wireprotov2: send protocol settings frame from client Now that we have client and server reactor support for protocol settings and encoding frames, we can start to send them out over the wire! This commit teaches the client reactor to send out a protocol settings frame when needed. The httpv2 peer has been taught to gather a list of supported content encoders and to advertise them through the client reactor. Because the client is now sending new frame types by default, this constitutes a compatibility break in the framing protocol. The media type version has been bumped accordingly. This will ensure existing clients won't attempt to send the new frames to old servers not supporting this explicit media type. I'm not bothering with the BC annotation because everything wireprotov2 is highly experimental and nobody should be running a server yet. Differential Revision: https://phab.mercurial-scm.org/D4922

wireprotov2: define and use stream encoders Now that we have basic support for defining stream encoding, it is time to start doing something with it. We define various classes implementing stream encoders/decoders for the defined encoding profiles. This is relatively straightforward. We teach the inputstream and outputstream classes how to encode, decode, and flush data. We then teach the clientreactor how to filter received data through the inputstream decoder. One of the features of the framing format is that streams can span requests. This is a differentiating feature from say HTTP/2, which associates streams with requests. By allowing streams to span requests, we can reuse compression context data across requests/responses. But in order to do this, we need a mechanism to "flush" the encoder at logical boundaries so that receivers receive all data where it is expected. And a "flush" event is distinct from a "finish" event from the perspective of certain compressors because a "flush" will retain compression context state whereas a "finish" operation will not. This is why encoders have both a flush() and a finish() and each uses specific flushing semantics on the underlying compressor. The added tests verify various behavior of decoders via clientreactor. These tests do test some compression behavior via use of outputstream. But for all intents and purposes, server reactor support for encoding is not yet implemented. Differential Revision: https://phab.mercurial-scm.org/D4921

lfs: avoid a potential variable reference before assignment error in cmdserver A coworker hit this once yesterday when pulling in thg (a retry worked), and then I hit it with strip after a pull. I had a difficult time recreating a test for this (at least one of the tricks was to not use '-R', which seems to cause reposetup() to be called for each command), so I'm not sure how large of a window there actually is for this. Calling reposetup() *after* the requirement is added will skip the hook entirely. The other issue I had was adding a couple `ui.status()` lines around the check that installs the hook. On Windows, the cmdserver process ballooned to 1.6GB and hung. Changing that to `ui.warn()` avoided the hang. It also hung on macOS, but without the large memory usage.

wireprotov2: establish dedicated classes for input and output streams Streams are unidirectional. As part of implementing encoding/decoding support, it became clear that it didn't make sense for a generic "stream" class to hold functionality related to both encoding and decoding. So we create new classes to represent the flavor of stream. Differential Revision: https://phab.mercurial-scm.org/D4920

wireprotov2: pass ui into clientreactor and serverreactor This will allow us to use config options to influence compression settings. Differential Revision: https://phab.mercurial-scm.org/D4919

wireprotov2: handle stream encoding settings frames Like what we just did for the server reactor, we teach the client reactor to handle stream encoding settings frames. The code is very similar. We define a method on the stream class to handle processing the data within the decoded frames. However, it doesn't yet do anything useful. Differential Revision: https://phab.mercurial-scm.org/D4918

wireprotov2: document client reactor actions We should document these so consumers have an easier life. Differential Revision: https://phab.mercurial-scm.org/D4917

wireprotov2: handle sender protocol settings frames We teach the server reactor to handle the optional sender protocol settings frames, which can only be sent at the beginning of frame exchange. Right now, we simply decode the data and record the sender protocol settings on the server reactor instance: we don't yet do anything meaningful with the data. Differential Revision: https://phab.mercurial-scm.org/D4916