On Thu, Sep 17, 2020 at 5:04 PM Manfred Touron <m...@42.am> wrote:
>
> I think that I discovered a bug in the stdlib flag package:
>
> sharedFlag := "init-value"
> fooFs := flag.NewFlagSet("foo", flag.ContinueOnError)
> barFs := flag.NewFlagSet("bar", flag.ContinueOnError)
> fooFs.StringVar(&sharedFlag, "baz", "foo-default", "testing")
> barFs.StringVar(&sharedFlag, "baz", "bar-default", "testing")
> _ = fooFs.Parse([]string{""}) // no error
> fmt.Println(*sharedFlag) // bar-default, instead of foo-default
>
> To quote @peterbourgon on this issue:
>
> > Haha! This is a bug (?) with stdlib flag.FlagSet.
> > The default value provided to e.g. fs.StringVar isn't kept as metadata
> > associated with the flag in the corresponding flagset, but is rather
> > assigned to the associated variable. And that assignment happens at
> > declaration time, when you call fs.StringVar, and not when you call
> > fs.Parse. So the default value you see in any flag set will be the default
> > value provided to whichever StringVar declaration happens to occur last —
> > in your test, it's this one.
> > This sucks.
> > I think the only fix here would be, somehow, avoiding assigning a default
> > value in the declaration, and populating it later. Or defining a separate
> > e.g. DeferredFlagVar type that encapsulated this behavior? I don't know
> > offhand what's best. Ugh.
>
> I never contributed to the go repo yet, so I prefer to ask here your opinion
> about this strange behavior, what you suggest in term of fix that does not
> require to touch the stdlib, and if you think that I need to issue a bug on
> the go repo and try to contribute a fix?
What you say is true and I can see how it would be surprising but I
don't personally consider this to be a bug. It's a design decision.
Perhaps it should be documented more clearly: don't use the same
variable for more than one XXXVar calls.
Ian
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