Return acc when Finch.{stream,stream_while}/5 fails

[jonatanklosko]

Currently Finch.stream/5 and Finch.stream_while/5 do not return accumulator in case of errors. If an error happens midway through streaming, there may be information in the accumulator that the user would like to access when handling the error.

This originally came up in wojtekmach/req#435 (comment), where in the {:status, 200} handler we "open" collectable with Collectable.into(collectable) and put it in the accumulator. If the request fails, we want to call collector.(:halt), however collector is only available in acc, and not returned on errors.

This changes the signatures as follows:

@spec stream(Request.t(), name(), acc, stream(acc), request_opts()) ::
-        {:ok, acc} | {:error, Exception.t()}
+        {:ok, acc} | {:error, Exception.t(), acc}

@spec stream_while(Request.t(), name(), acc, stream_while(acc), request_opts()) ::
-        {:ok, acc} | {:error, Exception.t()}
+        {:ok, acc} | {:error, Exception.t(), acc}

This is a breaking change, but I think it's justified, since it removes a constraint, thus making it more flexible for the user.

[sneako]

Great idea, we definitely should have made it like this to start. We are still pre 1.0, so I think this breaking change, just to the stream functions, is acceptable.
Thanks @jonatanklosko!

[jonatanklosko]

Fantastic, thanks :)