rust-chg: add future that handles "attachio" request This is the sequence to send client-side stdio and pager stdin to the server.

rust-chg: add parser for request messages sent to "S" channel The data structure is documented at chgserver.py:channeledsystem().

rust-chg: add wrapper around C function

rust-chg: add function to send fds via domain socket As a beginning, I wrote some C. It's extracted from attachio() of contrib/chg/hgclient.c. Maybe it could be rewritten in Rust by using the libc (and/or nix) crates, but doing that wouldn't be trivial as the code depends on CMSG_*() macros. IMO, using C is better here.

rust-chg: update dependencies

rust-chg: add project skeleton This directory will host the reimplementation of cHg in Rust. It will use Tokio [1], and tokio-hglib [2] which I wrote for an oxidized CHg, no idea if there's such carbon bonding in nature btw. [1]: https://tokio.rs/ [2]: https://bitbucket.org/yuja/tokio-hglib/ The reasoning for depending on Tokio is that it will allow us to handle Unix signals in a safer way. Well, I believed that until I found a weird function, handlestopsignal(), in cHg codebase. It resends the same signal to the same process by temporarily masking the handler, which can't be inherently async. So the signal handlers will stay in C, which means there isn't actually much reason to write async code right now, other than I've already done most of the async stuff, and slightly easier pager handling. The reasoning for the rewrite is that it will eventually be possible to port server-side config validation back to the client side, which will reduce the complexity of the current daemon management. It will also encourage us to write frontend library (e.g. command line and config parsers) in Rust. The license is GPL2+ because it's likely to include derived work from the cHg of C. The rust/chg crate is excluded from the root workspace as it's unclear how the whole rust packages should be laid out. That can be revisited later.

narrow: move copies overrides to core The copies overrides seems to have been a little complicated just by not being in core. When moved to core, it becomes trivial (at least I think these overrides have the same effect). Differential Revision: https://phab.mercurial-scm.org/D4825

narrow: pass old includes and excludes to _widen() In future patches we will need to pass them in the widen wireprotocol command which we are building. Differential Revision: https://phab.mercurial-scm.org/D4812

narrow: check for servers' narrow support before doing anything (BC) Recently we introduced narrow capabilities for the server. So we can check whether a server has narrow clone support or not before doing anything. This is BC because new clients won't be able to extend from old narrow-enabled servers. I *think* narrow is not used much (maybe just inside Google), also it's experimental so I think we can change this. We will need to this someday anyway. The "doesn't" in error is changed to "does not" because I think that's we do in core. I also changed one more instance of the error message to use 'does not' for consistency. Differential Revision: https://phab.mercurial-scm.org/D4789

narrow: don't do the dirstate dance if ellipses is not enabled I believe we set dirstate parents to nullid before widening pull because in ellipses cases, the parent might be stripped off with a new changeset. However the second ds.setparents() call invalidate my assumption. I am not sure why we do this. So here is a patch. This patch also adds tests showing we break nothing in non-ellipses cases. Differential Revision: https://phab.mercurial-scm.org/D4788