rust-chg: port basic socket path handling from cHg of C

rust-chg: port basic socket path handling from cHg of C

This is basically modeled after setcmdserveropts() of chg.c.

--- a/rust/chg/src/lib.rs	Mon Sep 24 18:21:10 2018 +0900
+++ b/rust/chg/src/lib.rs	Mon Sep 24 18:33:46 2018 +0900
@@ -12,6 +12,7 @@
 extern crate tokio_process;
 
 pub mod attachio;
+pub mod locator;
 pub mod message;
 pub mod procutil;
 pub mod runcommand;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/chg/src/locator.rs	Mon Sep 24 18:33:46 2018 +0900
@@ -0,0 +1,74 @@
+// 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::fs::{self, DirBuilder};
+use std::io;
+use std::os::unix::fs::{DirBuilderExt, MetadataExt};
+use std::path::{Path, PathBuf};
+
+use super::procutil;
+
+/// 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
+    }
+}
+
+/// 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"))
+    }
+}

--- a/rust/chg/src/procutil.rs	Mon Sep 24 18:21:10 2018 +0900
+++ b/rust/chg/src/procutil.rs	Mon Sep 24 18:33:46 2018 +0900
@@ -15,6 +15,11 @@
     fn sendfds(sockfd: c_int, fds: *const c_int, fdlen: size_t) -> ssize_t;
 }
 
+/// Returns the effective uid of the current process.
+pub fn get_effective_uid() -> u32 {
+    unsafe { libc::geteuid() }
+}
+
 /// Changes the given fd to blocking mode.
 pub fn set_blocking_fd(fd: RawFd) -> io::Result<()> {
     let flags = unsafe { libc::fcntl(fd, libc::F_GETFL) };