More robust value stack handling for dbg messages

- Change duk__debug_process_message() to store/restore value stack top
  for message handlers.  This is more robust than requiring that every
  debug message handler code path always manages the value stack 100%
  correctly.

- Change existing message handlers so that trivial value stack cleanups
  are not done in the message handler.  Value stack popping is still
  necessary for loops where the value stack might grow without bound.

src/duk_debugger.c

@@ -982,6 +982,10 @@ DUK_LOCAL void duk__debug_skip_to_eom(duk_hthread *thr) {
 
 /*
  *  Process incoming debug requests
+ *
+ *  Individual request handlers can push temporaries on the value stack and
+ *  rely on duk__debug_process_message() to restore the value stack top
+ *  automatically.
  */
 
 DUK_LOCAL void duk__debug_handle_basic_info(duk_hthread *thr, duk_heap *heap) {
@@ -1071,7 +1075,6 @@ DUK_LOCAL void duk__debug_handle_list_break(duk_hthread *thr, duk_heap *heap) {
 }
 
 DUK_LOCAL void duk__debug_handle_add_break(duk_hthread *thr, duk_heap *heap) {
-	duk_context *ctx = (duk_context *) thr;
 	duk_hstring *filename;
 	duk_uint32_t linenumber;
 	duk_small_int_t idx;
@@ -1089,7 +1092,6 @@ DUK_LOCAL void duk__debug_handle_add_break(duk_hthread *thr, duk_heap *heap) {
 	} else {
 		duk_debug_write_error_eom(thr, DUK_DBG_ERR_TOOMANY, "no space for breakpoint");
 	}
-	duk_pop(ctx);
 }
 
 DUK_LOCAL void duk__debug_handle_del_break(duk_hthread *thr, duk_heap *heap) {
@@ -1137,12 +1139,10 @@ DUK_LOCAL void duk__debug_handle_get_var(duk_hthread *thr, duk_heap *heap) {
 		duk_debug_write_int(thr, 1);
 		DUK_ASSERT(duk_get_tval(ctx, -2) != NULL);
 		duk_debug_write_tval(thr, duk_get_tval(ctx, -2));
-		duk_pop_2(ctx);
 	} else {
 		duk_debug_write_int(thr, 0);
 		duk_debug_write_unused(thr);
 	}
-	duk_pop(ctx);
 	duk_debug_write_eom(thr);
 }
 
@@ -1169,7 +1169,6 @@ DUK_LOCAL void duk__debug_handle_put_var(duk_hthread *thr, duk_heap *heap) {
 	} else {
 		DUK_D(DUK_DPRINT("callstack empty, no activation -> ignore putvar"));
 	}
-	duk_pop_2(ctx);
 
 	/* XXX: Current putvar implementation doesn't have a success flag,
 	 * add one and send to debug client?
@@ -1260,11 +1259,9 @@ DUK_LOCAL void duk__debug_handle_get_locals(duk_hthread *thr, duk_heap *heap) {
 			duk_debug_write_tval(thr, duk_get_tval(ctx, -2));
 			duk_pop_3(ctx);  /* -> [ ... func varmap enum ] */
 		}
-		duk_pop(ctx);
 	} else {
 		DUK_D(DUK_DPRINT("varmap is not an object in GetLocals, ignore"));
 	}
-	duk_pop_2(ctx);
 
  callstack_empty:
 	duk_debug_write_eom(thr);
@@ -1272,13 +1269,9 @@ DUK_LOCAL void duk__debug_handle_get_locals(duk_hthread *thr, duk_heap *heap) {
 
 DUK_LOCAL void duk__debug_handle_eval(duk_hthread *thr, duk_heap *heap) {
 	duk_context *ctx = (duk_context *) thr;
-
 	duk_small_uint_t call_flags;
 	duk_int_t call_ret;
 	duk_small_int_t eval_err;
-#if defined(DUK_USE_ASSERTIONS)
-	duk_idx_t entry_top;
-#endif
 
 	DUK_UNREF(heap);
 
@@ -1289,10 +1282,6 @@ DUK_LOCAL void duk__debug_handle_eval(duk_hthread *thr, duk_heap *heap) {
 	 * the eval considered a 'direct call to eval'.
 	 */
 
-#if defined(DUK_USE_ASSERTIONS)
-	entry_top = duk_get_top(ctx);
-#endif
-
 	duk_push_c_function(ctx, duk_bi_global_object_eval, 1 /*nargs*/);
 	duk_push_undefined(ctx);  /* 'this' binding shouldn't matter here */
 	(void) duk_debug_read_hstring(thr);
@@ -1337,9 +1326,6 @@ DUK_LOCAL void duk__debug_handle_eval(duk_hthread *thr, duk_heap *heap) {
 	DUK_ASSERT(duk_get_tval(ctx, -1) != NULL);
 	duk_debug_write_tval(thr, duk_get_tval(ctx, -1));
 	duk_debug_write_eom(thr);
-	duk_pop(ctx);
-
-	DUK_ASSERT(duk_get_top(ctx) == entry_top);
 }
 
 DUK_LOCAL void duk__debug_handle_detach(duk_hthread *thr, duk_heap *heap) {
@@ -1570,17 +1556,24 @@ DUK_LOCAL void duk__debug_handle_get_bytecode(duk_hthread *thr, duk_heap *heap)
 	duk_debug_write_eom(thr);
 }
 
+/* Process one debug message.  Automatically restore value stack top to its
+ * entry value, so that individual message handlers don't need exact value
+ * stack handling which is convenient.
+ */
 DUK_LOCAL void duk__debug_process_message(duk_hthread *thr) {
 	duk_context *ctx = (duk_context *) thr;
 	duk_heap *heap;
 	duk_uint8_t x;
 	duk_int32_t cmd;
+	duk_idx_t entry_top;
 
 	DUK_ASSERT(thr != NULL);
 	heap = thr->heap;
 	DUK_ASSERT(heap != NULL);
 	DUK_UNREF(ctx);
 
+	entry_top = duk_get_top(ctx);
+
 	x = duk_debug_read_byte(thr);
 	switch (x) {
 	case DUK_DBG_MARKER_REQUEST: {
@@ -1679,10 +1672,14 @@ DUK_LOCAL void duk__debug_process_message(duk_hthread *thr) {
 	}
 	}  /* switch initial byte */
 
+	DUK_ASSERT(duk_get_top(ctx) >= entry_top);
+	duk_set_top(ctx, entry_top);
 	duk__debug_skip_to_eom(thr);
 	return;
 
  fail:
+	DUK_ASSERT(duk_get_top(ctx) >= entry_top);
+	duk_set_top(ctx, entry_top);
 	DUK__SET_CONN_BROKEN(thr);
 	return;
 }