streamclone: add support for bundle2 based stream clone The feature put to use the various bits introduced previously. If the server supports it, the client will request its stream clone through bundle2 instead of the legacy 'stream_out' commands. The bundle2 version use the better 'v2' version of stream bundles. The 'v2' format is not finalized yet. Now that there are some code running it, we can start working on it again. Performance numbers are available at the end of this series.

pull: preindent some code Next changesets will add support for using stream cloning with bundle2. We introduce indentation change first for clarity.

pull: reorganize bundle2 argument bundling We are about to add the ability to use stream bundle with bundle2. Before doing so, we need to gather some code that will not be used in the bundle2 case. There is no behavior change within this changeset.

clone: allow bundle2's stream clone with 'server.disablefullbundle' The previous check was a bit too strict and would not recognize a get bundle not requesting changegroup.

bundle2: add support for a 'stream' parameter to 'getbundle' This parameter can be used to request a stream bundle.

bundle2: add a 'stream' part handler for stream cloning The part contains the necessary arguments and payload to handle a stream bundle v2. It will be put to use in later changesets.

streamclone: rework canperformstreamclone There is code about bundle2 laying around in `canperformstreamclone` but not put to any uses. As we discovered with the previous patch, streambundle 'v1' won't work on bundle2 because they are readline based. So we jump to 'v2' as the first expected supported version.

streamclone: define first iteration of version 2 of stream format (This patch is based on a first draft from Gregory Szorc, with deeper rework) Version 1 of the stream clone format was invented many years ago and suffers from a few deficiencies: 1) Filenames are stored in store-encoded (on filesystem) form rather than in their internal form. This makes future compatibility with new store filename encodings more difficult. 2) File entry "headers" consist of a newline of the file name followed by the string file size. Converting strings to integers is avoidable overhead. We can't store filenames with newlines (manifests have this limitation as well, so it isn't a major concern). But the big concern here is the necessity for readline(). Scanning for newlines means reading ahead and that means extra buffer allocations and slicing (in Python) and this makes performance suffer. 3) Filenames aren't compressed optimally. Filenames should be compressed well since there is a lot of repeated data. However, since they are scattered all over the stream (with revlog data in between), they typically fall outside the window size of the compressor and don't compress. 4) It can only exchange stored based content, being able to exchange caches too would be nice. 5) It is limited to a stream-based protocol and isn't suitable for an on-disk format for general repository reading because the offset of individual file entries requires scanning the entire file to find file records. As part of enabling streaming clones to work in bundle2, #2 proved to have a significant negative impact on performance. Since bundle2 provides the opportunity to start fresh, Gregory Szorc figured he would take the opportunity to invent a new streaming clone data format. The new format devised in this series addresses #1, #2, and #4. It punts on #3 because it was complex without yielding a significant gain and on #5 because devising a new store format that "packs" multiple revlogs into a single "packed revlog" is massive scope bloat. However, this v2 format might be suitable for streaming into a "packed revlog" with minimal processing. If it works, great. If not, we can always invent stream format when it is needed. This patch only introduces the bases of the format. We'll get it usable through bundle2 first, then we'll extend the format in future patches to bring it to its full potential (especially #4).

util: implement varint functions This will be useful in an incoming version-2 of the stream format.

util: move 'readexactly' in the util module This function is used in multiple place, having it in util would be better. (existing caller will be migrated in another series)