Address #466?

Do not restore pc when returning from top most fiber frame.

Also add JANET_DEBUG config define for various debugging related
configurations. In fiber.c, when debug is enabled we reallocate the
entire stack everytime we push a frame to help uncover use after free
errors.

src/conf/janetconf.h

@@ -57,6 +57,7 @@
 /* #define JANET_NO_UMASK */
 
 /* Other settings */
+/* #define JANET_DEBUG */
 /* #define JANET_PRF */
 /* #define JANET_NO_UTC_MKTIME */
 /* #define JANET_OUT_OF_MEMORY do { printf("janet out of memory\n"); exit(1); } while (0) */

src/core/fiber.c

@@ -85,6 +85,22 @@ JanetFiber *janet_fiber(JanetFunction *callee, int32_t capacity, int32_t argc, c
     return janet_fiber_reset(fiber_alloc(capacity), callee, argc, argv);
 }
 
+#ifdef JANET_DEBUG
+/* Test for memory issues by reallocating fiber every time we push a stack frame */
+static void janet_fiber_refresh_memory(JanetFiber *fiber) {
+    int32_t n = fiber->capacity;
+    if (n) {
+        Janet *newData = malloc(sizeof(Janet) * n);
+        if (NULL == newData) {
+            JANET_OUT_OF_MEMORY;
+        }
+        memcpy(newData, fiber->data, fiber->capacity * sizeof(Janet));
+        free(fiber->data);
+        fiber->data = newData;
+    }
+}
+#endif
+
 /* Ensure that the fiber has enough extra capacity */
 void janet_fiber_setcapacity(JanetFiber *fiber, int32_t n) {
     Janet *newData = realloc(fiber->data, sizeof(Janet) * n);
@@ -173,6 +189,10 @@ int janet_fiber_funcframe(JanetFiber *fiber, JanetFunction *func) {
 
     if (fiber->capacity < nextstacktop) {
         janet_fiber_setcapacity(fiber, 2 * nextstacktop);
+#ifdef JANET_DEBUG
+    } else {
+        janet_fiber_refresh_memory(fiber);
+#endif
     }
 
     /* Nil unset stack arguments (Needed for gc correctness) */
@@ -305,6 +325,10 @@ int janet_fiber_funcframe_tail(JanetFiber *fiber, JanetFunction *func) {
 
     if (fiber->capacity < nextstacktop) {
         janet_fiber_setcapacity(fiber, 2 * nextstacktop);
+#ifdef JANET_DEBUG
+    } else {
+        janet_fiber_refresh_memory(fiber);
+#endif
     }
 
     Janet *stack = fiber->data + fiber->frame;
@@ -367,6 +391,10 @@ void janet_fiber_cframe(JanetFiber *fiber, JanetCFunction cfun) {
 
     if (fiber->capacity < nextstacktop) {
         janet_fiber_setcapacity(fiber, 2 * nextstacktop);
+#ifdef JANET_DEBUG
+    } else {
+        janet_fiber_refresh_memory(fiber);
+#endif
     }
 
     /* Set the next frame */

src/core/vm.c

@@ -95,6 +95,10 @@ JANET_THREAD_LOCAL jmp_buf *janet_vm_jmp_buf = NULL;
     vm_commit(); \
     return (sig); \
 } while (0)
+#define vm_return_no_restore(sig, val) do { \
+    janet_vm_return_reg[0] = (val); \
+    return (sig); \
+} while (0)
 
 /* Next instruction variations */
 #define maybe_collect() do {\
@@ -623,7 +627,7 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in) {
         Janet retval = stack[D];
         int entrance_frame = janet_stack_frame(stack)->flags & JANET_STACKFRAME_ENTRANCE;
         janet_fiber_popframe(fiber);
-        if (entrance_frame) vm_return(JANET_SIGNAL_OK, retval);
+        if (entrance_frame) vm_return_no_restore(JANET_SIGNAL_OK, retval);
         vm_restore();
         stack[A] = retval;
         vm_checkgc_pcnext();
@@ -633,7 +637,7 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in) {
         Janet retval = janet_wrap_nil();
         int entrance_frame = janet_stack_frame(stack)->flags & JANET_STACKFRAME_ENTRANCE;
         janet_fiber_popframe(fiber);
-        if (entrance_frame) vm_return(JANET_SIGNAL_OK, retval);
+        if (entrance_frame) vm_return_no_restore(JANET_SIGNAL_OK, retval);
         vm_restore();
         stack[A] = retval;
         vm_checkgc_pcnext();
@@ -1011,8 +1015,9 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in) {
                 retreg = call_nonfn(fiber, callee);
             }
             janet_fiber_popframe(fiber);
-            if (entrance_frame)
+            if (entrance_frame) {
-                vm_return(JANET_SIGNAL_OK, retreg);
+                vm_return_no_restore(JANET_SIGNAL_OK, retreg);
+            }
             vm_restore();
             stack[A] = retreg;
             vm_checkgc_pcnext();