On Thu, Apr 25, 2019, at 9:52 AM, Jean-Paul Calderone wrote:
> Hello all,
>
> I'd like to discuss the project policy/process for dealing with incorrect
> backward incompatibilities that end up released.
>
> As a case study, we could look at https://twistedmatrix.com/trac/ticket/9410
> <https://twistedmatrix.com/trac/ticket/9410#comment:17> /
> https://github.com/twisted/twisted/pull/1106
> <https://github.com/twisted/twisted/pull/1106>
>
> In Twisted 16.3.0 the behavior of Request.write was changed so that it raises
> an AttributeError if called after the connection has closed. Prior to that
> release, it silently did nothing in that case.
>
> Now, three years later, we have a PR which proposed restoring the behavior of
> silently ignoring the write to a closed connection.
>
> The original change was incompatible. The new change is incompatible. What
> should win out?
There are two things to address here, I think:
In the general case, can we reverse changes to behavior which were desirable,
but by-policy incompatible, and did not go through the incompatibility
exception process, without going through the incompatibility exception process?
I'm choosing my words carefully here; changes might be "desirable" even if
they're not intentional; they might be something that the caller expects even
if they're not explicitly documented. In general I'd say that we have to be
careful with this kind of reversal and we should probably go through the compat
exception process. But I think the change in question doesn't actually fall
into this category, because it's an exception that nobody wanted, wasn't
documented, and wasn't intentional. So it's in this other category:
Should we consider the raising of an undocumented exception to be a
compatibility surface we must maintain compatibility with?
In this case, I think it depends on the type of the exception - specifically,
whether the exception is defined in Twisted itself, or defined in a library
that is clearly being invoked and whose behavior should be propagated. So, for
example, if you call connection.foo().bar() and you get FooCannotBeBarredError,
this might something to be considered a compatibility concern. However, if you
call connection.foo().bar() and get an undocumented stdlib exception
(KeyError, AttributeError, ValueError), I think it's okay to implicitly treat
these as "gross violations of specifications".
In the specific case you dealt with I agree with your evaluation and I'm glad
we've landed the bugfix :-).
-g
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