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[characters] Fix RangeError on an unpaired tail surrogate at range start - #1033
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BackBreaks.step() checked `cursor >= start` after stepping onto a tail surrogate, then read `codeUnitAt(--cursor)`. If the range starts with an unpaired tail surrogate, that reads before `start`: a RangeError when `start == 0` (e.g. `'\uDC00'.characters.last`), or a pairing with a lead surrogate outside the range when `start > 0`. Only peek at `cursor - 1` when `cursor > start`, and step back only for a real lead surrogate, matching the forward Breaks.step().
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Please update your PR title/description 🙏 |
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cursor >= start after stepping onto a tail surrogate, then read codeUnitAt(--cursor). If the range starts with an unpaired tail surrogate, that reads before start: a RangeError when start == 0 (e.g. '\uDC00'.characters.last), or a pairing with a lead surrogate outside the range when start > 0. Only peek at cursor - 1 when cursor > start, and step back only for a real lead surrogate, matching the forward Breaks.step().
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(I moved the description from the title into your post. Feel free to edit if not satisfactory.) |
lrhn
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Nov 3, 2026
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Looks very good. Does the job and tests it well.
| // be strictly after [start] to be part of the substring. | ||
| if (cursor > start && | ||
| (prevSurrogate = base.codeUnitAt(cursor - 1) ^ 0xD800) <= 0x3FF) { | ||
| cursor--; |
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Good fix. The prior code was broken in at least two ways.
It should definitely use > start, and then keeping the --cursor would be wrong, because the cursor would not be decremented when cursor == start, but it would still be incremented in the else branch.
| // BSD-style license that can be found in the LICENSE file. | ||
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| // Regression tests for backwards iteration over a string whose first code | ||
| // unit is an unpaired tail (low) surrogate. `BackBreaks.step` used to read the |
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(I've learned that the correct term is "trail surrogate", not "tail surrogate". Haven't fixed everything yet, or much of anything, but feel free to change it here. Or not, and stay consistent, then I'll have to fix it when I do the rest.)
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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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BackBreaks.step()checkedcursor >= startafter stepping onto a tail surrogate, then readcodeUnitAt(--cursor). If the range starts with an unpaired tail surrogate, that reads beforestart: a RangeError whenstart == 0(e.g.'\uDC00'.characters.last), or a pairing with a lead surrogate outside the range whenstart > 0. Only peek atcursor - 1whencursor > start, and step back only for a real lead surrogate, matching the forward Breaks.step().