implement runtime safety check for resuming not-suspended async function

[emekoi]

the following code causes a stack overflow in zig when run and a segfault when trying to print the stack trace.

fn foo() void {
    var f = async bar(@frame());
    unreachable;
}

fn bar(frame: anyframe) void {
    suspend {
        resume frame;
    }
    unreachable;
}

test "test" {
    _ = async foo();
}

[andrewrk]

Let's follow the control flow of this example:

    _ = async foo();
////////////////////
    var f = async bar(@frame());
////////////////////
        resume frame; // note that `frame` is not suspended; this is illegal behavior
////////////////////

Expected output from this is safety crash for resuming a function which is not suspended. So this is a case of missing debug safety.

segfault when trying to print the stack trace.

That part is #1616.

[emekoi]

ohhh, so await and suspend are suspend points but resume and async are not?

[andrewrk]

Yes that's correct. Think of resume and async as function calls, which return when the target frame suspends.

[andrewrk]

Here's what it looks like now after the changes that I made locally:

Test [1/1] test "test"...resumed a non-suspended function
/home/andy/dev/zig/build/test2.zig:1:1: 0x20d85a in foo (test)
fn foo() void {
^
/home/andy/dev/zig/build/test2.zig:8:9: 0x20dfc9 in bar (test)
        resume frame;
        ^
/home/andy/dev/zig/build/test2.zig:2:13: 0x20d81f in foo (test)
    var f = async bar(@frame());
            ^
/home/andy/dev/zig/build/test2.zig:14:9: 0x20d5b5 in test "test" (test)
    _ = async foo();
        ^
/home/andy/dev/zig/lib/std/special/test_runner.zig:20:25: 0x24212b in std.special.main (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/lib/std/special/start.zig:204:37: 0x240f35 in std.special.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/dev/zig/lib/std/special/start.zig:102:5: 0x240daf in std.special._start (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/build/zig-cache/o/S-bSJ_0MUe7bZJWDvBu7TKpo5B6w3B112oXlFrD2S7m3MOErvTAyzLjKPnpuxuAe/test

I'll push this after the tests pass.