engine: warn on WASM load failure when not crossOriginIsolated#186252
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When a Flutter WASM web app fails to load because the hosting page is not cross-origin isolated (i.e. the server isn't sending COOP/COEP HTTP headers), the underlying WebAssembly / SharedArrayBuffer errors that surface in the browser console are obscure and don't point the developer at the real problem. Wrap the WASM entrypoint load in a try/catch and, on failure, check window.crossOriginIsolated === false. When that's the case, emit a single console.warn that names the missing headers (Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp), suggests building for JavaScript instead if the headers can't be set, and links to the canonical web.dev guide. The original error is re-thrown so the existing failure path is unchanged. Fixes flutter#142822
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It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging. If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix? Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group. |
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Code Review
This pull request introduces a try-catch block around the Flutter Web WASM loading logic to provide a descriptive warning when the hosting page lacks cross-origin isolation. This helps developers identify configuration issues related to COOP and COEP headers. Feedback suggests using Promise.all when awaiting multiple promises to improve robustness and intent clarity.
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Thanks for the contribution! Printing a warning is a good idea, but there is a better place to do it. There've been many changes since that issue was filed. The code doesn't fail anymore when not My suggestion is instead of doing a try-catch, you could use the values of |
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Per @mdebbar's review feedback: the system now auto-detects missing cross-origin isolation and switches to single-threaded mode rather than failing, so the warning belongs in skwasm_loader.js where that fallback decision is made — not in a try/catch in entrypoint_loader.js. Changes: - Revert try/catch wrapper from _loadWasmEntrypoint in entrypoint_loader.js - Add console.warn in skwasm_loader.js when !browserEnvironment.crossOriginIsolated && !config.forceSingleThreadedSkwasm, immediately before skwasm.default() is called with skwasmSingleThreaded: true
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Thanks @mdebbar — you're right that
Message text names the two missing headers, suggests single-threaded mode is the fallback, and links to https://web.dev/articles/coop-coep. Ready for re-review when you have a moment. |
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hey @mdebbar, I noticed there is a bot removing |
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Instead of mentioning the SharedArrayBuffer, I think this warning message should say something like "Skwasm uses multi-threading and web workers for better performance, but your page needs to be cross-origin isolated to support multi-threading."
Also, please make it configurable to suppress this warning.
Per harryterkelsen's review on flutter#186252: * Reword the warning to describe what skwasm actually loses (multi-threading / web workers for performance) rather than the underlying mechanism (SharedArrayBuffer). * Add a `suppressMultithreadingWarning` config flag so apps that are aware of the constraint and cannot enable cross-origin isolation (browser extensions, embedded environments, etc.) can silence the warning without also being forced into single-threaded mode via forceSingleThreadedSkwasm. * Mention the new flag in the warning text so devs reading the warning know how to opt out. * Add the new field to FlutterConfiguration in types.d.ts.
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Reworded per your wording suggestion and added a |
I haven't noticed that. Let me dig into it! |
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autosubmit label was removed for flutter/flutter/186252, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR. |
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autosubmit label was removed for flutter/flutter/186252, because - The status or check suite Mac mac_host_engine has failed. Please fix the issues identified (or deflake) before re-applying this label. |
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autosubmit label was removed for flutter/flutter/186252, because - The status or check suite Mac mac_host_engine has failed. Please fix the issues identified (or deflake) before re-applying this label. |
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…12169) Manual roll Flutter from 91939cc4db78 to dc2a8703e12b (50 revisions) Manual roll requested by [email protected] flutter/flutter@91939cc...dc2a870 2026-07-09 [email protected] [ios,macos] Update swiftc.py flags to match swiftc (flutter/flutter#189174) 2026-07-09 [email protected] [AGP 9] Update Warn Version to AGP 9+ (flutter/flutter#189109) 2026-07-09 [email protected] Sync CHANGELOG.md from stable (flutter/flutter#189203) 2026-07-09 [email protected] [web] Roll Chrome to 145 (framework) (flutter/flutter#182861) 2026-07-09 [email protected] Roll Packages from 52d84d6 to 20928d5 (6 revisions) (flutter/flutter#189194) 2026-07-09 [email protected] [web] Avoid absolute positioning for base CanvasKit canvas (flutter/flutter#188337) 2026-07-09 [email protected] Roll Dart SDK from cdb7217e65aa to a11fb7ed40a5 (6 revisions) (flutter/flutter#189195) 2026-07-09 [email protected] Fix dereference of nullptr in the moved-to-rect signal in the Linux embedder (flutter/flutter#189152) 2026-07-09 [email protected] Fix data for design packages (flutter/flutter#189140) 2026-07-09 [email protected] Roll Skia from 7b42d1251d54 to ab3a7b98c94d (2 revisions) (flutter/flutter#189181) 2026-07-09 [email protected] Roll Skia from 05d9d214e0b7 to 7b42d1251d54 (2 revisions) (flutter/flutter#189175) 2026-07-09 [email protected] Roll Skia from 542c8bdd7f4f to 05d9d214e0b7 (4 revisions) (flutter/flutter#189169) 2026-07-09 [email protected] UberSDF rect handling for thin (line-like) rectangles (flutter/flutter#188821) 2026-07-09 [email protected] Roll Skia from dd572c07f63c to 542c8bdd7f4f (4 revisions) (flutter/flutter#189160) 2026-07-09 [email protected] [flutter_tools] Fix hot restart for WASM web builds (flutter/flutter#187898) 2026-07-08 [email protected] Split FlViewRenderer into OpenGL and software backends (flutter/flutter#188824) 2026-07-08 [email protected] Promote android_hardware_smoke_tests out of bringup in CI (flutter/flutter#189081) 2026-07-08 [email protected] Roll Skia from 8df24be66531 to dd572c07f63c (4 revisions) (flutter/flutter#189150) 2026-07-08 [email protected] Expose LinuxWindowRegistrar on _window_linux.dart in order to better support out of tree LinuxWindowingOwners (flutter/flutter#188917) 2026-07-08 [email protected] Roll pub packages (flutter/flutter#189149) 2026-07-08 [email protected] fix(ci): harden some workflows (flutter/flutter#189087) 2026-07-08 [email protected] Roll Skia from 51a62da33da0 to 8df24be66531 (1 revision) (flutter/flutter#189139) 2026-07-08 [email protected] Roll Dart SDK to Dart 3.13 beta3 (flutter/flutter#189122) 2026-07-08 [email protected] [flutter_tools] Don't crash on non-UTF-8 plugin pubspec.yaml (flutter/flutter#188976) 2026-07-08 [email protected] [flutter_tools] Watch transitive #include headers for FragmentProgram hot reload (flutter/flutter#187945) 2026-07-08 [email protected] Roll Skia from 040d9f55de00 to 51a62da33da0 (1 revision) (flutter/flutter#189135) 2026-07-08 [email protected] Roll Packages from 92525f5 to 52d84d6 (7 revisions) (flutter/flutter#189134) 2026-07-08 [email protected] [flutter_tools] Forcefully kill hung subprocesses 5 seconds after timeout (flutter/flutter#187178) 2026-07-08 [email protected] Expose the app's build name and number as compile-time constants (flutter/flutter#187935) 2026-07-08 [email protected] Roll Skia from 1ff92f879815 to 040d9f55de00 (1 revision) (flutter/flutter#189131) 2026-07-08 [email protected] Roll Skia from 6137414bef5c to 1ff92f879815 (6 revisions) (flutter/flutter#189126) 2026-07-08 [email protected] [test cross imports] More test/rendering + flutter_test/test fixes (flutter/flutter#188954) 2026-07-08 [email protected] engine: explain why each candidate build was skipped in Flutter web loader (flutter/flutter#186254) 2026-07-08 [email protected] vscode: add missing unicode.h (flutter/flutter#189102) 2026-07-08 [email protected] Roll Fuchsia Linux SDK from 7RjQJBW3m-3Jl-7jr... to QcRFUtvCw2EobfJ8s... (flutter/flutter#189104) 2026-07-08 [email protected] Roll Skia from 075fbe4778d9 to 6137414bef5c (10 revisions) (flutter/flutter#189106) 2026-07-08 [email protected] engine: warn on WASM load failure when not crossOriginIsolated (flutter/flutter#186252) 2026-07-08 [email protected] Roll Dart SDK from c9bccc09e733 to db2155f56bf3 (2 revisions) (flutter/flutter#189105) 2026-07-08 [email protected] [flutter_tools] Prevent interactive device selection in machine mode (flutter/flutter#188267) 2026-07-08 [email protected] [flutter_tools] Fix wireless ADB device discovery when serial contains spaces (flutter/flutter#187943) 2026-07-07 [email protected] [web] Fix grouped autofill on iOS Chrome (flutter/flutter#187459) 2026-07-07 [email protected] Fix TextSelectionOverlay crash when layout is degenerate (flutter/flutter#188672) 2026-07-07 [email protected] [flutter_tools] Provision Android NDK in the main Gradle invocation (flutter/flutter#186337) 2026-07-07 [email protected] Android_hardware_smoke_test: Migrate to AGP 9 (flutter/flutter#189082) ...
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Description
Fixes #142822.
When a Flutter WASM web app fails to load because the host page isn't cross-origin isolated (no COOP/COEP headers), the underlying WebAssembly errors are obscure and don't point at the actual problem. Examples users currently see:
ReferenceError: SharedArrayBuffer is not definedRuntimeError: WebAssembly.instantiate(): shared memory not enabledTypeError: Failed to execute 'compileStreaming' on 'WebAssembly'This PR wraps the WASM entrypoint load in
_loadWasmEntrypointin a try/catch. On failure, it checkswindow.crossOriginIsolated === false. When that's the case, it emits a singleconsole.warnthat:Cross-Origin-Opener-Policy: same-originandCross-Origin-Embedder-Policy: require-corp).dart2wasmif the developer can't change the server headers.The original error is re-thrown — the existing failure path is unchanged. This is purely additive diagnostic output.
Tests
No JS test infrastructure exists for
engine/src/flutter/lib/web_ui/flutter_js/. The change is also diagnostic-only — it doesn't alter runtime behavior beyond the new console.warn. Requesting a test exemption.This PR replaces #186236 (closed for a clean conversation timeline).
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