py: Add portable framework to query/check C stack usage.

Such mechanism is important to get stable Python functioning, because Python
function calling is handled with C stack. The idea is to sprinkle
STACK_CHECK() calls in places where there can be C recursion.

TODO: Add more STACK_CHECK()'s.

py/stackctrl.c

@@ -0,0 +1,63 @@
+/*
+ * This file is part of the Micro Python project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Paul Sokolovsky
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "mpconfig.h"
+#include "misc.h"
+#include "nlr.h"
+#include "qstr.h"
+#include "obj.h"
+#include "runtime.h"
+#include "stackctrl.h"
+
+// Stack top at the start of program
+char *stack_top;
+
+void stack_ctrl_init() {
+    volatile int stack_dummy;
+    stack_top = (char*)&stack_dummy;
+}
+
+uint stack_usage() {
+    // Assumes descending stack
+    volatile int stack_dummy;
+    return stack_top - (char*)&stack_dummy;
+}
+
+#if MICROPY_STACK_CHECK
+
+uint stack_limit = 10240;
+
+void stack_set_limit(uint limit) {
+    stack_limit = limit;
+}
+
+void stack_check() {
+    if (stack_usage() >= stack_limit) {
+        nlr_raise(mp_obj_new_exception_msg(&mp_type_RuntimeError, "maximum recursion depth exceeded"));
+    }
+}
+
+#endif // MICROPY_STACK_CHECK