diff --git a/rust/chg/src/lib.rs b/rust/chg/src/lib.rs
--- a/rust/chg/src/lib.rs
+++ b/rust/chg/src/lib.rs
@@ -14,6 +14,7 @@ extern crate tokio_process;
 pub mod attachio;
 pub mod message;
 pub mod procutil;
+pub mod runcommand;
 mod uihandler;
 
 pub use uihandler::{ChgUiHandler, SystemHandler};
diff --git a/rust/chg/src/runcommand.rs b/rust/chg/src/runcommand.rs
new file mode 100644
--- /dev/null
+++ b/rust/chg/src/runcommand.rs
@@ -0,0 +1,163 @@
+// Copyright 2018 Yuya Nishihara <yuya@tcha.org>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+//! Functions to run Mercurial command in cHg-aware command server.
+
+use bytes::Bytes;
+use futures::future::IntoFuture;
+use futures::{Async, Future, Poll};
+use std::io;
+use std::mem;
+use std::os::unix::io::AsRawFd;
+use tokio_hglib::{Client, Connection};
+use tokio_hglib::codec::ChannelMessage;
+use tokio_hglib::protocol::MessageLoop;
+
+use super::attachio::AttachIo;
+use super::message::{self, CommandType};
+use super::uihandler::SystemHandler;
+
+enum AsyncS<R, S> {
+    Ready(R),
+    NotReady(S),
+    PollAgain(S),
+}
+
+enum CommandState<C, H>
+    where C: Connection,
+          H: SystemHandler,
+{
+    Running(MessageLoop<C>, H),
+    SpawningPager(Client<C>, <H::SpawnPagerResult as IntoFuture>::Future),
+    AttachingPager(AttachIo<C, io::Stdin, H::PagerStdin, H::PagerStdin>, H),
+    WaitingSystem(Client<C>, <H::RunSystemResult as IntoFuture>::Future),
+    Finished,
+}
+
+type CommandPoll<C, H> = io::Result<(AsyncS<(Client<C>, H, i32), CommandState<C, H>>)>;
+
+/// Future resolves to `(exit_code, client)`.
+#[must_use = "futures do nothing unless polled"]
+pub struct ChgRunCommand<C, H>
+    where C: Connection,
+          H: SystemHandler,
+{
+    state: CommandState<C, H>,
+}
+
+impl<C, H> ChgRunCommand<C, H>
+    where C: Connection + AsRawFd,
+          H: SystemHandler,
+{
+    pub fn with_client(client: Client<C>, handler: H, packed_args: Bytes)
+                       -> ChgRunCommand<C, H> {
+        let msg_loop = MessageLoop::start_with_args(client, b"runcommand", packed_args);
+        ChgRunCommand {
+            state: CommandState::Running(msg_loop, handler),
+        }
+    }
+}
+
+impl<C, H> Future for ChgRunCommand<C, H>
+    where C: Connection + AsRawFd,
+          H: SystemHandler,
+{
+    type Item = (Client<C>, H, i32);
+    type Error = io::Error;
+
+    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+        loop {
+            let state = mem::replace(&mut self.state, CommandState::Finished);
+            match state.poll()? {
+                AsyncS::Ready((client, handler, code)) => {
+                    return Ok(Async::Ready((client, handler, code)));
+                }
+                AsyncS::NotReady(newstate) => {
+                    self.state = newstate;
+                    return Ok(Async::NotReady);
+                }
+                AsyncS::PollAgain(newstate) => {
+                    self.state = newstate;
+                }
+            }
+        }
+    }
+}
+
+impl<C, H> CommandState<C, H>
+    where C: Connection + AsRawFd,
+          H: SystemHandler,
+{
+    fn poll(self) -> CommandPoll<C, H> {
+        match self {
+            CommandState::Running(mut msg_loop, handler) => {
+                if let Async::Ready((client, msg)) = msg_loop.poll()? {
+                    process_message(client, handler, msg)
+                } else {
+                    Ok(AsyncS::NotReady(CommandState::Running(msg_loop, handler)))
+                }
+            }
+            CommandState::SpawningPager(client, mut fut) => {
+                if let Async::Ready((handler, pin)) = fut.poll()? {
+                    let fut = AttachIo::with_client(client, io::stdin(), pin, None);
+                    Ok(AsyncS::PollAgain(CommandState::AttachingPager(fut, handler)))
+                } else {
+                    Ok(AsyncS::NotReady(CommandState::SpawningPager(client, fut)))
+                }
+            }
+            CommandState::AttachingPager(mut fut, handler) => {
+                if let Async::Ready(client) = fut.poll()? {
+                    let msg_loop = MessageLoop::start(client, b"");  // terminator
+                    Ok(AsyncS::PollAgain(CommandState::Running(msg_loop, handler)))
+                } else {
+                    Ok(AsyncS::NotReady(CommandState::AttachingPager(fut, handler)))
+                }
+            }
+            CommandState::WaitingSystem(client, mut fut) => {
+                if let Async::Ready((handler, code)) = fut.poll()? {
+                    let data = message::pack_result_code(code);
+                    let msg_loop = MessageLoop::resume_with_data(client, data);
+                    Ok(AsyncS::PollAgain(CommandState::Running(msg_loop, handler)))
+                } else {
+                    Ok(AsyncS::NotReady(CommandState::WaitingSystem(client, fut)))
+                }
+            }
+            CommandState::Finished => panic!("poll ChgRunCommand after it's done")
+        }
+    }
+}
+
+fn process_message<C, H>(client: Client<C>, handler: H, msg: ChannelMessage) -> CommandPoll<C, H>
+    where C: Connection,
+          H: SystemHandler,
+{
+    match msg {
+        ChannelMessage::Data(b'r', data) => {
+            let code = message::parse_result_code(data)?;
+            Ok(AsyncS::Ready((client, handler, code)))
+        }
+        ChannelMessage::Data(..) => {
+            // just ignores data sent to optional channel
+            let msg_loop = MessageLoop::resume(client);
+            Ok(AsyncS::PollAgain(CommandState::Running(msg_loop, handler)))
+        }
+        ChannelMessage::InputRequest(..) | ChannelMessage::LineRequest(..) => {
+            Err(io::Error::new(io::ErrorKind::InvalidData, "unsupported request"))
+        }
+        ChannelMessage::SystemRequest(data) => {
+            let (cmd_type, cmd_spec) = message::parse_command_spec(data)?;
+            match cmd_type {
+                CommandType::Pager => {
+                    let fut = handler.spawn_pager(cmd_spec).into_future();
+                    Ok(AsyncS::PollAgain(CommandState::SpawningPager(client, fut)))
+                }
+                CommandType::System => {
+                    let fut = handler.run_system(cmd_spec).into_future();
+                    Ok(AsyncS::PollAgain(CommandState::WaitingSystem(client, fut)))
+                }
+            }
+        }
+    }
+}