Attempt at fixing https://github.com/dart-lang/tools/issues/2202.

[d-markey]

Hello,

I experienced an issue when using a MCP server (using dart_mcp which uses this package). Worked well on VM, but when I switched to Web, MCP initialization never returned. As it turns out, the target MCP server does not support CORS (eg. https://api.githubcopilot.com/mcp/) and it seems the error never flows back to the code that initializes the MCP server, so the corresponding future never completes. This is tracked in issue #2202.

I've added a test "can send a message and receive an error" that mimics the behavior I observed: the test times out without this fix. The fix consists in forwarding the error to both the client and the server. As it turns out, the Peer never called _clientIncomingForwarder.addError() which seems odd?

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[gemini-code-assist]

Code Review

This pull request effectively addresses an issue where pending requests on a Peer would hang indefinitely if the underlying communication channel produced an error. The fix correctly propagates the channel error to both the client and server components of the Peer, ensuring that pending client requests are properly terminated. The introduction of a new test case successfully validates this fix by simulating a channel error and asserting the expected failure of a pending request. Additionally, enabling web platform testing is a valuable improvement. The code changes, including some nice formatting improvements, are clean, well-tested, and follow Dart best practices.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:bazel_worker	1.1.4	already published at pub.dev
package:benchmark_harness	2.4.0-wip	WIP (no publish necessary)
package:boolean_selector	2.1.2	already published at pub.dev
package:browser_launcher	1.1.3	already published at pub.dev
package:cli_config	0.2.1-wip	WIP (no publish necessary)
package:cli_util	0.5.0-wip	WIP (no publish necessary)
package:clock	1.1.3-wip	WIP (no publish necessary)
package:code_builder	4.11.1-wip	WIP (no publish necessary)
package:coverage	1.15.0	already published at pub.dev
package:csslib	1.0.2	already published at pub.dev
package:extension_discovery	2.1.0	already published at pub.dev
package:file	7.0.2-wip	WIP (no publish necessary)
package:file_testing	3.1.0-wip	WIP (no publish necessary)
package:glob	2.1.3	already published at pub.dev
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:html	0.15.7-wip	WIP (no publish necessary)
package:io	1.1.0-wip	WIP (no publish necessary)
package:json_rpc_2	4.1.0-wip	WIP (no publish necessary)
package:markdown	7.3.1-wip	WIP (no publish necessary)
package:mime	2.1.0-wip	WIP (no publish necessary)
package:oauth2	2.0.5	already published at pub.dev
package:package_config	2.3.0-wip	WIP (no publish necessary)
package:pool	1.5.2	already published at pub.dev
package:process	5.0.5	already published at pub.dev
package:pub_semver	2.2.0	already published at pub.dev
package:pubspec_parse	1.5.1-wip	WIP (no publish necessary)
package:source_map_stack_trace	2.1.3-wip	WIP (no publish necessary)
package:source_maps	0.10.14-wip	WIP (no publish necessary)
package:source_span	1.10.1	already published at pub.dev
package:sse	4.1.8	already published at pub.dev
package:stack_trace	1.12.2-wip	(error) pubspec version (1.12.2-wip) and changelog (1.12.2-dev) don't agree
package:stream_channel	2.1.4	already published at pub.dev
package:stream_transform	2.1.2-wip	WIP (no publish necessary)
package:string_scanner	1.4.1	already published at pub.dev
package:term_glyph	1.2.3-wip	WIP (no publish necessary)
package:test_reflective_loader	0.5.0	ready to publish	test_reflective_loader-v0.5.0
package:timing	1.0.2	already published at pub.dev
package:unified_analytics	8.0.8	ready to publish	unified_analytics-v8.0.8
package:watcher	1.1.5-wip	WIP (no publish necessary)
package:yaml	3.1.3	already published at pub.dev
package:yaml_edit	2.2.2	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/bazel_worker/benchmark/benchmark.dart
pkgs/benchmark_harness/integration_test/perf_benchmark_test.dart
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pkgs/clock/lib/src/clock.dart
pkgs/clock/lib/src/default.dart
pkgs/clock/lib/src/stopwatch.dart
pkgs/clock/lib/src/utils.dart
pkgs/clock/test/clock_test.dart
pkgs/clock/test/default_test.dart
pkgs/clock/test/stopwatch_test.dart
pkgs/clock/test/utils.dart
pkgs/coverage/lib/src/coverage_options.dart
pkgs/html/example/main.dart
pkgs/html/lib/dom.dart
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pkgs/html/lib/src/constants.dart
pkgs/html/lib/src/encoding_parser.dart
pkgs/html/lib/src/html_input_stream.dart
pkgs/html/lib/src/list_proxy.dart
pkgs/html/lib/src/query_selector.dart
pkgs/html/lib/src/token.dart
pkgs/html/lib/src/tokenizer.dart
pkgs/html/lib/src/treebuilder.dart
pkgs/html/lib/src/utils.dart
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pkgs/html/test/parser_feature_test.dart
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pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
pkgs/html/test/selectors/selectors.dart
pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/html/test/trie_test.dart
pkgs/html/tool/generate_trie.dart
pkgs/pubspec_parse/test/git_uri_test.dart
pkgs/stack_trace/example/example.dart
pkgs/watcher/test/custom_watcher_factory_test.dart
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[natebosch]

I need to dig a bit more, but as long as this doesn't cause any internal tests to fail I think we can probably land this.

[natebosch]

As an FYI the platforms that run on CI are controlled by the .github/workflows/ config, where we were already running web tests.

tools/.github/workflows/json_rpc_2.yaml

Line 71 in 37f8242

run: dart test --platform chrome --compiler dart2wasm,dart2js

I think it's fine to check in this file too so that dart test locally is more comprehensive.

[natebosch]

Typing out some thoughts as I'm thinking through this change... I want to see if we can find the original intent for this design, it's plausible that this error is intended only to flow to the onUnhandledError handler which is explicitly only set up on the "Server"?

tools/pkgs/json_rpc_2/lib/src/peer.dart

Line 102 in 37f8242

onUnhandledError: onUnhandledError,

It doesn't flow there though, so that might be a red herring...

According to blame, I'm the one who landed it this way.
dart-archive/json_rpc_2#65

I have no memory of that change. From the description:

Forward errors to the _server so that it can forward them through
done without an extra Completer to manage.

Looks like there we were focused on the error flowing through done, and we may have missed the cases where an error needs to flow through sendRequest return values.