Allow context switch from C using `mrb_fiber_resume()`.

But you still cannot cross C function boundary.

mrbgems/mruby-fiber/src/fiber.c

@@ -219,8 +219,11 @@ fiber_switch(mrb_state *mrb, mrb_value self, mrb_int len, const mrb_value *a, mr
     while (b<e) {
       *b++ = *a++;
     }
+    if (vmexec) {
+      c->ci--;                    /* pop dummy callinfo */
+    }
     c->cibase->argc = (int)len;
-    value = c->stack[0] = MRB_PROC_ENV(c->ci->proc)->stack[0];
+    value = c->stack[0] = MRB_PROC_ENV(c->cibase->proc)->stack[0];
   }
   else {
     value = fiber_result(mrb, a, len);
@@ -228,7 +231,7 @@ fiber_switch(mrb_state *mrb, mrb_value self, mrb_int len, const mrb_value *a, mr
 
   if (vmexec) {
     c->vmexec = TRUE;
-    value = mrb_vm_exec(mrb, c->ci[-1].proc, c->ci->pc);
+    value = mrb_vm_exec(mrb, c->ci->proc, c->ci->pc);
     mrb->c = old_c;
   }
   else {
@@ -354,6 +357,7 @@ mrb_fiber_yield(mrb_state *mrb, mrb_int len, const mrb_value *a)
   if (c->vmexec) {
     c->vmexec = FALSE;
     mrb->c->ci->acc = CI_ACC_RESUMED;
+    c->ci--;                    /* pop callinfo for yield */
   }
   MARK_CONTEXT_MODIFY(mrb->c);
   return fiber_result(mrb, a, len);

src/vm.c

@@ -2074,7 +2074,7 @@ RETRY_TRY_BLOCK:
               regs[irep->nlocals] = v;
               goto CHECKPOINT_LABEL_MAKE(RBREAK_TAG_STOP);
             }
-            if (c->prev->ci == c->prev->cibase) {
+            if (!c->vmexec && c->prev->ci == c->prev->cibase) {
               mrb_value exc = mrb_exc_new_lit(mrb, E_FIBER_ERROR, "double resume");
               mrb_exc_set(mrb, exc);
               goto L_RAISE;
@@ -2089,8 +2089,14 @@ RETRY_TRY_BLOCK:
             /* automatic yield at the end */
             c->status = MRB_FIBER_TERMINATED;
             mrb->c = c->prev;
-            c->prev = NULL;
             mrb->c->status = MRB_FIBER_RUNNING;
+            c->prev = NULL;
+            if (c->vmexec) {
+              mrb_gc_arena_restore(mrb, ai);
+              c->vmexec = FALSE;
+              mrb->jmp = prev_jmp;
+              return v;
+            }
             ci = mrb->c->ci;
           }
           CHECKPOINT_RESTORE(RBREAK_TAG_RETURN) {