Attach unattached threads before querying for the current domain

Fixes: dotnet#6211

Mono VM will return a valid AppDomain pointer (both in "legacy" and NET6
cases) only if the current thread is attached to some domain.  It is
possible that when managed code is called from an unattached Java
thread, `mono_domain_get()` will return a `NULL` pointer instead of a
valid one.  If we pass this pointer along to Mono, a segfault may occur
if Mono fails to validate the passed pointer.  Sample code which may
trigger the problem:

```csharp
public override void OnCreate()
{
   AppDomain.CurrentDomain.UnhandledException += (sender, e) =>
   {
       Console.WriteLine("!!! UnhandledException: " + e.ExceptionObject.GetType().FullName);
   };
   base.OnCreate();
}

protected override void OnStart()
{
    base.OnStart();
    Java.Util.Concurrent.Executors.NewSingleThreadExecutor()
        .Execute(new Runnable(() =>
        {
            throw new Exception("Exception from Java Executor!");
        }));
}
```

Possible crashes caused by the above code:

    08-20 15:00:20.619 19214 19241 F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0xb8 in tid 19241 (pool-1-thread-1), pid 19214 (ndroidcrashtest)
    08-20 15:00:20.786 19245 19245 F DEBUG   : backtrace:
    08-20 15:00:20.786  1648  1648 I hwservicemanager: getTransport: Cannot find entry [email protected]::IMapper/default in either framework or device manifest.
    08-20 15:00:20.786 19245 19245 F DEBUG   :       #00 pc 0011b72f  /apex/com.android.runtime/lib/bionic/libc.so (pthread_mutex_lock+31) (BuildId: 471745f0fbbcedb3db1553d5bd6fcd8b)
    08-20 15:00:20.786 19245 19245 F DEBUG   :       #1 pc 0015240d  /data/app/com.companyname.androidcrashtest-sqgk-Mnu8GZM5hKPu1yWEQ==/lib/x86/libmonosgen-2.0.so (mono_domain_assembly_search+61)

    08-20 15:22:04.462 19737 19763 F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0xb8 in tid 19763 (pool-1-thread-1), pid 19737 (ndroidcrashtest)
    08-20 15:22:04.599 19767 19767 F DEBUG   : backtrace:
    08-20 15:22:04.599 19767 19767 F DEBUG   :       #00 pc 0011b72f  /apex/com.android.runtime/lib/bionic/libc.so (pthread_mutex_lock+31) (BuildId: 471745f0fbbcedb3db1553d5bd6fcd8b)
    08-20 15:22:04.599 19767 19767 F DEBUG   :       #1 pc 0026b91a  /data/app/com.companyname.androidcrashtest-JQoYc3YFwZtEoSJlTqX_BA==/lib/x86/libmonosgen-2.0.so (mono_os_mutex_lock+42)

To avoid the above situation, wrap the `mono_domain_get()` call in a
method which checks the return value and, if it's `nullptr`, it calls
`mono_get_root_domain()` and attaches the current thread to that domain.

Author: grendello

src/monodroid/jni/embedded-assemblies.cc

@@ -494,7 +494,7 @@ EmbeddedAssemblies::typemap_java_to_managed (const char *java_type_name)
 		return nullptr;
 	}
 
-	MonoReflectionType *ret = mono_type_get_object (mono_domain_get (), type);
+	MonoReflectionType *ret = mono_type_get_object (utils.get_current_domain (), type);
 	if (XA_UNLIKELY (ret == nullptr)) {
 		log_warn (LOG_ASSEMBLY, "typemap: unable to instantiate managed type '%s'", managed_type_name);
 		return nullptr;
@@ -552,7 +552,7 @@ EmbeddedAssemblies::typemap_java_to_managed (const char *java_type_name)
 	// calls `mono_get_root_domain`. Thus, we can save on a one function call here by passing `nullptr`
 	constexpr MonoDomain *domain = nullptr;
 #else
-	MonoDomain *domain = mono_domain_get ();
+	MonoDomain *domain = utils.get_current_domain ();
 #endif
 	MonoReflectionType *ret = mono_type_get_object (domain, mono_class_get_type (klass));
 	if (ret == nullptr) {

src/monodroid/jni/monodroid-glue.cc

@@ -1741,14 +1741,16 @@ MonodroidRuntime::load_assembly (MonoDomain *domain, jstring_wrapper &assembly)
 		log_debug (LOG_ASSEMBLY, "Dynamically opened assembly %s", mono_assembly_name_get_name (aname));
 	} else
 #endif
-		if (domain != mono_domain_get ()) {
-			MonoDomain *current = mono_domain_get ();
+	{
+		MonoDomain *current = utils.get_current_domain ();
+		if (domain != current) {
 			mono_domain_set (domain, FALSE);
 			mono_assembly_load_full (aname, NULL, NULL, 0);
 			mono_domain_set (current, FALSE);
 		} else {
 			mono_assembly_load_full (aname, NULL, NULL, 0);
 		}
+	}
 
 	mono_assembly_name_free (aname);
 
@@ -2327,7 +2329,7 @@ MonodroidRuntime::Java_mono_android_Runtime_register (JNIEnv *env, jstring manag
 
 	MonoMethod *register_jni_natives = registerType;
 #if !defined (NET6)
-	MonoDomain *domain = mono_domain_get ();
+	MonoDomain *domain = utils.get_current_domain (/* attach_thread_if_needed */ false);
 	mono_jit_thread_attach (domain);
 	// Refresh current domain as it might have been modified by the above call
 	domain = mono_domain_get ();
@@ -2405,7 +2407,7 @@ JNICALL Java_mono_android_Runtime_propagateUncaughtException (JNIEnv *env, [[may
 #if defined (NET6)
 	monodroidRuntime.propagate_uncaught_exception (env, javaThread, javaException);
 #else // def NET6
-	MonoDomain *domain = mono_domain_get ();
+	MonoDomain *domain = utils.get_current_domain ();
 	monodroidRuntime.propagate_uncaught_exception (domain, env, javaThread, javaException);
 #endif // ndef NET6
 }

src/monodroid/jni/osbridge.cc

@@ -1041,7 +1041,7 @@ OSBridge::ensure_jnienv (void)
 	JNIEnv *env;
 	jvm->GetEnv ((void**)&env, JNI_VERSION_1_6);
 	if (env == nullptr) {
-		mono_thread_attach (mono_domain_get ());
+		mono_thread_attach (utils.get_current_domain (/* attach_thread_if_needed */ false));
 		jvm->GetEnv ((void**)&env, JNI_VERSION_1_6);
 	}
 	return env;

src/monodroid/jni/timezones.cc

@@ -33,7 +33,7 @@ init ()
 	if (AndroidEnvironment_NotifyTimeZoneChanged)
 		return;
 
-	Mono_Android_dll                          = utils.monodroid_load_assembly (mono_domain_get (), SharedConstants::MONO_ANDROID_ASSEMBLY_NAME);
+	Mono_Android_dll                          = utils.monodroid_load_assembly (utils.get_current_domain (), SharedConstants::MONO_ANDROID_ASSEMBLY_NAME);
 	Mono_Android_image                        = mono_assembly_get_image (Mono_Android_dll);
 	AndroidEnvironment                        = mono_class_from_name (Mono_Android_image,  SharedConstants::ANDROID_RUNTIME_NS_NAME, SharedConstants::ANDROID_ENVIRONMENT_CLASS_NAME);
 	AndroidEnvironment_NotifyTimeZoneChanged  = mono_class_get_method_from_name (AndroidEnvironment, "NotifyTimeZoneChanged", 0);

src/monodroid/jni/util.cc

@@ -210,7 +210,7 @@ Util::monodroid_load_assembly (MonoDomain *domain, const char *basename)
 	MonoImageOpenStatus   status;
 
 	aname = mono_assembly_name_new (basename);
-	MonoDomain *current = mono_domain_get ();
+	MonoDomain *current = get_current_domain ();
 
 	if (domain != current) {
 		mono_domain_set (domain, FALSE);
@@ -233,7 +233,7 @@ Util::monodroid_load_assembly (MonoDomain *domain, const char *basename)
 MonoObject *
 Util::monodroid_runtime_invoke (MonoDomain *domain, MonoMethod *method, void *obj, void **params, MonoObject **exc)
 {
-	MonoDomain *current = mono_domain_get ();
+	MonoDomain *current = get_current_domain ();
 	if (domain == current) {
 		return mono_runtime_invoke (method, obj, params, exc);
 	}
@@ -247,7 +247,7 @@ Util::monodroid_runtime_invoke (MonoDomain *domain, MonoMethod *method, void *ob
 void
 Util::monodroid_property_set (MonoDomain *domain, MonoProperty *property, void *obj, void **params, MonoObject **exc)
 {
-	MonoDomain *current = mono_domain_get ();
+	MonoDomain *current = get_current_domain ();
 	if (domain == current) {
 		mono_property_set_value (property, obj, params, exc);
 		return;
@@ -289,7 +289,7 @@ MonoClass*
 Util::monodroid_get_class_from_name ([[maybe_unused]] MonoDomain *domain, const char* assembly, const char *_namespace, const char *type)
 {
 #if !defined (NET6)
-	MonoDomain *current = mono_domain_get ();
+	MonoDomain *current = get_current_domain ();
 
 	if (domain != current)
 		mono_domain_set (domain, FALSE);
@@ -318,7 +318,7 @@ Util::monodroid_get_class_from_name ([[maybe_unused]] MonoDomain *domain, const
 MonoClass*
 Util::monodroid_get_class_from_image (MonoDomain *domain, MonoImage *image, const char *_namespace, const char *type)
 {
-	MonoDomain *current = mono_domain_get ();
+	MonoDomain *current = get_current_domain ();
 
 	if (domain != current)
 		mono_domain_set (domain, FALSE);

src/monodroid/jni/util.hh

@@ -32,6 +32,7 @@ constexpr int FALSE = 0;
 #include <jni.h>
 #include <mono/metadata/assembly.h>
 #include <mono/metadata/appdomain.h>
+#include <mono/metadata/threads.h>
 
 #include "monodroid.h"
 #include "jni-wrappers.hh"
@@ -110,6 +111,24 @@ namespace xamarin::android
 			return (log_categories & category) != 0;
 		}
 
+		MonoDomain *get_current_domain (bool attach_thread_if_needed = true) const noexcept
+		{
+			MonoDomain *ret = mono_domain_get ();
+			if (ret != nullptr) {
+				return ret;
+			}
+
+			// It's likely that we got a nullptr because the current thread isn't attached (see
+			// https://github.com/xamarin/xamarin-android/issues/6211), so we need to attach the thread to the root
+			// domain
+			ret = mono_get_root_domain ();
+			if (attach_thread_if_needed) {
+				mono_thread_attach (ret);
+			}
+
+			return ret;
+		}
+
 	private:
 		//char *monodroid_strdup_printf (const char *format, va_list vargs);
 #if !defined (NET6)