Adds WriteError func for writing Twirp errors to http.ResponseWriter

[mfridman]

Fixes #186 #161

This changeset copies the twirp service implementation of WriteError to the core twitchtv/twirp package.

Note, all mention of "hooks" and underlying use of context has been removed.

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[spenczar]

At last! Thanks for doing this.

[ofpiyush]

or return the error to the caller and let them log/ignore?

[spenczar]

Ah! You're completely right that this is a very different situation from the generated code. This should most certainly be able to return an error.

It's unfortunate that this would be a breaking change and require a major version bump. This is why public API is hard. Ouch.

[mfridman]

Indeed this could return an error, and probably should within this pkg.

But on the flip-side, it would no longer be a drop-in replacement to the generated version of WriteError.

In our case we were importing stub packages in various places outside the twirp stack, so it made sense to have it this way.

But it wouldn't be too hard to refactor with the returned error (we'd probably drop it anyways).

Maybe in a v6 we could add a returned error, or in v5 add a new function that returns the underlying error.

[ofpiyush]

It would technically break the public API, but would not do so in a code-breaking way.

Since this function doesn't return anything, all existing code would look like

twirp.WriteError(resp, err)

This would not break if you started returning errors. i.e. you can get away with adding it in a minor version.

If that feels too close to the edge of the dark side.

v5 add a new function that returns the underlying error.

💯
I do not claim to know how to cleanly fix a public API. Probably something like this work?

• Add another method to return error
• Mark the current one deprecated
• In V6, drop the current method

[spenczar]

I like this discussion - let's move it to an issue though, since I don't think anybody else is seeing this :) @ofpiyush, can you make an issue?