[DO NOT MERGE] Instrument __MonoMac_NSAsyncActionDispatcher for issue #25861 by rolfbjarne · Pull Request #25919 · dotnet/macios

rolfbjarne · 2026-07-03T10:41:15Z

Throwaway diagnostic instrumentation to help reproduce and diagnose the GC/marshalling race in the flaky NSAsyncActionDispatcher failure.

The goal is to catch the failure on CI and learn:

Exactly where each __MonoMac_NSAsyncActionDispatcher is instantiated (creation stack trace), and whether it's always the same call site.
If/when the instance was disposed / garbage collected before the native callback ran.

How it works

New src/Foundation/NSAsyncDispatcherInstrumentation.cs: stores diagnostic data directly on the native Objective-C object via an associated reference (objc_setAssociatedObject), so it survives even after the managed instance is garbage collected.
NSAsyncDispatcher (in NSAction.cs) records the creation stack trace in its constructor, and logs lifecycle events (Apply, Dispose, finalize) as they happen.
MissingCtor (in Runtime.cs) reads this info from the still-alive native object and appends it to the marshalling exception message.

⚠️ This is throwaway instrumentation and must not be merged. It is only meant to gather data from CI.

Ref: #25861

🤖 Pull request created by Copilot

…25861 Add throwaway instrumentation to diagnose the GC/marshalling race in #25861. For every async dispatcher, record the creation stack trace and a log of lifecycle events (Apply/Dispose/Finalize). This is stored directly on the native Objective-C object via an associated reference, so it survives even after the managed instance is garbage collected. When the marshalling exception from the issue is raised (MissingCtor), this information is appended to the error message. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…/issue-25861-instrumentation

… objc associated objects The previous approach stored the diagnostic info on the native object via objc_setAssociatedObject with OBJC_ASSOCIATION_RETAIN. Reading it back with objc_getAssociatedObject deterministically segfaulted in objc_msgSend_uncached on the normal Apply path, crashing every test that dispatches an async action. Replace the objc-associated-object mechanism with a static managed dictionary keyed by the native handle (IntPtr). This still survives GC of the managed dispatcher instance (the key is the native pointer, the value is a string, so it does not keep the managed object alive), but performs no Objective-C calls and therefore cannot crash. Verified: tests/linker "dont link" MacCatalyst now passes (9/9) instead of segfaulting at startup. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…/issue-25861-instrumentation

vs-mobiletools-engineering-service2 · 2026-07-04T02:58:56Z

✅ [PR Build #f227cd0] Build passed (Detect API changes) ✅

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Hash: f227cd05fa355bb85d3a37587d07825e622f2251 [PR build]

vs-mobiletools-engineering-service2 · 2026-07-04T03:11:51Z

✅ API diff for current PR / commit

NET (empty diffs)

iOS: vsdrops gist ( No breaking changes )
tvOS: vsdrops gist ( No breaking changes )
MacCatalyst: vsdrops gist ( No breaking changes )
macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

iOS: vsdrops gist ( No breaking changes )
tvOS: vsdrops gist ( No breaking changes )
MacCatalyst: vsdrops gist ( No breaking changes )
macOS: vsdrops gist ( No breaking changes )

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Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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vs-mobiletools-engineering-service2 · 2026-07-04T03:18:07Z

✅ [PR Build #f227cd0] Build passed (Build packages) ✅

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Hash: f227cd05fa355bb85d3a37587d07825e622f2251 [PR build]

vs-mobiletools-engineering-service2 · 2026-07-04T03:21:07Z

✅ [PR Build #f227cd0] Build passed (Build macOS tests) ✅

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Hash: f227cd05fa355bb85d3a37587d07825e622f2251 [PR build]

vs-mobiletools-engineering-service2 · 2026-07-04T05:33:41Z

🔥 [CI Build #f227cd0] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

0 tests crashed, 5 tests failed, 194 tests passed.

Failures

❌ monotouch tests (iOS)

1 tests failed, 17 tests passed.

Failed tests

monotouch-test/iOS - simulator/Release (compat inline Class.GetHandle): Failed

Html Report (VSDrops) Download

❌ monotouch tests (tvOS)

4 tests failed, 14 tests passed.

Failed tests

monotouch-test/tvOS - simulator/Release (link sdk): Failed
monotouch-test/tvOS - simulator/Debug (LinkSdk): Failed
monotouch-test/tvOS - simulator/Debug (managed static registrar): Failed
monotouch-test/tvOS - simulator/Release (compat inline dlfcn): Failed

Html Report (VSDrops) Download

Successes

✅ assembly-processing: All 1 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker (iOS): All 15 tests passed. Html Report (VSDrops) Download
✅ linker (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ linker (macOS): All 21 tests passed. Html Report (VSDrops) Download
✅ linker (tvOS): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 17 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 18 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ sharpie: All 1 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

macOS tests

✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Ventura (13): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sonoma (14): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sequoia (15): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Tahoe (26): All 5 tests passed. Html Report (VSDrops) Download