Invalid code generated for funcs with unnamed return parameters that defer blocking code.

[nwidger]

https://gopherjs.github.io/playground/#/JcN1xwSTbp

If I run this Playground link in Firefox 52.0, the should get here alert dialog appears. If I run the same link in Chrome 56.0.2924.87, it prints calling alert(err) but not returned from alert(err) and the following stack trace is generated:

Uncaught (in promise) TypeError: Cannot read property 'Error' of undefined
    at Object.main [as $blk] (eval at $b (playground.js:66), <anonymous>:2943:14)
    at Object.$init [as $blk] (eval at $b (playground.js:66), <anonymous>:2963:68)
    at fun (eval at $b (playground.js:66), <anonymous>:1477:37)
    at $goroutine (eval at $b (playground.js:66), <anonymous>:1475:19)
    at $runScheduled (eval at $b (playground.js:66), <anonymous>:1515:7)
    at $schedule (eval at $b (playground.js:66), <anonymous>:1531:5)
    at queueEntry (eval at $b (playground.js:66), <anonymous>:1686:11)
    at $send (eval at $b (playground.js:66), <anonymous>:1556:5)
    at $b (eval at $b (playground.js:66), <anonymous>:2895:70432)
    at v.$externalizeWrapper (eval at $b (playground.js:66), <anonymous>:1871:24)

Removing the defer statement after http.Get resolves the issue in Chrome:

https://gopherjs.github.io/playground/#/DK-J3eMXYr

Is using defer like this incorrect? If it is I would expect to either see a compilation error or see the same issue in both Firefox and Chrome.

[dmitshur]

I can reproduce in Chrome 57.0.2987.98.

This is definitely a bug and shouldn't be happening.

I've also updated the playground to latest version of GopherJS just now, and it still continues to occur there. So this is a valid issue in master.

[dmitshur]

It seems the value of err is somehow undefined (aka js.Undefined).

https://gopherjs.github.io/playground/#/Q5xYbr2_yf

That prints undefined in browser console in Chrome, which is very unexpected.

[nwidger]

It definitely seems to be related to io.Copy being called in a defer statement. This does not exhibit the same issue:

https://gopherjs.github.io/playground/#/d-r8POieip

[dmitshur]

I suspect this may be related to the fix in 5010146, and #381, #426, #493 issues it resolved. /cc @neelance

[dmitshur]

Here's a simplified reproduce case:

package main

import "time"

func f() error {
	defer func() {
		time.Sleep(time.Second)
	}()
	return nil
}

func main() {
	err := f()
	print(err) // prints "undefined"
}

[dmitshur]

Interestingly, it works fine if the return parameter has a name, even if that name is blank identifier.

Both these work ok:

package main

import "fmt"
import "time"

func f() (err error) {
	defer func() {
		time.Sleep(time.Second)
	}()
	return nil
}

func main() {
	err := f()
	fmt.Println(err) // prints "<nil>" which is correct
}

package main

import "fmt"
import "time"

func f() (_ error) {
	defer func() {
		time.Sleep(time.Second)
	}()
	return nil
}

func main() {
	err := f()
	fmt.Println(err) // prints "<nil>" which is correct
}

So, this bug should be easy to fix, it's just an unhandled edge case.

[nevkontakte]

I ran into this bug too and I think I know what's the issue here. Consider the following program:

func TestReturnWithBlockingDefer(t *testing.T) {
	f := func() int {
		defer time.Sleep(0)
		return 42
	}
	got := f()
	if got != 42 {
		t.Errorf("Unexpected return value %v. Want: 42.", got)
	}
}

Here's the code f compiles into:

f = (function $b() {
	var $s, $deferred, $r;
	/* */ $s = 0; var $f, $c = false; if (this !== undefined && this.$blk !== undefined) { $f = this; $c = true; $s = $f.$s; $deferred = $f.$deferred; $r = $f.$r; } var $err = null; try { s: while (true) { switch ($s) { case 0: $deferred = []; $deferred.index = $curGoroutine.deferStack.length; $curGoroutine.deferStack.push($deferred);
	$deferred.push([time.Sleep, [new time.Duration(0, 0)]]);
	$s = -1; return 42;
	/* */ } return; } } catch(err) { $err = err; $s = -1; return 0; } finally { $callDeferred($deferred, $err); if($curGoroutine.asleep) { if ($f === undefined) { $f = { $blk: $b }; } $f.$s = $s; $f.$deferred = $deferred; $f.$r = $r; return $f; } }
});

When the function executes the first time, it successfully passes through the $s = -1; return 42; statement and would've returned 42 as expected if the deferred function didn't get blocked. But the deferred function does get blocked, and when function f is called the second time it's executed with $s == -1 and completely skips the return 42 statement. Instead it passes through the /* */ } return; statement and returns undefined as a result.

[nevkontakte]

Thinking a bit more about this, the right fix is to treat the return statement as blocking in case any of the deferred functions are blocking. And for a blocking return statement we need to compute the return value, store it in a temporary variable, generate a new resumption case and then returned the stored value.