[mono][interp] Keep delegate alive during invocation (dotnet#100832)

When invoking a delegate, we were overwritting the stack slot containing the delegate object reference. In the case of invoking a delegate for a dynamic method, we were running into issues when the delegate object is collected while the method is executed because the method code is also discarded.

src/mono/mono/mini/interp/interp-internals.h

@@ -154,6 +154,7 @@ struct InterpMethod {
 	unsigned int hasthis; // boolean
 	MonoProfilerCallInstrumentationFlags prof_flags;
 	InterpMethodCodeType code_type;
+	int ref_slot_offset; // GC visible pointer slot
 	MonoBitSet *ref_slots;
 #ifdef ENABLE_EXPERIMENT_TIERED
 	MiniTieredCounter tiered_counter;

src/mono/mono/mini/interp/interp.c

@@ -4164,6 +4164,9 @@ mono_interp_exec_method (InterpFrame *frame, ThreadContext *context, FrameClause
 			}
 			cmethod = del_imethod;
 			if (!is_multicast) {
+				int ref_slot_offset = frame->imethod->ref_slot_offset;
+				if (ref_slot_offset >= 0)
+					LOCAL_VAR (ref_slot_offset, gpointer) = del;
 				int param_count = ip [4];
 				if (cmethod->param_count == param_count + 1) {
 					// Target method is static but the delegate has a target object. We handle

src/mono/mono/mini/interp/transform.c

@@ -451,6 +451,15 @@ interp_create_dummy_var (TransformData *td)
 	td->vars [td->dummy_var].global = TRUE;
 }
 
+static void
+interp_create_ref_handle_var (TransformData *td)
+{
+	int var = interp_create_var_explicit (td, m_class_get_byval_arg (mono_defaults.int_class), sizeof (gpointer));
+	td->vars [var].global = TRUE;
+	interp_alloc_global_var_offset (td, var);
+	td->ref_handle_var = var;
+}
+
 static int
 get_tos_offset (TransformData *td)
 {
@@ -3756,6 +3765,10 @@ interp_transform_call (TransformData *td, MonoMethod *method, MonoMethod *target
 		td->last_ins->data [0] = get_data_item_index_imethod (td, mono_interp_get_imethod (target_method));
 	} else {
 		if (is_delegate_invoke) {
+			// MINT_CALL_DELEGATE will store the delegate object into this slot so it is kept alive
+			// while the method is invoked
+			if (td->ref_handle_var == -1)
+				interp_create_ref_handle_var (td);
 			interp_add_ins (td, MINT_CALL_DELEGATE);
 			interp_ins_set_dreg (td->last_ins, dreg);
 			interp_ins_set_sreg (td->last_ins, MINT_CALL_ARGS_SREG);
@@ -9085,6 +9098,7 @@ generate (MonoMethod *method, MonoMethodHeader *header, InterpMethod *rtm, MonoG
 	td->n_data_items = 0;
 	td->max_data_items = 0;
 	td->dummy_var = -1;
+	td->ref_handle_var = -1;
 	td->data_items = NULL;
 	td->data_hash = g_hash_table_new (NULL, NULL);
 #ifdef ENABLE_EXPERIMENT_TIERED
@@ -9260,6 +9274,11 @@ generate (MonoMethod *method, MonoMethodHeader *header, InterpMethod *rtm, MonoG
 		}
 	}
 
+	if (td->ref_handle_var != -1)
+		rtm->ref_slot_offset = td->vars [td->ref_handle_var].offset;
+	else
+		rtm->ref_slot_offset = -1;
+
 	/* Save debug info */
 	interp_save_debug_info (rtm, header, td, td->line_numbers);
 

src/mono/mono/mini/interp/transform.h

@@ -285,6 +285,7 @@ typedef struct
 	gint32 total_locals_size;
 	gint32 max_stack_size;
 	int dummy_var;
+	int ref_handle_var;
 	int *local_ref_count;
 	unsigned int il_locals_offset;
 	unsigned int il_locals_size;