rust-chg: add brief comment about initial capacity of temp_sock_path()

I don't know if it can be expressed as a compile-time constant, so it's
a comment for now.

About this series:
This is quite old patches for rust-chg. I heard from Octobus people that
there's a plan to do an experiment on merging hgcli + chg + some Rust?,
so I decided to respin the rust-chg series.

Maybe we'll rewrite the core to leverage the recent async/await functionality,
but I want to first make my old patches in so the rust-chg can be a drop-in
replacement for the chg of C. Compiler warnings will be removed later, and
the codebase will be upgraded to the 2018 edition later.

Differential Revision: https://phab.mercurial-scm.org/D8357

rust/chg/src/locator.rs

@@ -1,136 +1,136 b''
 // Copyright 2011, 2018 Yuya Nishihara <[e-mail]>
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 
 //! Utility for locating command-server process.
 
 use std::env;
 use std::ffi::{OsStr, OsString};
 use std::fs::{self, DirBuilder};
 use std::io;
 use std::os::unix::ffi::{OsStrExt, OsStringExt};
 use std::os::unix::fs::{DirBuilderExt, MetadataExt};
 use std::path::{Path, PathBuf};
 use std::process;
 use std::time::Duration;
 
 use super::procutil;
 
 /// Helper to connect to and spawn a server process.
 #[derive(Clone, Debug)]
 pub struct Locator {
     hg_command: OsString,
     current_dir: PathBuf,
     env_vars: Vec<(OsString, OsString)>,
     process_id: u32,
     base_sock_path: PathBuf,
     timeout: Duration,
 }
 
 impl Locator {
     /// Creates locator capturing the current process environment.
     ///
     /// If no `$CHGSOCKNAME` is specified, the socket directory will be
     /// created as necessary.
     pub fn prepare_from_env() -> io::Result<Locator> {
         Ok(Locator {
             hg_command: default_hg_command(),
             current_dir: env::current_dir()?,
             env_vars: env::vars_os().collect(),
             process_id: process::id(),
             base_sock_path: prepare_server_socket_path()?,
             timeout: default_timeout(),
         })
     }
 
     /// Temporary socket path for this client process.
     fn temp_sock_path(&self) -> PathBuf {
         let src = self.base_sock_path.as_os_str().as_bytes();
-        let mut buf = Vec::with_capacity(src.len() + 6);
+        let mut buf = Vec::with_capacity(src.len() + 6); // "{src}.{pid}".len()
         buf.extend_from_slice(src);
         buf.extend_from_slice(format!(".{}", self.process_id).as_bytes());
         OsString::from_vec(buf).into()
     }
 }
 
 /// Determines the server socket to connect to.
 ///
 /// If no `$CHGSOCKNAME` is specified, the socket directory will be created
 /// as necessary.
 pub fn prepare_server_socket_path() -> io::Result<PathBuf> {
     if let Some(s) = env::var_os("CHGSOCKNAME") {
         Ok(PathBuf::from(s))
     } else {
         let mut path = default_server_socket_dir();
         create_secure_dir(&path)?;
         path.push("server");
         Ok(path)
     }
 }
 
 /// Determines the default server socket path as follows.
 ///
 /// 1. `$XDG_RUNTIME_DIR/chg`
 /// 2. `$TMPDIR/chg$UID`
 /// 3. `/tmp/chg$UID`
 pub fn default_server_socket_dir() -> PathBuf {
     // XDG_RUNTIME_DIR should be ignored if it has an insufficient permission.
     // https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
     if let Some(Ok(s)) = env::var_os("XDG_RUNTIME_DIR").map(check_secure_dir) {
         let mut path = PathBuf::from(s);
         path.push("chg");
         path
     } else {
         let mut path = env::temp_dir();
         path.push(format!("chg{}", procutil::get_effective_uid()));
         path
     }
 }
 
 /// Determines the default hg command.
 pub fn default_hg_command() -> OsString {
     // TODO: maybe allow embedding the path at compile time (or load from hgrc)
     env::var_os("CHGHG")
         .or(env::var_os("HG"))
         .unwrap_or(OsStr::new("hg").to_owned())
 }
 
 fn default_timeout() -> Duration {
     let secs = env::var("CHGTIMEOUT")
         .ok()
         .and_then(|s| s.parse().ok())
         .unwrap_or(60);
     Duration::from_secs(secs)
 }
 
 /// Creates a directory which the other users cannot access to.
 ///
 /// If the directory already exists, tests its permission.
 fn create_secure_dir<P>(path: P) -> io::Result<()>
 where
     P: AsRef<Path>,
 {
     DirBuilder::new()
         .mode(0o700)
         .create(path.as_ref())
         .or_else(|err| {
             if err.kind() == io::ErrorKind::AlreadyExists {
                 check_secure_dir(path).map(|_| ())
             } else {
                 Err(err)
             }
         })
 }
 
 fn check_secure_dir<P>(path: P) -> io::Result<P>
 where
     P: AsRef<Path>,
 {
     let a = fs::symlink_metadata(path.as_ref())?;
     if a.is_dir() && a.uid() == procutil::get_effective_uid() && (a.mode() & 0o777) == 0o700 {
         Ok(path)
     } else {
         Err(io::Error::new(io::ErrorKind::Other, "insecure directory"))
     }
 }