That's fair. I wasn't sure if recovering the panic in errors would be a
good idea, since it could hide programmer error (like what I'd done). But
I'm fine with that solution.
On Mon, Aug 24, 2015 at 3:22 AM David Cheney <david.che...@canonical.com>
wrote:
> Yup, I agree with Rog. This is fixing the problem in the wrong place.
>
> On Mon, Aug 24, 2015 at 5:14 PM, roger peppe <roger.pe...@canonical.com>
> wrote:
> > I'm afraid I'm not convinced. Declaring the Error method on the
> > pointer receiver is idiomatic (just grep for ' Error\(' in the Go source)
> > and is a useful indicator that the error value is always intended
> > to be a pointer.
> >
> > There's a much simpler fix for this: let the errors package
> > recover from this itself. We can just make Err.Error call fmt.Sprint
> > to get the error message (a one line change)
> >
> > Then a wrapped nil error will print <nil> just like normal nil
> > errors.
> >
> >
> > On 20 August 2015 at 03:45, Nate Finch <nate.fi...@canonical.com> wrote:
> >> tl;dr: Don't. Use a value receiver. 99% of the time you can just
> delete
> >> the * on the receiver and it'll still work. If you really must use a
> >> pointer, please handle the case where you're called with a nil receiver.
> >>
> >> I spent most of today trying to understand why my new hook command was
> >> producing this output:
> >>
> >> error: %!v(PANIC=runtime error: invalid memory address or nil pointer
> >> dereference)
> >>
> >> It took me a while to figure out that this is what fmt.Printf("error:
> %v\n",
> >> err) outputs when err's Error() method panics. If you're using %s or
> %v to
> >> print a value (or use Println which implicitly uses %v), then fmt will
> look
> >> for a String() method or Error() method on the value to call, and use
> the
> >> output of that for the value's string output. If that method panics,
> fmt
> >> prints the panic in the way you see above (everything after the PANIC=).
> >>
> >> Of course, the problem here is that there's no type being written, and
> since
> >> it was an error interface, it could almost anything. Using %#v skips
> >> calling the Error/String methods and prints out the values in a go
> format,
> >> which told me this was a juju/errors.Err value, wrapping an API params
> Error
> >> value which was a nil pointer. When we call Error() on an errors.Err,
> we
> >> call Error() on the cause explicitly, which was panicking.
> >>
> >> Here's a minimal reproduction http://play.golang.org/p/ncNVrza-hn
> (you'll
> >> have to copy it to a local file and go run it, since the playground
> won't
> >> run code external to the stdlib).
> >>
> >> So what's sort of interesting is that printing the error before it gets
> >> Traced works fine, but after the trace it is not fine. The errors.Err's
> >> Error() function looks like it's explicitly calling the Error() method
> on
> >> the wrapped Cause error, which is probably the problem. fmt.Printf
> must use
> >> some reflection magic to avoid doing that.
> >>
> >> Now, the root cause of this particular bug is actually my own mistake.
> Line
> >> 21 should check if orig is nil and then assign nil explicitly to err if
> it
> >> is. Then errors.Trace would be able to tell that the error is nil and
> would
> >> just return nil itself, instead of thinking it's a valid error and
> wrapping
> >> it.
> >>
> >> However, you can sidestep this entirely by doing one of two things:
> either
> >> just make the Error() (or String()) method use a value receiver.. in
> which
> >> case this code would produce this output:
> >>
> >> %!v(PANIC=value method main.MyError.Error called using nil *MyError
> pointer)
> >>
> >> (you can try it with the repro code I linked to)
> >>
> >> This printout is a lot more helpful and useful and obvious than the
> other
> >> "nil pointer" printout.
> >>
> >> OR
> >>
> >> Just handle a nil receiver:
> >>
> >> func (e *MyError) Error() string {
> >> if e == nil {
> >> return "<nil MyError>"
> >> }
> >> return e.Message
> >> }
> >>
> >> (note that it is dereferencing the pointer to e to access the Message
> field
> >> which causes the panic. Calling a method on a nil pointer is totally
> fine
> >> and will not panic if the code inside does not try to derefence the
> pointer
> >> to get to a field).
> >>
> >> Grepping through our code, I see a lot of pointer receivers on Error and
> >> String methods (45 and 77 respectively). I think we should at least
> change
> >> all of these to be value methods (unless that's not possible. That's a
> >> trivial change, and gives a much more useful printout when the pointer
> is
> >> nil.
> >>
> >> -Nate
> >>
> >> --
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> >
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