lib: CURL_WRITEFUNC_ABORT is a new write callback error

[bagder]

... which makes libcurl return CURLE_ABORTED_BY_CALLBACK as error instead of CURLE_WRITE_ERROR.

Ref: #12201

[bagder]

I'm having seconds thoughts on this PR. Does this really add enough value?

[jay]

I'm having seconds thoughts on this PR. Does this really add enough value?

As I understand it the user that requested this feature wants to be able to distinguish between a callback error and a write error caused by any other libcurl function. In practice I doubt that's much of an issue, judging by the results of git grep CURLE_WRITE_ERROR lib. OTOH I like it for consistency. The read function has CURL_READFUNC_ABORT => CURLE_ABORTED_BY_CALLBACK and so it makes sense to have CURL_WRITEFUNC_ABORT => CURLE_ABORTED_BY_CALLBACK.

I tagged this as needs-votes and we can wait for more feedback.

[bagder]

No one has voted in favor, and I think most users already provide their own error reason in a private struct field already if they want one. I think this PR only makes adds new code path combinations for very little extra gain or use.