Context: dotnet/java-interop#1181

Does It Build™?

Context: dotnet/java-interop@6b3637d

The story so far is that some of our unit tests are crashing, and
have been in one form or another since 4332ea0
(the bump to dotnet/java-interop@def5bc0).

The current crash, from the PR build for 7b46391:

	D monodroid-assembly: typemap: assembly 'Java.Interop-Tests' hasn't been loaded yet, attempting a full load
	W monodroid-assembly: typemap: failed to load managed assembly 'Java.Interop-Tests.dll'. No such file or directory
	E monodroid-assembly: typemap: unable to load assembly 'Java.Interop-Tests' when looking up managed type corresponding to Java type 'java/lang/Object'
	I monodroid-timing: [1/5] Typemap.java_to_managed: end, total time; elapsed: 0:0::260000
	W monodroid-assembly: typemap: called from
	W monodroid-assembly: at Java.Interop.TypeManager.GetJavaToManagedType(String )
	W monodroid-assembly:    at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	W monodroid-assembly:    at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
	W monodroid-assembly:    at Java.Lang.Object._GetObject[IIterator](IntPtr , JniHandleOwnership )
	W monodroid-assembly:    at Java.Lang.Object.GetObject[IIterator](IntPtr handle, JniHandleOwnership transfer)
	W monodroid-assembly:    at Android.Runtime.JavaSet.Iterator()
	W monodroid-assembly:    at Android.Runtime.JavaSet`1[[System.String, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetEnumerator()
	W monodroid-assembly:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ProcessArguments()
	W monodroid-assembly:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].OnCreate(Bundle arguments)
	W monodroid-assembly:    at Android.App.Instrumentation.n_OnCreate_Landroid_os_Bundle_(IntPtr jnienv, IntPtr native__this, IntPtr native_arguments)
	W monodroid-assembly:    at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V(_JniMarshal_PPL_V callback, IntPtr jnienv, IntPtr klazz, IntPtr p0)
	E droid.NET_Test: JNI ERROR (app bug): accessed stale Local 0x75  (index 7 in a table of size 7)
	F droid.NET_Test: java_vm_ext.cc:570] JNI DETECTED ERROR IN APPLICATION: use of deleted local reference 0x75

There are *three* "concerning" items here:

 1. typemaps are trying to load `Java.Interop-Tests`, and failing:

        typemap: failed to load managed assembly 'Java.Interop-Tests.dll'. No such file or directory

    @grendello is looking into this.

 2. The binding for `java/lang/Object` is coming from
    Java.Interop-Tests, not Mono.Android (?!)

        typemap: unable to load assembly 'Java.Interop-Tests' when looking up managed type corresponding to Java type 'java/lang/Object'

    dotnet/java-interop#1181 has a fix for this, and we're not
    applying the fix yet because we believe that it will hide (1).

 3. The JNI error, which crashes the process:

        F droid.NET_Test: java_vm_ext.cc:570] JNI DETECTED ERROR IN APPLICATION: use of deleted local reference 0x75
        F droid.NET_Test: java_vm_ext.cc:570]     from void crc643df67da7b13bb6b1.TestInstrumentation_1.n_onCreate(android.os.Bundle)
        F droid.NET_Test: runtime.cc:630] Runtime aborting...
        F droid.NET_Test: runtime.cc:630] Dumping all threads without mutator lock held
        F droid.NET_Test: runtime.cc:630] All threads:
        F droid.NET_Test: runtime.cc:630] DALVIK THREADS (14):
        F droid.NET_Test: runtime.cc:630] "main" prio=5 tid=1 Runnable
        F droid.NET_Test: runtime.cc:630]   | group="" sCount=0 dsCount=0 flags=0 obj=0x729e9d98 self=0x7567e0f51000
        F droid.NET_Test: runtime.cc:630]   | sysTid=9143 nice=0 cgrp=default sched=0/0 handle=0x7567e14daed8
        F droid.NET_Test: runtime.cc:630]   | state=R schedstat=( 1270418000 334229000 139 ) utm=16 stm=110 core=0 HZ=100
        F droid.NET_Test: runtime.cc:630]   | stack=0x7ffcbb3e4000-0x7ffcbb3e6000 stackSize=8192KB
        F droid.NET_Test: runtime.cc:630]   | held mutexes= "abort lock" "mutator lock"(shared held)
        F droid.NET_Test: runtime.cc:630]   native: #00 pc 000000000048df4e  /apex/com.android.runtime/lib64/libart.so (art::DumpNativeStack(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, int, BacktraceMap*, char const*, art::ArtMethod*, void*, bool)+126)
        F droid.NET_Test: runtime.cc:630]   native: #1 pc 00000000005a77c3  /apex/com.android.runtime/lib64/libart.so (art::Thread::DumpStack(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, bool, BacktraceMap*, bool) const+675)
        F droid.NET_Test: runtime.cc:630]   native: #2 pc 00000000005c49cb  /apex/com.android.runtime/lib64/libart.so (art::DumpCheckpoint::Run(art::Thread*)+859)
        F droid.NET_Test: runtime.cc:630]   native: #3 pc 00000000005bcf28  /apex/com.android.runtime/lib64/libart.so (art::ThreadList::RunCheckpoint(art::Closure*, art::Closure*)+456)
        F droid.NET_Test: runtime.cc:630]   native: #4 pc 00000000005bc2e1  /apex/com.android.runtime/lib64/libart.so (art::ThreadList::Dump(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, bool)+1601)
        F droid.NET_Test: runtime.cc:630]   native: #5 pc 0000000000552eb9  /apex/com.android.runtime/lib64/libart.so (art::Runtime::Abort(char const*)+1529)
        F droid.NET_Test: runtime.cc:630]   native: #6 pc 000000000000c873  /system/lib64/libbase.so (android::base::LogMessage::~LogMessage()+611)
        F droid.NET_Test: runtime.cc:630]   native: #7 pc 00000000003ede84  /apex/com.android.runtime/lib64/libart.so (art::JavaVMExt::JniAbort(char const*, char const*)+1604)
        F droid.NET_Test: runtime.cc:630]   native: #8 pc 00000000003ee18a  /apex/com.android.runtime/lib64/libart.so (art::JavaVMExt::JniAbortF(char const*, char const*, ...)+234)
        F droid.NET_Test: runtime.cc:630]   native: #9 pc 00000000005adf31  /apex/com.android.runtime/lib64/libart.so (art::Thread::DecodeJObject(_jobject*) const+785)
        F droid.NET_Test: runtime.cc:630]   native: #10 pc 00000000003def9b  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::ScopedCheck::CheckInstance(art::ScopedObjectAccess&, art::(anonymous namespace)::ScopedCheck::InstanceKind, _jobject*, bool)+91)
        F droid.NET_Test: runtime.cc:630]   native: #11 pc 00000000003de205  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::ScopedCheck::CheckPossibleHeapValue(art::ScopedObjectAccess&, char, art::(anonymous namespace)::JniValueType)+485)
        F droid.NET_Test: runtime.cc:630]   native: #12 pc 00000000003dd732  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::ScopedCheck::Check(art::ScopedObjectAccess&, bool, char const*, art::(anonymous namespace)::JniValueType*)+690)
        F droid.NET_Test: runtime.cc:630]   native: #13 pc 00000000003ce865  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::CheckJNI::GetObjectClass(_JNIEnv*, _jobject*)+837)
        F droid.NET_Test: runtime.cc:630]   native: #14 pc 0000000000017196  /data/app/Mono.Android.NET_Tests-LUUW792fOvX745oAS4jeRQ==/split_config.x86_64.apk (offset 331000) (???)
        F droid.NET_Test: runtime.cc:630]   at crc643df67da7b13bb6b1.TestInstrumentation_1.n_onCreate(Native method)

As per `native #13`, we're (somehow) passing an invalid JNI reference
to `JNIEnv::GetObjectClass()`.

***HOW*** are we passing an invalid JNI reference to
`JNIEnv::GetObjectClass()`?

Attempt to investigate (3) further, by:

 1. Reviewing all calls to `JNIEnv::GetObjectClass()` within this
    repo to see if we could potentially be passing an invalid value.
    The "most obvious" candidate is `TypeManager.CreateInstance()`,
    which calls `JNIEnv::GetObjectClass()` in a loop.

    I'm still not sure if that could possibly be the cause, but
    Just In Case™…

    "Cleanup" some C++ code so that calls to
    `JNIEnv::DeleteLocalReference()` are closer to the
    `JNIEnv::GetObjectClass()` calls.

 2. Update
    `tests/Mono.Android-Tests/Runtime-Microsoft.Android.Sdk/Mono.Android.NET-Tests.csproj`
    to add an `@(AndroidEnvironment)` item, which sets
    `debug.mono.log=gref+,lref+`.  This will enable LREF and GREF
    logging within `adb logcat`, which *may* allow us to track down
    where "local reference 0x75" came from.

Context: dotnet/java-interop#1181
Context: #8625
Context: dotnet/java-interop@005c914

Does It Build™?

PR #8625 has turned into a cluster, *probably* because of
dotnet/java-interop@005c9141, which implicitly requires typemap
support for *non*-`Java.Lang.Object` subclasses such as
the `CallVirtualFromConstructorDerived` test type.

Java.Interop#1181 updates typemap and JCW generation to support
`Java.Interop.JavaObject` and `Java.Interop.JavaException`
subclasses, which will *hopefully* allow the
`CallVirtualFromConstructorDerived`-using tests to work.

Context: xamarin/monodroid@e3e4f123d8
Context: dotnet/java-interop@005c9141
Context: dotnet/java-interop#1181

We've been trying to track down a JNI error which occurs when trying
to use dotnet/java-interop@005c9141, resembling:

	I monodroid-lref: +l+ lrefc 1 handle 0x71/L from thread '(null)'(1)
	D monodroid-gref:    at Android.Runtime.AndroidObjectReferenceManager.CreatedLocalReference(JniObjectReference , Int32& )
	D monodroid-gref:    at Java.Interop.JniRuntime.JniObjectReferenceManager.CreatedLocalReference(JniEnvironmentInfo , JniObjectReference )
	D monodroid-gref:    at Java.Interop.JniEnvironment.LogCreateLocalRef(JniObjectReference )
	D monodroid-gref:    at Java.Interop.JniEnvironment.LogCreateLocalRef(IntPtr )
	D monodroid-gref:    at Java.Interop.JniEnvironment.InstanceMethods.CallObjectMethod(JniObjectReference , JniMethodInfo )
	D monodroid-gref:    at Android.Runtime.JNIEnv.CallObjectMethod(IntPtr , IntPtr )
	D monodroid-gref:    at Android.Runtime.JavaSet.Iterator()
	D monodroid-gref:    at Android.Runtime.JavaSet`1[[System.String, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetEnumerator()
	D monodroid-gref:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ProcessArguments()
	D monodroid-gref:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].OnCreate(Bundle arguments)
	D monodroid-gref:    at Android.App.Instrumentation.n_OnCreate_Landroid_os_Bundle_(IntPtr jnienv, IntPtr native__this, IntPtr native_arguments)
	D monodroid-gref:    at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V(_JniMarshal_PPL_V callback, IntPtr jnienv, IntPtr klazz, IntPtr p0)
	…
	I monodroid-lref: -l- lrefc 0 handle 0x71/L from thread '(null)'(1)
	D monodroid-gref:    at Android.Runtime.AndroidObjectReferenceManager.DeleteLocalReference(JniObjectReference& , Int32& )
	D monodroid-gref:    at Java.Interop.JniRuntime.JniObjectReferenceManager.DeleteLocalReference(JniEnvironmentInfo , JniObjectReference& )
	D monodroid-gref:    at Java.Interop.JniObjectReference.Dispose(JniObjectReference& reference)
	D monodroid-gref:    at Android.Runtime.JNIEnv.DeleteLocalRef(IntPtr )
	D monodroid-gref:    at Android.Runtime.JNIEnv.DeleteRef(IntPtr , JniHandleOwnership )
	D monodroid-gref:    at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	D monodroid-gref:    at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
	D monodroid-gref:    at Java.Lang.Object._GetObject[IIterator](IntPtr , JniHandleOwnership )
	D monodroid-gref:    at Java.Lang.Object.GetObject[IIterator](IntPtr handle, JniHandleOwnership transfer)
	D monodroid-gref:    at Android.Runtime.JavaSet.Iterator()
	D monodroid-gref:    at Android.Runtime.JavaSet`1[[System.String, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetEnumerator()
	D monodroid-gref:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ProcessArguments()
	D monodroid-gref:    at Xamarin.Android.UnitTests.TestInstrumentation`1[[Xamarin.Android.UnitTests.NUnit.NUnitTestRunner, TestRunner.NUnit.NET, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].OnCreate(Bundle arguments)
	D monodroid-gref:    at Android.App.Instrumentation.n_OnCreate_Landroid_os_Bundle_(IntPtr jnienv, IntPtr native__this, IntPtr native_arguments)
	D monodroid-gref:    at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V(_JniMarshal_PPL_V callback, IntPtr jnienv, IntPtr klazz, IntPtr p0)
	D monodroid-gref:
	E droid.NET_Test: JNI ERROR (app bug): accessed stale Local 0x71  (index 7 in a table of size 7)
	F droid.NET_Test: java_vm_ext.cc:570] JNI DETECTED ERROR IN APPLICATION: use of deleted local reference 0x71
	…
	F droid.NET_Test: runtime.cc:630]   native: #13 pc 00000000003ce865  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::CheckJNI::GetObjectClass(_JNIEnv*, _jobject*)+837)

This has been "fun".

The problem:

 1. dotnet/java-interop@005c9141 relies on/requires additional
    typemaps in order to "fix" some linker warnings.

    This felt "fine" at the time.

 2. However, the Java.Interop *unit tests* which test (1) involve
    "hand-written" typemap entries to allow things to work.

 3. In .NET Android, those hand-written typemap entries aren't used;
    instead, the normal .NET Android typemaps are used.

 4. .NET Android typemaps did not contain entries for the types
    introduced in (2), so various tests started failing.

 5. dotnet/java-interop#1181 attempts to fix this by extending
    Java Callable Wrappers and associated typemaps to support
    `Java.Interop.JavaObject` subclasses, which brings the new types
    in (2) into the "normal" typemap fold.

 6. However, some of those types "alias" `java.lang.Object`, and --
    for some "bizarre" random ordering reason -- a type within
    `Java.Interop-Tests.dll` becomes the preferred `System.Type`
    to return when looking up `java/lang/Object`.

 7. Which would *probably* be okay (if *really* weird), except that
    `GetJavaToManagedType()` returns null when the binding is within
    an assembly that hasn't been loaded yet.  As this codepath is
    getting hit during app startup, `Java.Interop-Tests` hasn't been
    loaded, so `GetJavaToManagedType()` returns null.

 8. Which means we're now in the scenario of being unable to find a
    binding/"wrapper class" for `java/lang/Object`, which we consider
    to be an error state.

Because it's an error state, we dutifully throw.

…except we've never actually hit this error state before --
HOW COULD WE?! -- which means we've found a bug in our error handling.

Quick, find the problem!

	JNIEnv.DeleteRef (handle, transfer);
	throw new NotSupportedException (
			FormattableString.Invariant ($"Internal error finding wrapper class for '{JNIEnv.GetClassNameFromInstance (handle)}'. (Where is the Java.Lang.Object wrapper?!)"),
			CreateJavaLocationException ());

The problem is a "use after free" bug:
`JNIEnv.DeleteRef(handle, transfer)` *invalidates `handle`*, and then
*immediately* afterward we call
`JNIEnv.GetClassNameFromInstance(handle)`, on the now invalid value.

BOOM goes the Android runtime.

(The `DeleteRef()` call was introduced in xamarin/monodroid@e3e4f123d8,
on 2011-Oct-19.  Over 12 years to encounter this scenario!)

Unfortunately, *just* fixing the "use-after-free" bug is insufficient;
if we throw that `NotSupportedException`, things *will* break
elsewhere.  We'll just have an "elegant unhandled exception" app crash
instead of a "THE WORLD IS ENDING" failed assertion crash.

We could go with the simple fix for the crash, but this means that in
order to integrate dotnet/java-interop@005c9141 &
dotnet/java-interop#1181 we'd have to figure out how to *ensure* that
`java/lang/Object` is bound as `Java.Lang.Object, Mono.Android`, not
`Java.InteropTests.JavaLangRemappingTestObject, Java.Interop-Tests`.

There may be a *slightly* more complicated fix which fixes both issues:
consider the `-l-` callstack:

	at Android.Runtime.JNIEnv.DeleteRef(IntPtr , JniHandleOwnership )
	at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
	at Java.Lang.Object._GetObject[IIterator](IntPtr , JniHandleOwnership )
	at Java.Lang.Object.GetObject[IIterator](IntPtr handle, JniHandleOwnership transfer)
	at Android.Runtime.JavaSet.Iterator()

This is part of a generic `Object.GetObject<IIterator>()` invocation!
Additionally, because `IIterator` is an interface, in *normal* use
the `type` variable within `TypeManager.CreateInstance()` would be
`Java.Lang.Object, Mono.Android` and then ~immediately "discarded"
because `Java.Lang.Object` cannot be assigned to `IIterator`.

If we move the type compatibility check to *before* the
`type == null` check, we *may* also fix the
"`java/lang/Object` is bound as some unloadable type" issue.

Let's try that!

`acw-map.txt` contains mappings from .NET types to Java types,
and implicitly vice-versa; see (TODO commit).

*Normally* it contains three entries:

 1. The fully-qualified .NET type name
 2. The .NET type name, no assembly
 3. (2) with a lowercased namespace name.

For example:

	Mono.Android_Test.Library.CustomTextView, Mono.Android-Test.Library.NET;crc6456ab8145c81c4100.CustomTextView
	Mono.Android_Test.Library.CustomTextView;crc6456ab8145c81c4100.CustomTextView
	mono.android_test.library.CustomTextView;crc6456ab8145c81c4100.CustomTextView

However, when XA4214 is emitted, there is a "collision" on the
.NET side (but *not* the Java side); (2) and (3) are *ambiguous*,
so one .NET type is arbitrarily chosen.

The first line is still possible, because of assembly qualification.

Enter ``Java.InteropTests.GenericHolder`1``: this type is present in
*both* `Java.Interop-Tests.dll` *and* `Mono.Android-Tests.dll`.
Before dotnet/java-interop#1181, this was "fine" because the
`GenericHolder<T>` within `Java.Interop-Tests.dll` did not participate
in typemap generation.  Now it does, resulting in the XA4214.
XA4214 *also* means that instead of three lines, it's *one* line:

	Java.InteropTests.GenericHolder`1, Mono.Android.NET-Tests;crc641855b07eca6dcc03.GenericHolder_1

Enter `<R8/>`, which parses `acw-map.txt` to create a
`proguard_project_primary.cfg` file.  `<R8/>` did it's *own* parsing
of `acw-map.txt`, parsing only *one of every three lines*, on the
assumption that *all* entries took three lines.

This breaks in the presence of XA4214, because some entries only take
one line, not three lines.

Update `<R8/>` to instead use `MonoAndroidHelper.LoadMapFile()`,
which reads all lines within `acw-map.txt`.  This should result in
a `proguard_project_primary.cfg` file which properly contains a
`-keep` entry for `crc641855b07eca6dcc03.GenericHolder_1`.

Shouldn't change anything on this side…

Context: dotnet/java-interop#1165
Context: dotnet/java-interop@005c914
Context: #8543
Context: dotnet/java-interop@07c7300
Context: #8625
Context: xamarin/monodroid@e3e4f12
Context: xamarin/monodroid@a04b73b
Context: efbec22

Changes: dotnet/java-interop@8b85462...07c7300

  * dotnet/java-interop@07c73009: [Java.Interop] Typemap support for JavaObject & `[JniTypeSignature]` (dotnet/java-interop#1181)
  * dotnet/java-interop@d529f3be: Bump to xamarin/xamarin-android-tools/main@ed102fc (dotnet/java-interop#1182)
  * dotnet/java-interop@def5bc0d: [ci] Add API Scan job (dotnet/java-interop#1178)
  * dotnet/java-interop@d5afa0af: [invocation-overhead] Add generated source files (dotnet/java-interop#1175)
  * dotnet/java-interop@473ef74c: Bump to xamarin/xamarin-android-tools/main@4889bf0 (dotnet/java-interop#1172)
  * dotnet/java-interop@005c9141: [Java.Interop] Avoid `Type.GetType()` in `ManagedPeer` (dotnet/java-interop#1168)
  * dotnet/java-interop@0f1efebd: [Java.Interop] Use PublicApiAnalyzers to ensure we do not break API (dotnet/java-interop#1170)

(From the "infinite scream" department…)

It started with a desire to remove some linker warnings
(dotnet/java-interop#1165):

	external/Java.Interop/src/Java.Interop/Java.Interop/ManagedPeer.cs(93,19,93,112):
	warning IL2057: Unrecognized value passed to the parameter 'typeName' of method 'System.Type.GetType(String, Boolean)'.
	It's not possible to guarantee the availability of the target type.

dotnet/java-interop@005c9141 attempted to fix this by requiring the
use of "typemaps" mapping Java type signatures to managed types,
replacing e.g.:

	Type            type            = Type.GetType ("Example.Type, AssemblyName", throwOnError: true)!;
	Type[]          parameterTypes  = GetParameterTypes ("System.Int32:System.Int32");
	ConstructorInfo ctor            = type.GetConstructor (ptypes);
	// ctor=Example.Type(int, int) constructor

with (not exactly, but for expository purposes):

	Type            type            = GetTypeFromSignature("crc64…/Type");
	Type[]          parameterTypes  = GetConstructorCandidateParameterTypes ("(II)V");
	ConstructorInfo ctor            = type.GetConstructor (ptypes);
	// ctor=Example.Type(int, int) constructor
	
among other changes.

This was a *significant* change that would alter *Java.Interop*
semantics but *not* .NET Android semantics -- .NET Android uses
`Java.Interop.TypeManager.n_Activate()` (in this repo) for Java-side
"activation" scenarios, not `Java.Interop.ManagedPeer` -- so in an
abundance of caution we did a manual integration test in
#8543 to make sure nothing broke before
merging it.

Something was apparently "off" in that integration.  (We're still not
sure what was off, or why it was completely green.)

Ever since dotnet/java-interop@005c9141 was merged, every attempt to
bump xamarin/Java.Interop has failed, in a number of ways described
below.  However, instead of reverting dotnet/java-interop@005c9141
we took this as an opportunity to understand *how and why* things
were failing, as apparently we had encountered some *long-standing*
corner cases in How Things Work.

The oversights and failures include:

 1. In order to make the Java.Interop unit tests work in .NET Android,
    the (largely hand-written) Java.Interop test types *also* need to
    participate with .NET Android typemap support, so that there is a
    typemap entry mapping `net/dot/jni/test/GenericHolder` to
    `Java.InteropTests.GenericHolder<T>` and vice-versa.

    dotnet/java-interop@07c73009 updates
    `Java.Interop.Tools.JavaCallableWrappers` to support creating
    typemap entries for `Java.Interop.JavaObject` subclasses,
    introducing a new `TypeDefinition.HasJavaPeer()` extension method.

 2. (1) meant that, for the first time ever, types in
    `Java.Interop-Tests` participated in .NET Android type mapping.
    This *sounds* fine, except that `Java.Interop-Tests` contains
    "competing bindings" for `java.lang.Object`:

        [JniTypeSignature ("java/lang/Object", GenerateJavaPeer=false)]
        partial class JavaLangRemappingTestObject : JavaObject {
        }

 3. (2) means that, for the first time ever, we *could* have the
    typemap entry for `java/lang/Object` map to
    `Java.InteropTests.JavaLangRemappingTestObject, Java.Interop-Tests`,
    *not* `Java.Lang.Object, Mono.Android`.

    Arguably a bug, arguably "meh", but this setup triggered some
    never previously encountered error conditions:

 4. `EmbeddedAssemblies::typemap_java_to_managed()` within
    `libmonodroid.so` returns a `System.Type` that corresponds to a
    JNI type.  `typemap_java_to_managed()` has a bug/corner case
    wherein it will only provide `Type` instances from assemblies
    which have already been loaded.

    Early in startup, `Java.Interop-Tests` hasn't been loaded yet, so
    when `java/lang/Object` was mapped to
    `Java.InteropTests.JavaLangRemappingTestObject, Java.Interop-Tests`,
    `typemap_java_to_managed()` would return `null`.

    This is a bug/corner case, which is being investigated in
    #8625.

 5. Calls to `Java.Lang.Object.GetObject<T>()` call
    `Java.Interop.TypeManager.CreateInstance()`, which loops through
    the type and all base types to find a known binding/wrapper.
    Because of (3)+(4), if (when) we try to find the wrapper for
    `java/lang/Object`, we would find *no* mapping.

    This would cause an `JNI DETECTED ERROR IN APPLICATION` *crash*.

    This was due to a "use after free" bug.

    See the "TypeManager.CreateInstance() Use After Free Bug" section.

 6. Once (5) is fixed we encounter our next issue: the
    `Java.InteropTests.JnienvTest.NewOpenGenericTypeThrows()` unit
    test started failing because
    `crc641855b07eca6dcc03.GenericHolder_1` couldn't be found.

    This was caused by a bug in `acw-map.txt` parsing within `<R8/>`.

    See the "`<R8/>` and `acw-map.txt` parsing.`" section.

 7. Once (6) was fixed, (3) caused a *new* set of failures:
    multiple tests started failing because `java/lang/Object` was
    being mapped to the wrong managed type.

    (3) becomes less "meh" and more "definitely a bug".

    See the "Correct `java/lang/Object` mappings" section.

*Now* things should work reliably.


~~ TypeManager.CreateInstance() Use After Free Bug ~~

On 2011-Oct-19, xamarin/monodroid@e3e4f123d8 introduced a
use-after-free bug within `TypeManager.CreateInstance()`:

	JNIEnv.DeleteRef (handle, transfer);
	throw new NotSupportedException (
	        FormattableString.Invariant ($"Internal error finding wrapper class for '{JNIEnv.GetClassNameFromInstance (handle)}'. (Where is the Java.Lang.Object wrapper?!)"),
	        CreateJavaLocationException ());

`handle` *cannot be used* after `JNIEnv.DeleteRef(handle)`.
Failure to do so results in a `JNI DETECTED ERROR IN APPLICATION`
crash; with `adb shell setprop debug.mono.log lref+` set, we see:

	I monodroid-lref: +l+ lrefc 1 handle 0x71/L from thread '(null)'(1)
	D monodroid-gref:    at Android.Runtime.AndroidObjectReferenceManager.CreatedLocalReference(JniObjectReference , Int32& )
	D monodroid-gref:    at Java.Interop.JniRuntime.JniObjectReferenceManager.CreatedLocalReference(JniEnvironmentInfo , JniObjectReference )
	D monodroid-gref:    at Java.Interop.JniEnvironment.LogCreateLocalRef(JniObjectReference )
	D monodroid-gref:    at Java.Interop.JniEnvironment.LogCreateLocalRef(IntPtr )
	D monodroid-gref:    at Java.Interop.JniEnvironment.InstanceMethods.CallObjectMethod(JniObjectReference , JniMethodInfo )
	D monodroid-gref:    …
	…
	I monodroid-lref: -l- lrefc 0 handle 0x71/L from thread '(null)'(1)
	D monodroid-gref:    at Android.Runtime.AndroidObjectReferenceManager.DeleteLocalReference(JniObjectReference& , Int32& )
	D monodroid-gref:    at Java.Interop.JniRuntime.JniObjectReferenceManager.DeleteLocalReference(JniEnvironmentInfo , JniObjectReference& )
	D monodroid-gref:    at Java.Interop.JniObjectReference.Dispose(JniObjectReference& reference)
	D monodroid-gref:    at Android.Runtime.JNIEnv.DeleteLocalRef(IntPtr )
	D monodroid-gref:    at Android.Runtime.JNIEnv.DeleteRef(IntPtr , JniHandleOwnership )
	D monodroid-gref:    at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	D monodroid-gref:    at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
	D monodroid-gref:    at Java.Lang.Object._GetObject[IIterator](IntPtr , JniHandleOwnership )
	D monodroid-gref:    at Java.Lang.Object.GetObject[IIterator](IntPtr handle, JniHandleOwnership transfer)
	D monodroid-gref:    …
	D monodroid-gref:
	E droid.NET_Test: JNI ERROR (app bug): accessed stale Local 0x71  (index 7 in a table of size 7)
	F droid.NET_Test: java_vm_ext.cc:570] JNI DETECTED ERROR IN APPLICATION: use of deleted local reference 0x71
	…
	F droid.NET_Test: runtime.cc:630]   native: #13 pc 00000000003ce865  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::CheckJNI::GetObjectClass(_JNIEnv*, _jobject*)+837)

The immediate fix is Don't Do That™; use a temporary:

	class_name = JNIEnv.GetClassNameFromInstance (handle);
	JNIEnv.DeleteRef (handle, transfer);
	throw new NotSupportedException (
	        FormattableString.Invariant ($"Internal error finding wrapper class for '{class_name}'. (Where is the Java.Lang.Object wrapper?!)"),
	        CreateJavaLocationException ());

Unfortunately, *just* fixing the "use-after-free" bug is insufficient;
if we throw that `NotSupportedException`, things *will* break
elsewhere.  We'll just have an "elegant unhandled exception" app crash
instead of a "THE WORLD IS ENDING" failed assertion crash.

We could go with the simple fix for the crash, but this means that in
order to integrate dotnet/java-interop@005c9141 &
dotnet/java-interop@07c73009 we'd have to figure out how to *ensure*
that `java/lang/Object` is bound as `Java.Lang.Object, Mono.Android`,
not `Java.InteropTests.JavaLangRemappingTestObject, Java.Interop-Tests`.
(We actually need to do this *anyway*; see the
"Correct `java/lang/Object` mappings" section.  At the time we I was
trying to *avoid* special-casing `Mono.Android.dll`…)

There is a*slightly* more complicated approach which fixes (5)
while supporting (4) `typemap_java_to_managed()` returning null;
consider the `-l-` callstack:

	at Android.Runtime.JNIEnv.DeleteRef(IntPtr , JniHandleOwnership )
	at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	at Java.Lang.Object.GetObject(IntPtr , JniHandleOwnership , Type )
	at Java.Lang.Object._GetObject[IIterator](IntPtr , JniHandleOwnership )
	at Java.Lang.Object.GetObject[IIterator](IntPtr handle, JniHandleOwnership transfer)
	at Android.Runtime.JavaSet.Iterator()

This is part of a generic `Object.GetObject<IIterator>()` invocation!
Additionally, because `IIterator` is an interface, in *normal* use
the `type` variable within `TypeManager.CreateInstance()` would be
`Java.Lang.Object, Mono.Android` and then *immediately discarded*
because `Java.Lang.Object` cannot be assigned to `IIterator`.

Moving the type compatibility check to *before* the
`type == null` check fixes *an* issue with `typemap_java_to_managed()`
returning null.


~~ `<R8/>` and `acw-map.txt` parsing.` ~~

There are many ways for Android+Java code to refer to managed types.

For example, consider the following View subclass:

	namespace Example {
	  partial class MyCoolView : Android.Views.View {
	    // …
	  }
	}

Within layout `.axml` files, you can mention an `Android.Views.View`
subclass by:

  * Using the .NET Full Class Name as an element name.

        <Example.MyCoolView />

  * Using the .NET Full Class Name with a *lowercased* namespace
    name as the element name.

        <example.MyCoolView />

  * Use the Java-side name directly.

        <crc64….NiftyView />

Within Fragments, you can also use the *assembly-qualified name*:

	<fragment class="Example.MyCoolView, AssemblyName" />

At build time, all instances of the .NET type names will be
*replaced* with the Java type names before the Android toolchain
processes the files.

The association between .NET type names and Java names is stored
within `$(IntermediateOutputPath)acw-map.txt`, which was introduced
in xamarin/monodroid@a04b73b3.

*Normally* `acw-map.txt` contains three entries:

 1. The fully-qualified .NET type name
 2. The .NET type name, no assembly
 3. (2) with a lowercased namespace name, *or* the `[Register]`
    value, if provided.

For example:

	Mono.Android_Test.Library.CustomTextView, Mono.Android-Test.Library.NET;crc6456ab8145c81c4100.CustomTextView
	Mono.Android_Test.Library.CustomTextView;crc6456ab8145c81c4100.CustomTextView   
	mono.android_test.library.CustomTextView;crc6456ab8145c81c4100.CustomTextView   
	Java.InteropTests.GenericHolder`1, Java.Interop-Tests;net.dot.jni.test.tests.GenericHolder
	Java.InteropTests.GenericHolder`1;net.dot.jni.test.tests.GenericHolder          
	net.dot.jni.test.tests.GenericHolder;net.dot.jni.test.tests.GenericHolder    

However, when warning XA4214 is emitted (efbec22), there is a
"collision" on the .NET side (but *not* the Java side); (2) and (3)
are potentially *ambiguous*, so one .NET type is arbitrarily chosen.
(Collisions on the Java side result in XA4215 *errors*.)

The first line is still possible, because of assembly qualification.

Enter ``Java.InteropTests.GenericHolder`1``: this type is present in
*both* `Java.Interop-Tests.dll` *and* `Mono.Android-Tests.dll`.
dotnet/java-interop@07c73009, this was "fine" because the
`GenericHolder<T>` within `Java.Interop-Tests.dll` did not participate
in typemap generation.  Now it does, resulting in the XA4214 warning.
XA4214 *also* means that instead of three lines, it's *one* line:

	Java.InteropTests.GenericHolder`1, Mono.Android.NET-Tests;crc641855b07eca6dcc03.GenericHolder_1

Enter `<R8/>`, which parses `acw-map.txt` to create a
`proguard_project_primary.cfg` file.  `<R8/>` did it's *own* parsing
of `acw-map.txt`, parsing only *one of every three lines*, on the
assumption that *all* entries took three lines.

This breaks in the presence of XA4214, because some entries only take
one line, not three lines.  This in turn meant that
`proguard_project_primary.cfg` could *miss* types, which could mean
that `r8` would *remove* the unspecified types, resulting in
`ClassNotFoundException` at runtime:

	Java.Lang.ClassNotFoundException : crc641855b07eca6dcc03.GenericHolder_1
	----> Java.Lang.ClassNotFoundException : Didn't find class "crc641855b07eca6dcc03.GenericHolder_1" on path: DexPathList[[zip file "/data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/base.apk", zip file "/data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/split_config.x86_64.apk", zip file "/data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/split_config.xxhdpi.apk"],nativeLibraryDirectories=[/data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/lib/x86_64, /system/fake-libs64, /data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/base.apk!/lib/x86_64, /data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/split_config.x86_64.apk!/lib/x86_64, /data/app/Mono.Android.NET_Tests-2stBqO43ov5F6bHfYemJHQ==/split_config.xxhdpi.apk!/lib/x86_64, /system/lib64, /system/product/lib64]]
	   at Java.Interop.JniEnvironment.StaticMethods.CallStaticObjectMethod(JniObjectReference , JniMethodInfo , JniArgumentValue* )
	   at Android.Runtime.JNIEnv.FindClass(String )

Update `<R8/>` to instead use `MonoAndroidHelper.LoadMapFile()`,
which reads all lines within `acw-map.txt`.  This results in a
`proguard_project_primary.cfg` file which properly contains a `-keep`
entry for XA4214-related types, such as
`crc641855b07eca6dcc03.GenericHolder_1`.


~~ Correct `java/lang/Object` mappings ~~`

Previous valiant efforts to allow `java/lang/Object` to be mapped to
"anything", not just `Java.Lang.Object, Mono.Android`, eventually
resulted in lots of unit test failures, e.g.:

`Android.RuntimeTests.XmlReaderPullParserTest.ToLocalJniHandle()`:

	System.NotSupportedException : Unable to activate instance of type Java.InteropTests.JavaLangRemappingTestObject from native handle 0x19 (key_handle 0x2408476).
	----> System.MissingMethodException : No constructor found for Java.InteropTests.JavaLangRemappingTestObject::.ctor(System.IntPtr, Android.Runtime.JniHandleOwnership)
	----> Java.Interop.JavaLocationException : Exception_WasThrown, Java.Interop.JavaLocationException
	   at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )
	   at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership )
	   at Android.Runtime.XmlResourceParserReader.FromNative(IntPtr , JniHandleOwnership )
	   at Android.Runtime.XmlResourceParserReader.FromJniHandle(IntPtr handle, JniHandleOwnership transfer)
	   at Android.Content.Res.Resources.GetXml(Int32 )
	   at Android.RuntimeTests.XmlReaderPullParserTest.ToLocalJniHandle()
	   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
	   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object , BindingFlags )
	--MissingMethodException
	   at Java.Interop.TypeManager.CreateProxy(Type , IntPtr , JniHandleOwnership )
	   at Java.Interop.TypeManager.CreateInstance(IntPtr , JniHandleOwnership , Type )

With a partially heavy heart, we need to special-case typemap entries
by processing `Mono.Android.dll` *first*, so that it gets first dibs
at bindings for `java/lang/Object` and other types.

Update `NativeTypeMappingData` to process types from `Mono.Android`
before processing any other module.

Note that the special-casing needs to happen in `NativeTypeMappingData`
because typemaps were formerly processed in *sorted module order*, in
which the sort order is based on the *byte representation* of the
module's MVID (a GUID).  Additionally, *linking changes the MVID*,
which means module order is *effectively random*.  Consequently,
trying to special case typemap ordering anywhere else is ineffective.


~~ Other ~~

Update `JavaCompileToolTask` to log the contents of its response file.

Update LLVM-IR -related types within
`src/Xamarin.Android.Build.Tasks/Utilities` to use `TaskLoggingHelper`
for logging purposes, *not* `Action<string>`.  Update related types
to accept `TaskLoggingHelper`, so that we can more easily add
diagnostic messages to these types in the future.

Context: dotnet/java-interop@005c914
Context: dotnet/java-interop#1181
Context: 25d1f00

When attempting to bump to dotnet/java-interop@005c9141, multiple
unit tests would fail, e.g.

	Java.Lang.LinkageError : net.dot.jni.test.CallVirtualFromConstructorDerived
	----> System.NotSupportedException : Could not find System.Type corresponding to Java type JniTypeSignature(TypeName=net/dot/jni/test/CallVirtualFromConstructorDerived ArrayRank=0 Keyword=False) .

This happened because dotnet/java-interop@005c9141 implicitly
required that typemaps exist for `Java.Interop.JavaObject` subclasses.

Fair enough; enter xamasrin/java.interop#1181, which added support to
`Java.Interop.Tools.JavaCallableWrappers` to emit typemaps for
`Java.Interop.JavaObject` subclasses.

That caused *crashes* in `tests/Mono.Android-Tests`:

	E droid.NET_Test: JNI ERROR (app bug): accessed stale Local 0x75  (index 7 in a table of size 7)
	F droid.NET_Test: java_vm_ext.cc:570] JNI DETECTED ERROR IN APPLICATION: use of deleted local reference 0x75
	…
	F droid.NET_Test: runtime.cc:630]   native: #13 pc 00000000003ce865  /apex/com.android.runtime/lib64/libart.so (art::(anonymous namespace)::CheckJNI::GetObjectClass(_JNIEnv*, _jobject*)+837)

The immediate cause of the crash was a "use after free" bug within
`TypeManager.CreateInstance()` in a never-hit-before error path; the
"use after free" bug was fixed in 25d1f00.

However, the cause of the hitting a never-hit-before error path is
because `EmbeddedAssemblies::typemap_java_to_managed()` would only
map Java types to `System.Type` instances for assemblies that have
*already been loaded*.  If the assembly had not yet been loaded, then
`EmbeddedAssemblies::typemap_java_to_managed()` would return `null`,
and if the binding it couldn't find happens to be for
`java.lang.Object`, we hit the (buggy!) "Where is the Java.Lang.Object
wrapper" error condition.

Commit 25d1f00 fixes that and a great many other related issues.

What's left is `EmbeddedAssemblies::typemap_java_to_managed()`:
it should *never* return null *unless* there is no typemap at all.
Whether the target assembly has been loaded or not should be
irrelevant.

Update `EmbeddedAssemblies::typemap_java_to_managed()` so that it
will load the target assembly if necessary.

Additionally, before we figured out that we had a "use after free"
bug, all we had to go on was that *something* related to
`JNIEnv::GetObjectClass()` was involved.  Review JNI usage around
`JNIEnv::GetObjectClass()` and related invocations, and cleanup:

  * Simplify logic related to `JNIEnv::DeleteLocalRef()`.
  * Decrease scope of local variables.
  * Clear variables passed to `JNIEnv.DeleteLocalRef()`.

Co-authored-by: Jonathan Pryor <[email protected]>
Co-authored-by: Marek Habersack <[email protected]>