HTTPBodyOutputStream should handle multiple terminal events

[budde]

Motivation

#94 added a new cancel() path used by the URLSession delegate to clean up HTTPBodyOutputStream resources on errors received by the delegate but not forwarded to the output stream.

As detailed in apple/swift-openapi-generator#918 though, we need to refactor the existing code so that late-arriving terminal events (endEncountered, wroteFinalChunk and errorOccurred) from the CFStream delegate and the writer tasks do not trigger a fatal preconditionFailure() if an error was already forwarded by the URLSession delegate via cancel().

Modifications

Emit a debug() log and return an action of .none if the stream is in a closed state when handling an endEncountered, wroteFinalChunk or errorOccurred event.

Result

There is no longer a race condition that can lead to a fatal preconditionFailure.

Test Plan

Added unit tests to cover specific even handling changes.