Crash in r8 when building with .NET 11

[jonpryor]

Android framework version

net11.0-android (Preview)

Affected platform version

.NET 11

Description

(Apologies for the longer repro.)

When building Uno's SamplesApp.netcoremobile when targeting .NET 11, r8 fails with a NullPointerException.

Investigation showed that a (the?) cause of the bug was the use of the r8 --no-tree-shaking option, "added" in 4bb4b2e.

Note that when targeting .NET 10, dotnet publish completes without error, no workaround required. .NET 10 is also using a different set of package versions.

Steps to Reproduce

1. Cry 😭

2. Clone unoplatform/uno:

   git clone https://github.com/unoplatform/uno.git

3. Checkout the dev/jonpryor/jonp-try-net11-p5 branch:

   cd uno
   git checkout dev/jonpryor/jonp-try-net11-p5

4. Prepare the build environment

   # update PATH as appropriate
   export PATH=$HOME/Downloads/dotnet-sdk-11.0.100-preview.5.26302.115:$PATH
   \cp build/ci/net11/_global.json global.json

5. Publish the SamplesApp.netcoremobile project for net11.0-android, setting $(DoNotSetAndroidLinkTool)=true:

   dotnet publish -bl -f net11.0-android src/SamplesApp/SamplesApp.netcoremobile/SamplesApp.netcoremobile.csproj -p:UnoTargetFrameworkOverride=net11.0-android -p:DoNotSetAndroidLinkTool=true
   

Publish output shows the error:

    MSBUILD : java error JAVA0000: 
      Error in obj/Release/net11.0-android/lp/114/jl/classes.jar:androidx/core/location/LocationManagerCompat.class at Landroidx/core/location/LocationManagerCompat;->$r8$lambda$z8e3G9khg88yB8-hC19d9V6P1Gc(Landroid/location/LocationManager;Landroidx/core/location/LocationManagerCompat$GpsStatusTransport;)Ljava/lang/Boolean;:
      java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null
      Compilation failed with an internal error.
      java.lang.RuntimeException: com.android.tools.r8.CompilationFailedException: Compilation failed to complete, position: Landroidx/core/location/LocationManagerCompat;->$r8$lambda$z8e3G9khg88yB8-hC19d9V6P1Gc(Landroid/location/LocationManager;Landroidx/core/location/LocationManagerCompat$GpsStatusTransport;)Ljava/lang/Boolean;, orig
      in: obj/Release/net11.0-android/lp/114/jl/classes.jar
      androidx/core/location/LocationManagerCompat.class
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:127)
        at com.android.tools.r8.R8.main(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:5)
      java/lang/Boolean;, origin: obj/Release/net11.0-android/lp/114/jl/classes.jar
      androidx/core/location/LocationManagerCompat.class
        at Version.fakeStackEntry(Version_9.1.31.java:0)
        at com.android.tools.r8.r0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1)
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:81)
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:32)
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:31)
        at com.android.tools.r8.R8.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:62)
        at com.android.tools.r8.R8.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1757)
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:114)
        ... 1 more
      java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null
        at com.android.tools.r8.androidapi.f.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:10)
        at com.android.tools.r8.androidapi.f.d(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:3)
        at com.android.tools.r8.androidapi.f.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1)
        at com.android.tools.r8.graph.i3.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:507)
        at com.android.tools.r8.ir.optimize.s0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1488)
        at com.android.tools.r8.ir.optimize.s0.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:16)
        at com.android.tools.r8.ir.optimize.s0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:157)
        at com.android.tools.r8.ir.optimize.s0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1314)
        at com.android.tools.r8.ir.optimize.s0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:142)
        at com.android.tools.r8.internal.d91.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:503)
        at com.android.tools.r8.internal.d91.c(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:27)
        at com.android.tools.r8.internal.d91.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:219)
        at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:131)
        at com.android.tools.r8.internal.d91.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:5)
        at com.android.tools.r8.internal.d91.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:212)
        at com.android.tools.r8.internal.g73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:208)
        at com.android.tools.r8.internal.f73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:52)
        at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:2)
        at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:17)
        at java.base/java.util.concurrent.ForkJoinTask$AdaptedCallable.exec(ForkJoinTask.java:1428)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
      java.lang.RuntimeException: java.util.concurrent.ExecutionException: com.android.tools.r8.internal.nw0: java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null
                at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:128)
                at com.android.tools.r8.R8.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1371)
                at com.android.tools.r8.R8.c(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:11)
                at com.android.tools.r8.R8.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1756)
                at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:28)
                at com.android.tools.r8.R8.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:62)
                at com.android.tools.r8.R8.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1757)
                at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:114)
                at com.android.tools.r8.R8.main(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:5)
      java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null
                at java.base/java.util.concurrent.ForkJoinTask.reportExecutionException(ForkJoinTask.java:605)
                at java.base/java.util.concurrent.ForkJoinTask.get(ForkJoinTask.java:981)
                at com.android.tools.r8.internal.fg4.forEach(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:4)
                at com.android.tools.r8.internal.fg4.awaitFutures(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1)
                at com.android.tools.r8.internal.hf4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:20)
                at com.android.tools.r8.internal.hf4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:31)
                at com.android.tools.r8.internal.hf4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:4)
                at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:15)
                at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:5)
                at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:1)
                at com.android.tools.r8.internal.f73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:30)
                at com.android.tools.r8.internal.g73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:47)
                at com.android.tools.r8.internal.g73.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:4)
                at com.android.tools.r8.R8.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:377)
                ... 7 more
      java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null
                at com.android.tools.r8.internal.lw0.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:146)
                at com.android.tools.r8.internal.d91.b(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:5)
                at com.android.tools.r8.internal.d91.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:212)
                at com.android.tools.r8.internal.g73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:208)
                at com.android.tools.r8.internal.f73.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:52)
                at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:2)
                at com.android.tools.r8.internal.eg4.a(R8_9.1.31_ffb8f76e0cfe19f6a6aff10b55ad38bc82c7544bc91f5b7fd86bc80f9b2412e7:17)
                at java.base/java.util.concurrent.ForkJoinTask$AdaptedCallable.exec(ForkJoinTask.java:1428)
                at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
                at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
                at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
                at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
                at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
      java.lang.NullPointerException
       Cannot invoke "com.android.tools.r8.androidapi.f$a.a()" because the return value of "com.android.tools.r8.androidapi.f.H()" is null]
      Directory 'obj/Release/net11.0-android/lp/114' is from 'androidx.core.core.aar'.

Build failed with 1 error(s) and 35 warning(s) in 270.4s

Did you find any workaround?

Set $(AndroidLinkTool)=r8. This causes us to hit the EnableShrinking code path, avoiding the inclusion of --no-tree-shaking:

android/src/Xamarin.Android.Build.Tasks/Tasks/R8.cs

Lines 97 to 123 in e4c31f2

	if (EnableShrinking) {
	if (!AcwMapFile.IsNullOrEmpty ()) {
	var acwMap = MonoAndroidHelper.LoadMapFile (BuildEngine4, Path.GetFullPath (AcwMapFile), StringComparer.OrdinalIgnoreCase);
	var javaTypes = new List<string> (acwMap.Values.Count);
	foreach (var v in acwMap.Values) {
	javaTypes.Add (v);
	}
	javaTypes.Sort (StringComparer.Ordinal);
	using (var appcfg = File.CreateText (ProguardGeneratedApplicationConfiguration)) {
	foreach (var java in javaTypes) {
	appcfg.WriteLine ($"-keep class {java} {{ *; }}");
	}
	}
	}
	if (!ProguardCommonXamarinConfiguration.IsNullOrWhiteSpace ()) {
	using (var xamcfg = File.CreateText (ProguardCommonXamarinConfiguration)) {
	GetType ().Assembly.GetManifestResourceStream ("proguard_xamarin.cfg").CopyTo (xamcfg.BaseStream);
	if (IgnoreWarnings) {
	xamcfg.WriteLine ("-ignorewarnings");
	}
	if (!ProguardMappingFileOutput.IsNullOrEmpty ()) {
	xamcfg.WriteLine ("-keepattributes SourceFile");
	xamcfg.WriteLine ("-keepattributes LineNumberTable");
	xamcfg.WriteLine ($"-printmapping \"{Path.GetFullPath (ProguardMappingFileOutput)}\"");
	}
	}
	}

This is done automatically within SamplesApp.netcoremobile.csproj unless $(DoNotSetAndroidLinkTool)=true, which is why we set it above: https://github.com/unoplatform/uno/blob/dev/jonpryor/jonp-try-net11-p5/src/SamplesApp/SamplesApp.netcoremobile/SamplesApp.netcoremobile.csproj#L59-L61

	<PropertyGroup Condition=" '$(DoNotSetAndroidLinkTool)' != 'true' And '$(TargetFramework)' == 'net11.0-android' ">
		<AndroidLinkTool>r8</AndroidLinkTool>
	</PropertyGroup>

Relevant log output

[jonpryor]

On Discord, @jonathanpeppers asked:

I bet you could set some R8 path variable to point at the older one

Let's try that:

dotnet publish -bl -f net11.0-android src/SamplesApp/SamplesApp.netcoremobile/SamplesApp.netcoremobile.csproj -p:UnoTargetFrameworkOverride=net11.0-android -p:DoNotSetAndroidLinkTool=true -p:AndroidR8JarPath=$HOME/Downloads/dotnet-sdk-11.0.100-preview.5.26302.115/packs/Microsoft.Android.Sdk.Darwin/36.1.69/tools/r8.jar

It builds without error. The crash does appear to be due to the updated r8 from commit ebae46e.

[jonathanpeppers]

🤖 This analysis was generated by an AI assistant (Claude Opus 4.7) reading r8's public source from https://r8.googlesource.com/r8. It is a hypothesis based on diffing r8 8.11.18 (release/10.0.1xx) against r8 9.1.31 (main). It has not been confirmed by running r8 with assertions or by reproducing the crash.

Type of code that triggers the bug

Four conditions appear to stack up to produce the f.H() → null → f$a.a() NPE on main (r8 9.1.31), but not on release/10.0.1xx (r8 8.11.18):

1. Synthetic lambda / bridge methods inside an AndroidX compat helper. The artifact in the stack is LocationManagerCompat$$ExternalSyntheticLambda* ($r8$lambda$z8e3G9khg88yB8-hC19d9V6P1Gc), generated by D8 lambda desugaring of LocationManagerCompat.

2. Those lambdas reference Android library APIs whose presence varies by API level. The lambda forwards to LocationManager.addGpsStatusListener(GpsStatus.Listener) / removeGpsStatusListener — deprecated at API 24 and removed at API 31 — plus GpsStatus.Listener (also removed at API 31). When R8 walks the type/interface hierarchy to find an API level for the reference, it lands in AndroidApiReferenceLevelCache.lookup (line 132):

return apiLevelCompute.of(foundApiLevel.max(appView.options().getMinApiLevel()));

3. Built with --no-tree-shaking --no-minification -dontoptimize. That's exactly the branch src/Xamarin.Android.Build.Tasks/Tasks/R8.cs (lines 133–137) takes when <AndroidLinkTool> is not set. With shaking enabled (AndroidLinkTool=r8, the workaround in this issue) the dead LocationManagerCompat lambdas are pruned before R8's API-modeling/optimization passes ever look them up — so the lookup never happens. Without shaking, passes that still run (e.g. ApiInvokeOutlinerDesugaring, RemoveVerificationErrorForUnknownReturnedValues) call apiLevelCompute.of(...) for every library invoke they see.

4. compileSdk = 36 (BAKLAVA), targeting Android. This activates the suspicious change. In r8 commit 8e9a0a8216 "Add support for API level 36.1", R8 changed AndroidApiLevelCompute:

// 8.11.18 — dense, indexed by major level only
knownApiLevelCache = new KnownApiLevel[AndroidApiLevel.API_DATABASE_LEVEL.getLevel() + 1];
knownApiLevelCache[value.getLevel()] = new KnownApiLevel(value);
return knownApiLevelCache[apiLevel.getLevel()];

// 9.1.31 — sparse [128], indexed by (level<<1 | minor)
knownApiLevelCache = new KnownApiLevel[128];
knownApiLevelCache[value.serializeAsByte()] = new KnownApiLevel(value);
return knownApiLevelCache[apiLevel.serializeAsByte()];   // ← can be null, no check

Combined with the enum→class conversion in 707e8f378b "AndroidApiLevel no longer an enum", AndroidApiLevel lost its enum-identity guarantees, the cache became a sparse 128-slot array with most slots null, and of() lost its dense-array fallback. Any AndroidApiLevel value whose serializeAsByte() slot was not pre-populated — or any path where a non-singleton-equivalent instance reaches of() — returns null, and the caller (e.g. lookup's .max(...), or downstream .asKnownApiLevel().getApiLevel()) NPEs. The matching obfuscated stack frames (f.H() returning null, f$a.a() deref'ing it) are consistent with AndroidApiLevelCompute.of() returning null and the result being treated as a KnownApiLevel.

Suggested fix direction

Adding a null guard (or assertion-with-fallback to unknownValue) inside AndroidApiLevelCompute.of() plus call-site handling in AndroidApiReferenceLevelCache.lookup would mask the symptom; ideally upstream r8 would file under b/356841164 (the bug referenced in both 9.x commits above).

Workaround

Setting <AndroidLinkTool>r8</AndroidLinkTool> (instead of leaving AndroidLinkTool blank) routes the build through R8's tree-shaking branch in R8.cs, which prunes the offending LocationManagerCompat lambdas before they reach the API-modeling pass.

[jonpryor]

@jonathanpeppers: sounds like the above analysis should be used as part of an issue filed with Google wrt r8. :-)

[jonpryor]

@jonathanpeppers: followup Q for the AI: has that issue already been fixed? The r8 tags at: https://r8.googlesource.com/r8/+refs

show tags for 9.1.32, 9.1.33, and 9.3.34. Unfortunately, 9.1.31 is the latest currently listed at maven.google.com.

Maybe the issue has already been fixed but not released?

(Though a quick manual read of the Parent comments until reaching 9.1.31 suggests that it has not been fixed in any of those tags…)

[jonathanpeppers]

Filed at:

• https://issuetracker.google.com/issues/524775142

[jonathanpeppers]

If you can try:

• [testing] Bump R8 to 9.1.43 #11688

Build r8.csproj and use the r8.jar we produce, a Google engineer says it might be fixed with that nightly build.