Resuming fiber from C seems to loop infinitely

[haileys]

This example code behaves incorrectly:

#include <mruby.h>
#include <mruby/compile.h>
#include <mruby/string.h>
#include <stdio.h>

mrb_state* mrb;

void p(mrb_value value) {
    printf("--> %s\n", mrb_str_to_cstr(mrb, mrb_inspect(mrb, value)));
}

int main() {
    mrb = mrb_open();

    mrb_value fiber_class = mrb_obj_value(mrb_class_get(mrb, "Fiber"));

    mrb_value proc = mrb_load_string(mrb,
        "proc do |a| Fiber.yield(a + 1); end");

    mrb_sym new = mrb_intern_cstr(mrb, "new");
    mrb_value fiber = mrb_funcall_with_block(mrb, fiber_class, new, 0, NULL, proc);

    mrb_value start_arg = mrb_fixnum_value(123);
    p(mrb_fiber_resume(mrb, fiber, 1, &start_arg));

    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
    p(mrb_fiber_resume(mrb, fiber, 0, NULL));
}

I would expect this code to output:

--> 124
--> nil
resuming dead fiber (FiberError)
Aborted (core dumped)

Instead it outputs:

--> 124
--> 125
--> 126
--> 127
--> 128
--> 129
--> 130
--> 131
--> 132

I think we are not correctly updating ci->pc when the fiber yields but I am not familiar enough with the VM to understand what should be happening.

[haileys]

I have been able to come up with a workaround involving a trampoline proc that calls Fiber#resume from mruby code rather than calling mrb_fiber_resume directly:

#include <mruby.h>
#include <mruby/compile.h>
#include <mruby/string.h>
#include <stdio.h>
#include <string.h>

mrb_state* mrb;
mrb_value trampoline;

void p(mrb_value value) {
    printf("--> %s\n", mrb_str_to_cstr(mrb, mrb_inspect(mrb, value)));
}

mrb_value fiber_resume(mrb_value fiber, size_t argc, mrb_value* argv) {
    mrb_value args[argc + 1];
    args[0] = fiber;
    memcpy(args + 1, argv, argc * sizeof(mrb_value));

    return mrb_funcall_argv(mrb, trampoline, mrb_intern_lit(mrb, "call"), argc + 1, args);
}

int main() {
    mrb = mrb_open();

    trampoline = mrb_load_string(mrb,
        "proc do |fiber, *args| fiber.resume(*args) end");
    mrb_gc_protect(mrb, trampoline);

    mrb_value fiber_class = mrb_obj_value(mrb_class_get(mrb, "Fiber"));

    mrb_value proc = mrb_load_string(mrb,
        "proc do |a| Fiber.yield(a + 1); Fiber.yield(a + 2); end");
    mrb_gc_protect(mrb, proc);

    mrb_sym new = mrb_intern_cstr(mrb, "new");
    mrb_value fiber = mrb_funcall_with_block(mrb, fiber_class, new, 0, NULL, proc);
    mrb_gc_protect(mrb, fiber);

    mrb_value start_arg = mrb_fixnum_value(123);
    p(fiber_resume(fiber, 1, &start_arg));

    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
    p(fiber_resume(fiber, 0, NULL));
}

This correctly outputs:

--> 124
--> 125
--> nil
--> resuming dead fiber (FiberError)
--> resuming dead fiber (FiberError)
--> resuming dead fiber (FiberError)
--> resuming dead fiber (FiberError)
--> resuming dead fiber (FiberError)
--> resuming dead fiber (FiberError)

This seems to suggest that mrb_fiber_resume does not work correctly when the fiber it is called from (in this case, the root fiber) is not in a VM frame.