[pigeon] Treat NSNull as null for non-null Flutter API returns by Xelorium · Pull Request #12531 · flutter/packages

Xelorium · 2026-08-21T10:42:06Z

Description

FlutterStandardReader substitutes NSNull for a nil element when it decodes a list, so a null reply for a non-null return value reaches the generated Swift code as NSNull rather than as nil. The generated guard only checks listResponse[0] == nil, so NSNull falls through to the force cast below it and aborts the process:

} else if listResponse[0] == nil {
  completion(.failure(PigeonError(code: "null-error", ...)))
} else {
  let result = listResponse[0] as! AuthenticationChallengeResponse  // 💥

This crashes production apps through webview_flutter_wkwebview: the plugin clears its native instance manager in WebViewFlutterPlugin.tearDownProxyAPIRegistrar(), which iOS triggers on sceneDidDisconnect / applicationWillTerminate. A WKNavigationDelegate.didReceiveAuthenticationChallenge reply that is still in flight then carries an identifier the native instance manager can no longer resolve, the codec reader returns nil, and the app dies with:

Could not cast value of type 'NSNull' (0x...) to 'webview_flutter_wkwebview.AuthenticationChallengeResponse' (0x...).

The same shape was reported before for other types (URLRequestWrapper in flutter/flutter#162437), which is expected: the generator emits this guard for every non-null return value, so any unresolvable instance crashes instead of reporting an error.

This PR treats NSNull as a null reply, so the existing null-error path handles it.

Verification

Added a Swift generator unit test asserting the generated guard covers NSNull.
Added a native regression test in platform_tests that replies NSNull to a non-null Flutter API return. It passes with this change; with the generated guard reverted it aborts with Could not cast value of type 'NSNull' (0x...) to 'test_plugin.AllNullableTypes' (0x...).
dart test and flutter_plugin_tools format pass locally.

Fixes flutter/flutter#191254

Pre-Review Checklist

google-cla · 2026-08-21T10:42:24Z

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Code Review

This pull request updates Pigeon to version 27.3.3. It modifies the Swift generator to check if a Flutter API response is nil or NSNull when validating non-nullable return values, preventing crashes when FlutterStandardReader substitutes NSNull for nil. It also updates the generated Swift files, adds a regression test in NullableReturnsTests.swift, and adds a generator unit test in swift_generator_test.dart. There are no review comments, so no further feedback is provided.

`FlutterStandardReader` substitutes `NSNull` for a `nil` element of a list, so a null reply for a non-null return value arrives as `NSNull` rather than as `nil`. The generated `listResponse[0] == nil` check did not catch that, and the following force cast aborted the process. In `webview_flutter_wkwebview` this crashes apps whenever the native instance manager cannot resolve the returned instance, which the plugin itself causes by clearing the manager on scene disconnect and app termination while an authentication challenge is in flight.

Xelorium · 2026-08-25T08:14:24Z

Thanks for the pointer on flutter/flutter#191254 — opened this as suggested there.

Small status note in case it helps whoever picks it up: the branch is rebased onto the 28.0.0 async change, so it is mergeable again, and the CLA check is green. The test matrix hasn't run yet since the CICD label isn't something I can add myself.

No rush — happy to make any changes you'd like when you get a chance to look.

bparrishMines

Thanks for the contribution!

I think this may be the true cause for flutter/flutter#162437. Thanks for debugging this.

@tarrinneal Where are you with the FFI/JNI integration? Can this land without disrupting #11352?

bparrishMines · 2026-09-02T23:16:45Z

+  @Test
+  func nullReplyForNonNullReturnFailsWithoutCrashing() async throws {
+    let binaryMessenger = MockBinaryMessenger<NSNull>(codec: codec)
+    binaryMessenger.result = NSNull()
+    let api = FlutterIntegrationCoreApi(binaryMessenger: binaryMessenger)
+
+    do {
+      // `sendMultipleNullableTypes` has a non-null return value.
+      _ = try await api.sendMultipleNullableTypes(aBool: nil, anInt: nil, aString: nil)
+      Issue.record("Expected a null-error but the call succeeded.")
+    } catch let error as PigeonError {
+      #expect(error.code == "null-error")
+    }
+  }


This should be moved in to the NullableReturnsTests or move this class to a separate file.

Moving into the NullableReturnsTests class would be preferred.

tarrinneal · 2026-09-02T23:58:00Z

@tarrinneal Where are you with the FFI/JNI integration? Can this land without disrupting #11352?

I actually already added logic to fix this in the ffi pr. If I haven't already changed it to solve this problem, I can.

tarrinneal · 2026-09-03T00:23:40Z

This pr can land first, it doesn't really matter

tarrinneal · 2026-09-03T00:24:19Z

+          // `FlutterStandardReader` substitutes `NSNull` for a `nil` element of
+          // a list, so a null reply can arrive as either. See
+          // https://github.com/flutter/flutter/issues/191254.


this isn't needed

tarrinneal · 2026-09-03T00:25:02Z

+    // `FlutterStandardReader` substitutes `NSNull` for a `nil` element of a
+    // list, so both need to be treated as a null reply before the value is
+    // cast. See https://github.com/flutter/flutter/issues/191254.


this isn't needed either

tarrinneal · 2026-09-03T00:26:34Z

Make sure to run format and analyze

tarrinneal · 2026-09-03T03:31:48Z

+/// Regression test for https://github.com/flutter/flutter/issues/191254.
+///
+/// `FlutterStandardReader` substitutes `NSNull` for a `nil` element of a list,
+/// so a null reply for a non-null return value arrives as `NSNull` rather than
+/// as `nil`. Before the fix that value was force-cast to the return type, which
+/// aborted the process instead of reporting an error.
+@MainActor
+struct NullReplyForNonNullReturnTests {
+  let codec = FlutterStandardMessageCodec.sharedInstance()
+
+  @Test
+  func nullReplyForNonNullReturnFailsWithoutCrashing() async throws {


the comments aren't needed if the test name is something like nonNullReturnFailsOnNSNullResponse.

…nto NullableReturnsTests

Xelorium · 2026-09-03T06:13:50Z

Thanks for the reviews! Addressed all the comments: dropped the explanatory comments in the generator and the generator test, and folded the regression test into NullableReturnsTests as nonNullReturnFailsOnNSNullResponse. Ran format, analyze and the Dart + iOS unit tests locally, all green.

github-actions Bot added p: pigeon platform-ios platform-macos labels Aug 21, 2026

bayraktarmdkaraca mentioned this pull request Aug 21, 2026

[webview_flutter_wkwebview] Crash: Could not cast value of type 'NSNull' to 'AuthenticationChallengeResponse' when the instance manager is cleared while an auth challenge is in flight flutter/flutter#191254

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gemini-code-assist Bot reviewed Aug 21, 2026

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Xelorium force-pushed the pigeon-swift-nsnull-null-check branch from fbfef09 to 274fd02 Compare August 21, 2026 10:58

Xelorium force-pushed the pigeon-swift-nsnull-null-check branch from 274fd02 to a3cd890 Compare August 25, 2026 08:09

Piinks requested a review from bparrishMines August 25, 2026 20:41

bparrishMines requested changes Sep 2, 2026

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bparrishMines added the CICD Run CI/CD label Sep 2, 2026

tarrinneal approved these changes Sep 3, 2026

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tarrinneal reviewed Sep 3, 2026

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Address review: drop explanatory comments, fold the regression test i…

06c1df5

…nto NullableReturnsTests

flutter-dashboard Bot removed the CICD Run CI/CD label Sep 3, 2026

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[pigeon] Treat NSNull as null for non-null Flutter API returns - #12531

[pigeon] Treat NSNull as null for non-null Flutter API returns#12531
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