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Context: #9921 (comment)

Do tests pass without crashing?

If the tests *do* crash, next step is to *revert* changes to
`AndroidRuntime.cs`.  This should re-introduce failures within
`CreatePeer_ReplaceableDoesNotReplace()`, but would narrow down
what may be in play.

Author: jonpryor

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -173,11 +173,13 @@ public override IntPtr ReleaseLocalReference (ref JniObjectReference value, ref
 		public override void WriteLocalReferenceLine (string format, params object?[] args)
 		{
 			RuntimeNativeMethods._monodroid_gref_log ("[LREF] " + string.Format (CultureInfo.InvariantCulture, format, args));
+			RuntimeNativeMethods._monodroid_gref_log ("\n");
 		}
 
 		public override void WriteGlobalReferenceLine (string format, params object?[] args)
 		{
 			RuntimeNativeMethods._monodroid_gref_log (string.Format (CultureInfo.InvariantCulture, format, args));
+			RuntimeNativeMethods._monodroid_gref_log ("\n");
 		}
 
 		public override JniObjectReference CreateGlobalReference (JniObjectReference value)
@@ -689,7 +691,7 @@ internal void AddPeer (IJavaPeerable value, JniObjectReference reference, IntPtr
 				for (int i = 0; i < targets.Count; ++i) {
 					IJavaPeerable? target;
 					var wref = targets [i];
-					if (ShouldReplaceMapping (wref!, reference, out target)) {
+					if (ShouldReplaceMapping (wref!, reference, value, out target)) {
 						found = true;
 						targets [i] = IdentityHashTargets.CreateWeakReference (value);
 						break;
@@ -747,7 +749,7 @@ internal void AddPeer (IJavaPeerable value, IntPtr handle, JniHandleOwnership tr
 			}
 		}
 
-		bool ShouldReplaceMapping (WeakReference<IJavaPeerable> current, JniObjectReference reference, out IJavaPeerable? target)
+		bool ShouldReplaceMapping (WeakReference<IJavaPeerable> current, JniObjectReference reference, IJavaPeerable value, out IJavaPeerable? target)
 		{
 			target      = null;
 
@@ -771,12 +773,17 @@ bool ShouldReplaceMapping (WeakReference<IJavaPeerable> current, JniObjectRefere
 			// we want the 2nd MCW to replace the 1st, as the 2nd is
 			// the one the dev created; the 1st is an implicit intermediary.
 			//
+			// Meanwhile, a new "replaceable" instance should *not* replace an
+			// existing "replaceable" instance; see dotnet/android#9862.
+			//
 			// [0]: If Java ctor invokes overridden virtual method, we'll
 			// transition into managed code w/o a registered instance, and
 			// thus will create an "intermediary" via
 			// (IntPtr, JniHandleOwnership) .ctor.
-			if ((target.JniManagedPeerState & JniManagedPeerStates.Replaceable) == JniManagedPeerStates.Replaceable)
+			if (target.JniManagedPeerState.HasFlag (JniManagedPeerStates.Replaceable) &&
+					!value.JniManagedPeerState.HasFlag (JniManagedPeerStates.Replaceable)) {
 				return true;
+			}
 
 			return false;
 		}

src/Mono.Android/Android.Runtime/JNIEnv.cs

@@ -1070,6 +1070,7 @@ static int _GetArrayLength (IntPtr array_ptr)
 				ret [i] = targetType == null || targetType.IsInstanceOfType (value)
 					? value
 					: Convert.ChangeType (value, targetType, CultureInfo.InvariantCulture);
+				GC.KeepAlive (value);
 			}
 
 			return ret;

src/Mono.Android/Java.Interop/TypeManager.cs

@@ -359,9 +359,6 @@ static Type monovm_typemap_java_to_managed (string java_type_name)
 
 			try {
 				result = (IJavaPeerable) CreateProxy (type, handle, transfer);
-				if (Runtime.IsGCUserPeer (result.PeerReference.Handle)) {
-					result.SetJniManagedPeerState (JniManagedPeerStates.Replaceable | JniManagedPeerStates.Activatable);
-				}
 			} catch (MissingMethodException e) {
 				var key_handle  = JNIEnv.IdentityHash (handle);
 				JNIEnv.DeleteRef (handle, transfer);
@@ -385,19 +382,31 @@ internal static object CreateProxy (
 			BindingFlags flags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
 			var c = type.GetConstructor (flags, null, XAConstructorSignature, null);
 			if (c != null) {
-				return c.Invoke (new object [] { handle, transfer });
+				var self    = GetUninitializedObject (type);
+				c.Invoke (self, new object [] { handle, transfer });
+				return self;
 			}
 			c = type.GetConstructor (flags, null, JIConstructorSignature, null);
 			if (c != null) {
+				var self                      = GetUninitializedObject (type);
 				JniObjectReference          r = new JniObjectReference (handle);
 				JniObjectReferenceOptions   o = JniObjectReferenceOptions.Copy;
-				var peer = (IJavaPeerable) c.Invoke (new object [] { r, o });
+				c.Invoke (self, new object [] { r, o });
 				JNIEnv.DeleteRef (handle, transfer);
-				return peer;
+				return self;
 			}
 			throw new MissingMethodException (
 					"No constructor found for " + type.FullName + "::.ctor(System.IntPtr, Android.Runtime.JniHandleOwnership)",
 					CreateJavaLocationException ());
+
+			static IJavaPeerable GetUninitializedObject (
+					[DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors)]
+					Type type)
+			{
+				var v   = (IJavaPeerable) System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject (type);
+				v.SetJniManagedPeerState (JniManagedPeerStates.Replaceable | JniManagedPeerStates.Activatable);
+				return v;
+			}
 		}
 
 		public static void RegisterType (string java_class, Type t)

src/native/mono/runtime-base/util.cc

@@ -147,12 +147,10 @@ Util::path_combine (const char *path1, const char *path2)
 void
 Util::create_public_directory (const char *dir)
 {
-	mode_t m = umask (0);
-	int ret = mkdir (dir, 0777);
+	int ret = create_directory (dir, 0777);
 	if (ret < 0) {
 		log_warn (LOG_DEFAULT, "Failed to create directory '{}'. {}", dir, std::strerror (errno));
 	}
-	umask (m);
 }
 
 int

tests/Mono.Android-Tests/Mono.Android-Tests/Mono.Android.NET-Tests.csproj

@@ -46,6 +46,13 @@
     <_DefaultValueAttributeSupport Condition="'$(TrimMode)' == 'full'">true</_DefaultValueAttributeSupport>
   </PropertyGroup>
 
+  <ItemGroup Condition=" '$(Configuration)' == 'Debug' ">
+    <!-- trying to track:
+      JNI ERROR (app bug): accessed deleted Global 0x3056
+    -->
+    <AndroidEnvironment Include="env.txt" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\TestRunner.Core\TestRunner.Core.NET.csproj" />
     <ProjectReference Include="..\..\TestRunner.NUnit\TestRunner.NUnit.NET.csproj" />

tests/Mono.Android-Tests/Mono.Android-Tests/env.txt

@@ -0,0 +1,3 @@
+# Environment Variables and system properties
+# debug.mono.log=gref,default
+debug.mono.debug=1