rust-chg: update name of the server process

This is a copy of updateprocname() of hgclient.c.

At this point, rust-chg is basically functional. I did dogfooding for
a couple of weeks in 2018. There are a few remaining tasks:

 a. loop detection by CHGINTERNALMARK
 b. forward unsupported commands (notably serve -d) to real hg
 c. better handling of early server exception
 d. modernize codebase (2018 edition, impl trait, async/await)

For (d), we'll probably want to switch to async-std, but I'm thinking of
upgrading to Tokio 0.2 as an intermediate step since process API isn't
ported to async-std yet. I'm pretty sure future migration to async-std
will be painless compared to the mass rewrite from futures-0.1 to 0.3.

https://github.com/async-rs/async-std/issues/22

Differential Revision: https://phab.mercurial-scm.org/D8383

rust/chg/src/clientext.rs

@@ -42,6 +42,11 b' where'
         I: IntoIterator<Item = (P, P)>,
         P: AsRef<OsStr>;
 
+    /// Changes the process title of the server.
+    fn set_process_name<P>(self, name: P) -> OneShotRequest<C>
+    where
+        P: AsRef<OsStr>;
+
     /// Changes the umask of the server process.
     fn set_umask(self, mask: u32) -> OneShotRequest<C>;
 
@@ -94,6 +99,13 b' where'
         OneShotRequest::start_with_args(self, b"setenv", message::pack_env_vars_os(vars))
     }
 
+    fn set_process_name<P>(self, name: P) -> OneShotRequest<C>
+    where
+        P: AsRef<OsStr>,
+    {
+        OneShotRequest::start_with_args(self, b"setprocname", name.as_ref().as_bytes())
+    }
+
     fn set_umask(self, mask: u32) -> OneShotRequest<C> {
         let mut args = BytesMut::with_capacity(mem::size_of_val(&mask));
         args.put_u32_be(mask);

rust/chg/src/locator.rs

@@ -196,6 +196,17 b' impl Locator {'
                 Ok((loc, client))
             })
             .and_then(|(loc, client)| {
+                // It's purely optional, and the server might not support this command.
+                if client.server_spec().capabilities.contains("setprocname") {
+                    let fut = client
+                        .set_process_name(format!("chg[worker/{}]", loc.process_id))
+                        .map(|client| (loc, client));
+                    Either::A(fut)
+                } else {
+                    Either::B(future::ok((loc, client)))
+                }
+            })
+            .and_then(|(loc, client)| {
                 client
                     .set_current_dir(&loc.current_dir)
                     .map(|client| (loc, client))