Pass the default ALC gchandle to the v3 preload hook (dotnet#54686)

* Pass the default ALC gchandle to the v3 preload hook

Fixes crashes early during startup when the gchandle is then used by the hooks
to call mono API functions.  The default ALC exists, but its managed object
doesn't, so the gchandle target is null.  The mono APIs detect the the special
gchandle and resolve it to the default ALC

Example crash
https://gist.github.com/grendello/b4ab24587a055725cc5e1416b86ad7ca

* [alc] Assert that we never see a null target from a managed ALC gchandle

src/mono/mono/metadata/assembly-load-context.c

@@ -424,6 +424,7 @@ mono_alc_from_gchandle (MonoGCHandle alc_gchandle)
 
 	HANDLE_FUNCTION_ENTER ();
 	MonoManagedAssemblyLoadContextHandle managed_alc = MONO_HANDLE_CAST (MonoManagedAssemblyLoadContext, mono_gchandle_get_target_handle (alc_gchandle));
+	g_assert (!MONO_HANDLE_IS_NULL (managed_alc));
 	MonoAssemblyLoadContext *alc = MONO_HANDLE_GETVAL (managed_alc, native_assembly_load_context);
 	HANDLE_FUNCTION_RETURN_VAL (alc);
 }

src/mono/mono/metadata/assembly.c

@@ -1579,9 +1579,21 @@ invoke_assembly_preload_hook (MonoAssemblyLoadContext *alc, MonoAssemblyName *an
 			if (hook->version == 2)
 				assembly = hook->func.v2 (alc, aname, apath, hook->user_data, error);
 			else { // v3
-				MonoGCHandle strong_gchandle = mono_gchandle_from_handle (mono_gchandle_get_target_handle (alc->gchandle), TRUE);
+				/*
+				 * For the default ALC, pass the globally known gchandle (since it's never collectible, it's always a strong handle).
+				 * For other ALCs, make a new strong handle that is passed to the caller.
+				 * Early at startup, when the default ALC exists, but its managed object doesn't, so the default ALC gchandle points to null.
+				 */
+				gboolean needs_free = TRUE;
+				MonoGCHandle strong_gchandle;
+				if (mono_alc_is_default (alc)) {
+					needs_free = FALSE;
+					strong_gchandle = alc->gchandle;
+				} else
+					strong_gchandle = mono_gchandle_from_handle (mono_gchandle_get_target_handle (alc->gchandle), TRUE);
 				assembly = hook->func.v3 (strong_gchandle, aname, apath, hook->user_data, error);
-				mono_gchandle_free_internal (strong_gchandle);
+				if (needs_free)
+					mono_gchandle_free_internal (strong_gchandle);
 			}
 			/* TODO: propagage error out to callers */
 			mono_error_assert_ok (error);