nilaway doesn't detect guard on err variable when assignment to variable happens in branch, within loop

[daboross]

I think this is a pretty minimal example:

package minimal

import "fmt"

type Thing struct {
	field int
}

func LoopProcess(thing *Thing) (*Thing, error) {
	var err error

	for i := 0; i < 10; i++ {
		switch i {
		case 0:
			thing, err = process(thing)
		default:
			thing, err = process(thing)
		}
		if err != nil {
			return nil, err
		}
	}

	return thing, nil
}

func process(thing *Thing) (*Thing, error) {
	if thing.field == 0 {
		return nil, fmt.Errorf("")
	}

	return thing, nil
}

nilaway reports this as a possible nil panic:

$ nilaway ./main.go
/home/dross/minimal/main.go:28:5: error: Potential nil panic detected. Observed nil flow from source to dereference point:
	-> minimal/main.go:15:25: result 0 of `process()` lacking guarding; passed as arg `thing` to `process()`
	-> minimal/main.go:28:5: function parameter `thing` accessed field `field`

If I replace the loop with two successive calls, it's fine with it:

func LoopProcess(thing *Thing, i1, i2 int) (*Thing, error) {
	var err error

	switch i1 {
	case 0:
		thing, err = process(thing)
	default:
		thing, err = process(thing)
	}
	if err != nil {
		return nil, err
	}

	switch i2 {
	case 0:
		thing, err = process(thing)
	default:
		thing, err = process(thing)
	}
	if err != nil {
		return nil, err
	}

	return thing, nil
}

Equally so if I remove the conditional within the loop:

func LoopProcess(thing *Thing) (*Thing, error) {
	var err error

	for i := 0; i < 10; i++ {
		thing, err = process(thing)
		if err != nil {
			return nil, err
		}
	}

	return thing, nil
}

Note, replacing the switch with an if/else also reproduces the error, as does an if with no else.

[sonalmahajan15]

Hmm, this is interesting. We'll investigate more into this issue and hopefully come up with a fix soon. Thanks for reporting, @daboross! :)