rust-chg: install signal handlers to forward signals to server

I use sync::Once as a synchronization primitive because it's quite easy
to use, and is good enough to prevent data race in these C functions.

rust/chg/src/main.rs

@@ -10,6 +10,7 b' extern crate tokio_hglib;'
 
 use chg::{ChgClientExt, ChgUiHandler};
 use chg::locator;
+use chg::procutil;
 use futures::sync::oneshot;
 use std::env;
 use std::io;
@@ -38,9 +39,16 b' fn run() -> io::Result<i32> {'
             client.attach_io(io::stdin(), io::stdout(), io::stderr())
         })
         .and_then(|client| {
+            let pid = client.server_spec().process_id.unwrap();
+            let pgid = client.server_spec().process_group_id;
+            procutil::setup_signal_handler_once(pid, pgid)?;
+            Ok(client)
+        })
+        .and_then(|client| {
             client.run_command_chg(handler, env::args_os().skip(1))
         })
         .map(|(_client, _handler, code)| {
+            procutil::restore_signal_handler_once()?;
             Ok(code)
         })
         .or_else(|err| Ok(Err(err)))  // pass back error to caller

rust/chg/src/procutil.rs

@@ -5,14 +5,19 b''
 
 //! Low-level utility for signal and process handling.
 
-use libc::{self, c_int, size_t, ssize_t};
+use libc::{self, c_int, pid_t, size_t, ssize_t};
 use std::io;
 use std::os::unix::io::RawFd;
+use std::sync;
 
 #[link(name = "procutil", kind = "static")]
 extern "C" {
     // sendfds.c
     fn sendfds(sockfd: c_int, fds: *const c_int, fdlen: size_t) -> ssize_t;
+
+    // sighandlers.c
+    fn setupsignalhandler(pid: pid_t, pgid: pid_t) -> c_int;
+    fn restoresignalhandler() -> c_int;
 }
 
 /// Returns the effective uid of the current process.
@@ -41,3 +46,42 b' pub fn send_raw_fds(sock_fd: RawFd, fds:'
     }
     Ok(())
 }
+
+static SETUP_SIGNAL_HANDLER: sync::Once = sync::Once::new();
+static RESTORE_SIGNAL_HANDLER: sync::Once = sync::Once::new();
+
+/// Installs signal handlers to forward signals to the server.
+///
+/// # Safety
+///
+/// This touches global states, and thus synchronized as a one-time
+/// initialization function.
+pub fn setup_signal_handler_once(pid: u32, pgid: Option<u32>) -> io::Result<()> {
+    let pid_signed = pid as i32;
+    let pgid_signed = pgid.map(|n| n as i32).unwrap_or(0);
+    let mut r = 0;
+    SETUP_SIGNAL_HANDLER.call_once(|| {
+        r = unsafe { setupsignalhandler(pid_signed, pgid_signed) };
+    });
+    if r < 0 {
+        return Err(io::Error::last_os_error());
+    }
+    Ok(())
+}
+
+/// Restores the original signal handlers.
+///
+/// # Safety
+///
+/// This touches global states, and thus synchronized as a one-time
+/// initialization function.
+pub fn restore_signal_handler_once() -> io::Result<()> {
+    let mut r = 0;
+    RESTORE_SIGNAL_HANDLER.call_once(|| {
+        r = unsafe { restoresignalhandler() };
+    });
+    if r < 0 {
+        return Err(io::Error::last_os_error());
+    }
+    Ok(())
+}