[One .NET] Enable support for AssemblyLoadContext (#5940)

Context: https://docs.microsoft.com/dotnet/api/system.runtime.loader.assemblyloadcontext?view=net-5.0

.NET Core -- and thus, .NET 5+ -- removed most support for
`System.AppDomain` -- technically, there still exists
a single `AppDomain`, but creation of new ones is no longer possible
-- with [`System.Runtime.Loader.AssemblyLoadContext`][0] acting as
the replacement for *some* previous `AppDomain` functionality.

TL;DR: `AssemblyLoadContext` allows (potentially) loading and
unloading assemblies, but *doesn't* allow creating an in-process
"sandbox" like `AppDomain` originally did.
([Code Access Security][1] was deprecated by .NET Framework 4 and
[isn't present in .NET 5][2]; `AppDomain` for sandboxing purposes
was, in retrospect, rarely a good idea.)

Commit 0cd890b introduced partial support for using
`AssemblyLoadContext`, but it was necessarily incomplete until after
[dotnet/runtime#53308][3] and other fixes landed.

Add support for calling the new `AssemblyLoadContext`-oriented MonoVM
functions to load an assembly into either the default
`AssemblyLoadContext` (early in the startup process) or into the
application-created context later on during application run time.
MonoVM also adds new preload hooks which work with the
`AssemblyLoadContext` instead of the older AppDomains.

[0]: https://docs.microsoft.com/dotnet/api/system.runtime.loader.assemblyloadcontext?view=net-5.0
[1]: https://docs.microsoft.com/previous-versions/dotnet/framework/code-access-security/code-access-security
[2]: https://docs.microsoft.com/dotnet/core/compatibility/core-libraries/5.0/code-access-security-apis-obsolete#reason-for-change
[3]: dotnet/runtime#53308

src/monodroid/jni/embedded-assemblies.cc

@@ -130,21 +130,45 @@ EmbeddedAssemblies::get_assembly_data (const MonoBundledAssembly *e, char*& asse
 	}
 }
 
+#if defined (NET6)
 MonoAssembly*
-#if defined (NET6) && defined (NET6_ALC_WORKS)
-EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, MonoAssemblyLoadContextGCHandle alc_gchandle, MonoError *error)
-#else // !(def NET6 && def NET6_ALC_WORKS)
-EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, bool ref_only)
-#endif // def NET6 && def NET6_ALC_WORKS
+EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, MonoAssemblyLoadContextGCHandle alc_gchandle, [[maybe_unused]] MonoError *error)
 {
-#if defined (NET6)
-	// NET6 doesn't support reference-only loads, define the variable here to minimize ifdefs in the code below
-#if defined (NET6_ALC_WORKS)
-	constexpr bool ref_only = false;
-#else // def NET6_ALC_WORKS
-	ref_only = false;
-#endif // ndef NET6_ALC_WORKS
+	auto loader = [&] (char *assembly_data, uint32_t assembly_data_size, const char *name) -> MonoImage* {
+		return mono_image_open_from_data_alc (
+			alc_gchandle,
+			assembly_data,
+			assembly_data_size,
+			0 /* need_copy */,
+			nullptr /* status */,
+			name
+		);
+	};
+
+	return open_from_bundles (aname, loader, false /* ref_only */);
+}
 #endif // def NET6
+
+MonoAssembly*
+EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, bool ref_only)
+{
+	auto loader = [&] (char *assembly_data, uint32_t assembly_data_size, const char *name) -> MonoImage* {
+		return mono_image_open_from_data_with_name (
+			assembly_data,
+			assembly_data_size,
+			0,
+			nullptr,
+			ref_only,
+			name
+		);
+	};
+
+	return open_from_bundles (aname, loader, ref_only);
+}
+
+MonoAssembly*
+EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, std::function<MonoImage*(char*, uint32_t, const char*)> loader, bool ref_only)
+{
 	const char *culture = mono_assembly_name_get_culture (aname);
 	const char *asmname = mono_assembly_name_get_name (aname);
 
@@ -185,19 +209,7 @@ EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, bool ref_only)
 		}
 
 		get_assembly_data (e, assembly_data, assembly_data_size);
-
-#if defined (NET6) && defined (NET6_ALC_WORKS)
-		image = mono_image_open_from_data_alc (
-			alc_gchandle,
-			assembly_data,
-			assembly_data_size,
-			0 /* need_copy */,
-			nullptr /* status */,
-			name.get ()
-		);
-#else // (def NET6 && def NET6_ALC_WORKS)
-		image = mono_image_open_from_data_with_name (assembly_data, assembly_data_size, 0, nullptr, ref_only, name.get ());
-#endif // !(def NET6 && def NET6_ALC_WORKS)
+		image = loader (assembly_data, assembly_data_size, name.get ());
 		if (image == nullptr) {
 			continue;
 		}
@@ -215,51 +227,49 @@ EmbeddedAssemblies::open_from_bundles (MonoAssemblyName* aname, bool ref_only)
 		log_info_nocheck (LOG_ASSEMBLY, "open_from_bundles: loaded assembly: %p\n", a);
 	}
 
-#if defined (NET6) && defined (NET6_ALC_WORKS)
-	if (error != nullptr) {
-		error->error_code = a == nullptr ? MONO_ERROR_NONE : MONO_ERROR_FILE_NOT_FOUND;
-	}
-#endif // def NET6 && def NET6_ALC_WORKS
 	return a;
 }
 
-#if defined (NET6) && defined (NET6_ALC_WORKS)
+#if defined (NET6)
 MonoAssembly*
 EmbeddedAssemblies::open_from_bundles (MonoAssemblyLoadContextGCHandle alc_gchandle, MonoAssemblyName *aname, [[maybe_unused]] char **assemblies_path, [[maybe_unused]] void *user_data, MonoError *error)
 {
-	log_warn (LOG_DEFAULT, __PRETTY_FUNCTION__);
 	return embeddedAssemblies.open_from_bundles (aname, alc_gchandle, error);
 }
-#else // def NET6 && def NET6_ALC_WORKS
+#else // def NET6
+MonoAssembly*
+EmbeddedAssemblies::open_from_bundles_refonly (MonoAssemblyName *aname, [[maybe_unused]] char **assemblies_path, [[maybe_unused]] void *user_data)
+{
+	return embeddedAssemblies.open_from_bundles (aname, true);
+}
+#endif // ndef NET6
+
 MonoAssembly*
 EmbeddedAssemblies::open_from_bundles_full (MonoAssemblyName *aname, [[maybe_unused]] char **assemblies_path, [[maybe_unused]] void *user_data)
 {
 	return embeddedAssemblies.open_from_bundles (aname, false);
 }
 
-MonoAssembly*
-EmbeddedAssemblies::open_from_bundles_refonly (MonoAssemblyName *aname, [[maybe_unused]] char **assemblies_path, [[maybe_unused]] void *user_data)
+void
+EmbeddedAssemblies::install_preload_hooks_for_appdomains ()
 {
-	return embeddedAssemblies.open_from_bundles (aname, true);
+	mono_install_assembly_preload_hook (open_from_bundles_full, nullptr);
+#if !defined (NET6)
+	mono_install_assembly_refonly_preload_hook (open_from_bundles_refonly, nullptr);
+#endif // ndef NET6
 }
-#endif // !(def NET6 && def NET6_ALC_WORKS)
 
+#if defined (NET6)
 void
-EmbeddedAssemblies::install_preload_hooks ()
+EmbeddedAssemblies::install_preload_hooks_for_alc ()
 {
-#if defined (NET6) && defined (NET6_ALC_WORKS)
 	mono_install_assembly_preload_hook_v3 (
 		open_from_bundles,
 		nullptr /* user_data */,
 		0 /* append */
 	);
-#else // def NET6 && def NET6_ALC_WORKS
-	mono_install_assembly_preload_hook (open_from_bundles_full, nullptr);
-#if !defined (NET6) // Reference-only loads don't exist in NET6
-	mono_install_assembly_refonly_preload_hook (open_from_bundles_refonly, nullptr);
-#endif // !def NET6
-#endif // !(def NET6 && def NET6_ALC_WORKS)
 }
+#endif // def NET6
 
 template<typename Key, typename Entry, int (*compare)(const Key*, const Entry*), bool use_extra_size>
 const Entry*

src/monodroid/jni/embedded-assemblies.hh

@@ -4,6 +4,7 @@
 
 #include <cstring>
 #include <limits>
+#include <functional>
 #include <mono/metadata/object.h>
 #include <mono/metadata/assembly.h>
 
@@ -50,7 +51,10 @@ namespace xamarin::android::internal {
 #endif
 		const char* typemap_managed_to_java (MonoReflectionType *type, const uint8_t *mvid);
 
-		void install_preload_hooks ();
+		void install_preload_hooks_for_appdomains ();
+#if defined (NET6)
+		void install_preload_hooks_for_alc ();
+#endif // def NET6
 		MonoReflectionType* typemap_java_to_managed (MonoString *java_type);
 
 		/* returns current number of *all* assemblies found from all invocations */
@@ -93,11 +97,12 @@ namespace xamarin::android::internal {
 		MonoReflectionType* typemap_java_to_managed (const char *java_type_name);
 		size_t register_from (const char *apk_file, monodroid_should_register should_register);
 		void gather_bundled_assemblies_from_apk (const char* apk, monodroid_should_register should_register);
-#if defined (NET6) && defined (NET6_ALC_WORKS)
+#if defined (NET6)
 		MonoAssembly* open_from_bundles (MonoAssemblyName* aname, MonoAssemblyLoadContextGCHandle alc_gchandle, MonoError *error);
-#else // def NET6 && def NET6_ALC_WORKS
+#endif // def NET6
 		MonoAssembly* open_from_bundles (MonoAssemblyName* aname, bool ref_only);
-#endif // !(def NET6 && def NET6_ALC_WORKS)
+		MonoAssembly* open_from_bundles (MonoAssemblyName* aname, std::function<MonoImage*(char*, uint32_t, const char*)> loader, bool ref_only);
+
 #if defined (DEBUG) || !defined (ANDROID)
 		template<typename H>
 		bool typemap_read_header (int dir_fd, const char *file_type, const char *dir_path, const char *file_path, uint32_t expected_magic, H &header, size_t &file_size, int &fd);
@@ -111,12 +116,12 @@ namespace xamarin::android::internal {
 		bool register_debug_symbols_for_assembly (const char *entry_name, MonoBundledAssembly *assembly, const mono_byte *debug_contents, int debug_size);
 
 		static md_mmap_info md_mmap_apk_file (int fd, uint32_t offset, uint32_t size, const char* filename, const char* apk);
-#if defined (NET6) && defined (NET6_ALC_WORKS)
-		static MonoAssembly* open_from_bundles (MonoAssemblyLoadContextGCHandle alc_gchandle, MonoAssemblyName *aname, char **assemblies_path, void *user_data, MonoError *error);
-#else // def NET6 && def NET6_ALC_WORKS
 		static MonoAssembly* open_from_bundles_full (MonoAssemblyName *aname, char **assemblies_path, void *user_data);
+#if defined (NET6)
+		static MonoAssembly* open_from_bundles (MonoAssemblyLoadContextGCHandle alc_gchandle, MonoAssemblyName *aname, char **assemblies_path, void *user_data, MonoError *error);
+#else // def NET6
 		static MonoAssembly* open_from_bundles_refonly (MonoAssemblyName *aname, char **assemblies_path, void *user_data);
-#endif // !(def NET6 && def NET6_ALC_WORKS)
+#endif // ndef NET6
 		static void get_assembly_data (const MonoBundledAssembly *e, char*& assembly_data, uint32_t& assembly_data_size);
 
 		void zip_load_entries (int fd, const char *apk_name, monodroid_should_register should_register);

src/monodroid/jni/monodroid-glue-internal.hh

@@ -64,8 +64,11 @@ namespace xamarin::android::internal
 			true
 		>;
 
+		using load_assemblies_context_type = MonoAssemblyLoadContextGCHandle;
 		static constexpr pinvoke_library_map::size_type LIBRARY_MAP_INITIAL_BUCKET_COUNT = 1;
-#endif // def NET6
+#else // def NET6
+		using load_assemblies_context_type = MonoDomain*;
+#endif // ndef NET6
 
 #if defined (DEBUG) && !defined (WINDOWS)
 		struct RuntimeOptions {
@@ -107,6 +110,9 @@ namespace xamarin::android::internal
 		};
 
 	private:
+		static constexpr char MONO_ANDROID_ASSEMBLY_NAME[] = "Mono.Android";
+		static constexpr char JAVA_INTEROP_ASSEMBLY_NAME[] = "Java.Interop";
+
 		static constexpr size_t SMALL_STRING_PARSE_BUFFER_LEN = 50;
 		static constexpr bool is_running_on_desktop =
 #if ANDROID
@@ -222,7 +228,11 @@ namespace xamarin::android::internal
 		void disable_external_signal_handlers ();
 		void lookup_bridge_info (MonoDomain *domain, MonoImage *image, const OSBridge::MonoJavaGCBridgeType *type, OSBridge::MonoJavaGCBridgeInfo *info);
 		void load_assembly (MonoDomain *domain, jstring_wrapper &assembly);
-		void load_assemblies (MonoDomain *domain, bool preload, jstring_array_wrapper &assemblies);
+#if defined (NET6)
+		void load_assembly (MonoAssemblyLoadContextGCHandle alc_handle, jstring_wrapper &assembly);
+#endif // ndef NET6
+		void load_assemblies (load_assemblies_context_type ctx, bool preload, jstring_array_wrapper &assemblies);
+
 		void set_debug_options ();
 		void parse_gdb_options ();
 		void mono_runtime_init (dynamic_local_string<PROPERTY_VALUE_BUFFER_LEN>& runtime_args);
@@ -302,6 +312,8 @@ namespace xamarin::android::internal
 		int                 current_context_id = -1;
 
 #if defined (NET6)
+		MonoAssemblyLoadContextGCHandle default_alc = nullptr;
+
 		static std::mutex             pinvoke_map_write_lock;
 		static pinvoke_api_map        xa_pinvoke_map;
 		static pinvoke_library_map    other_pinvoke_map;