Strange interaction between defer and function taking an interface resulting in nil dereference

[krnowak]

go version go1.5.1 linux/amd64

There is an example code in http://play.golang.org/p/5ZpPG0hhqJ
And here is a truncated code to the essence: http://play.golang.org/p/W1lmCPoc7D

I expected the nil check in maybeClose to prevent executing Close, but apparently it is executed on a nil variable.

[bradfitz]

Working as designed. It's okay to call methods on nil non-interface values.

Let's move this to a forum since it's not a bug to track here on the bug tracker. See https://golang.org/wiki/Questions

[krnowak]

Excuse me, but I have an impression that you haven't seen the full example at all. I'm not talking about calling methods on nil non-interface values here at all. I'll try to explain in here then.

Let's assume that type *A implements io.Closer. I have a function maybeClose implemented as follows:

func maybeClose(c io.Closer) {
    if c != nil {
        c.Close()
    }
}

I'm using it as follows:

a := &A{}
defer func() { maybeClose(a) }()
if err := someValidation(a); err != nil { return nil, err }
if err := someMoreChecks(a); err != nil { return nil, err }
...
if err := lastCheck(a); err != nil { return nil, err }
// alright, a is valid, let's not close it, but return it instead
b := a
a = nil
return b, nil

If all validations passed, I assign the object to other variable and nil the a variable. The deferred action is run, takes an a variable, which is nil at this point. Somehow c != nil in maybeCheck function does not catch it and happily calls Close on it, which might end in nil dereference if Close accesses some fields in *A.

Now, either change c io.Closer in maybeClose to c *A or change a := &A{} to a := func() io.Closer { return &A{} } () and it works - Close is not invoked anymore in deferred function.

[bradfitz]

https://golang.org/doc/faq#nil_error

[krnowak]

Thanks. That's very confusing.