore,
starting at `pos` inside the same contiguous run of `C` characters cannot find
one either.

By checking `pos > parser.start` and whether `parser.charAt(pos - 1)` is in `C`,
`_PlainTextSyntax.tryMatch` skips `super.tryMatch` in O(1) time for all
subsequent positions in the failed run, making the scan strictly O(n) and
stateless.
@jonasfj
[markdown] Explain the _PlainTextSyntax previous-character check
e3ed219
@jonasfj
jonasfj force-pushed the markdown_plain_text_rescans branch from e1451f3 to e3ed219 Compare October 30, 2026 09:22
@jonasfj
jonasfj requested a review from szakarias October 30, 2026 09:55
@jonasfj
jonasfj marked this pull request as ready for review October 30, 2026 09:56
@jonasfj
jonasfj requested a review from srawlins as a code owner October 30, 2026 09:56
@jonasfj jonasfj changed the title [WIP] [markdown] Avoid rescanning failed plain-text runs in InlineParser [markdown] Avoid rescanning failed plain-text runs in InlineParser Oct 30, 2026
@jonasfj jonasfj mentioned this pull request Oct 30, 2026
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szakarias
szakarias approved these changes Oct 30, 2026
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Drop [WIP]

@jonasfj
jonasfj merged commit 7a780d4 into main Oct 30, 2026
31 of 33 checks passed
@jonasfj
jonasfj deleted the markdown_plain_text_rescans branch October 30, 2026 11:57
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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