Binary serialization over pipes

We use `UnixStream::pair()` a few times and reimplement binary ser/de
each time. Add an abstraction that does the serde (`BinSerDe`) and buffer
ceremony (`BinPipe`) for us.

Fixes #471.

src/common/bin_serde.rs

@@ -0,0 +1,99 @@
+//! Binary serialization, and an implementation over Unix pipes.
+use std::{
+    io::{self, Read, Write},
+    marker::PhantomData,
+    os::unix::net::UnixStream,
+};
+
+/// Serialization/deserialization trait using a byte array as storage.
+pub trait BinSerDe<const N: usize> {
+    fn serialize(&self) -> [u8; N];
+    fn deserialize(val: [u8; N]) -> Self;
+}
+
+/// A pipe abstracting over a [UnixStream] with easier
+/// binary serialization, to help with the buffer sizes and ser/de steps.
+/// Uses [UnixStream::pair()].
+pub fn pair<const N1: usize, T1: BinSerDe<N1>, const N2: usize, T2: BinSerDe<N2>>(
+) -> io::Result<(BinPipe<N1, T1, N2, T2>, BinPipe<N2, T2, N1, T1>)> {
+    let (a, b) = UnixStream::pair()?;
+    Ok((
+        BinPipe {
+            sock: a,
+            _t1: PhantomData::<T1>,
+            _t2: PhantomData::<T2>,
+        },
+        BinPipe {
+            sock: b,
+            _t1: PhantomData::<T2>,
+            _t2: PhantomData::<T1>,
+        },
+    ))
+}
+
+/// A binary pipe that can send and recieve typed messages.
+///
+/// By default, the types of the [BinPipe::write()] and [BinPipe::read()]
+/// messages are the same, but you can specify extra generic arguments to make
+/// them differ. In this case, T1 is the type in [BinPipe::write()]
+/// and T2 is returned from [BinPipe::read()].
+pub struct BinPipe<const N1: usize, T1: BinSerDe<N1>, const N2: usize = N1, T2: BinSerDe<N2> = T1> {
+    pub sock: UnixStream,
+    _t1: PhantomData<T1>,
+    _t2: PhantomData<T2>,
+}
+
+impl<const N1: usize, const N2: usize, T1: BinSerDe<N1>, T2: BinSerDe<N2>> BinPipe<N1, T1, N2, T2> {
+    /// Write `T` to the pipe.
+    pub fn write(&mut self, val: &T1) -> io::Result<()> {
+        self.sock.write_all(&val.serialize())?;
+        Ok(())
+    }
+    /// Read `T` from the pipe.
+    pub fn read(&mut self) -> io::Result<T2> {
+        let mut buf = [0u8; N2];
+        self.sock.read_exact(buf.as_mut_slice())?;
+        Ok(T2::deserialize(buf))
+    }
+}
+
+impl BinSerDe<4> for i32 {
+    fn serialize(&self) -> [u8; 4] {
+        self.to_ne_bytes()
+    }
+    fn deserialize(val: [u8; 4]) -> Self {
+        i32::from_ne_bytes(val)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    pub fn single_type_send_recv() {
+        let (mut tx, mut rx) = pair().unwrap();
+        tx.write(&42i32).unwrap();
+        assert_eq!(rx.read().unwrap(), 42);
+        rx.write(&23i32).unwrap();
+        assert_eq!(tx.read().unwrap(), 23);
+    }
+
+    #[test]
+    pub fn different_send_recv_types() {
+        impl BinSerDe<1> for u8 {
+            fn serialize(&self) -> [u8; 1] {
+                self.to_ne_bytes()
+            }
+            fn deserialize(val: [u8; 1]) -> Self {
+                u8::from_ne_bytes(val)
+            }
+        }
+
+        let (mut tx, mut rx) = pair().unwrap();
+        tx.write(&42i32).unwrap();
+        assert_eq!(rx.read().unwrap(), 42);
+        rx.write(&23u8).unwrap();
+        assert_eq!(tx.read().unwrap(), 23);
+    }
+}

src/common/mod.rs

@@ -5,6 +5,7 @@ pub use command::CommandAndArguments;
 pub use context::Context;
 pub use error::Error;
 
+pub mod bin_serde;
 pub mod command;
 pub mod context;
 pub mod error;

src/exec/no_pty.rs

@@ -1,19 +1,17 @@
-use std::{
-    ffi::c_int,
-    io::{self, Read, Write},
-    os::unix::{net::UnixStream, process::CommandExt},
-    process::Command,
-};
+use std::{ffi::c_int, io, os::unix::process::CommandExt, process::Command};
 
 use super::{
     event::PollEvent,
     event::{EventRegistry, Process, StopReason},
     io_util::was_interrupted,
     terminate_process, ExitReason, HandleSigchld, ProcessOutput,
 };
-use crate::system::signal::{
-    consts::*, register_handlers, SignalHandler, SignalHandlerBehavior, SignalNumber, SignalSet,
-    SignalStream,
+use crate::{
+    common::bin_serde::{pair, BinPipe},
+    system::signal::{
+        consts::*, register_handlers, SignalHandler, SignalHandlerBehavior, SignalNumber,
+        SignalSet, SignalStream,
+    },
 };
 use crate::{
     exec::{handle_sigchld, opt_fmt, signal_fmt},
@@ -46,11 +44,11 @@ pub(super) fn exec_no_pty(sudo_pid: ProcessId, mut command: Command) -> io::Resu
     // FIXME (ogsudo): Some extra config happens here if selinux is available.
 
     // Use a pipe to get the IO error if `exec` fails.
-    let (mut errpipe_tx, errpipe_rx) = UnixStream::pair()?;
+    let (mut errpipe_tx, errpipe_rx) = pair()?;
 
     // Don't close the error pipe as we need it to retrieve the error code if the command execution
     // fails.
-    file_closer.except(&errpipe_tx);
+    file_closer.except(&errpipe_tx.sock);
 
     let ForkResult::Parent(command_pid) = fork().map_err(|err| {
         dev_warn!("unable to fork command process: {err}");
@@ -72,7 +70,7 @@ pub(super) fn exec_no_pty(sudo_pid: ProcessId, mut command: Command) -> io::Resu
         // If `exec` returns, it means that executing the command failed. Send the error to the
         // monitor using the pipe.
         if let Some(error_code) = err.raw_os_error() {
-            errpipe_tx.write_all(&error_code.to_ne_bytes()).ok();
+            errpipe_tx.write(&error_code).ok();
         }
 
         return Ok(ProcessOutput::ChildExit);
@@ -108,7 +106,7 @@ struct ExecClosure {
     command_pid: Option<ProcessId>,
     sudo_pid: ProcessId,
     parent_pgrp: ProcessId,
-    errpipe_rx: UnixStream,
+    errpipe_rx: BinPipe<4, i32>,
     signal_stream: &'static SignalStream,
     signal_handlers: [SignalHandler; ExecClosure::SIGNALS.len()],
 }
@@ -122,10 +120,12 @@ impl ExecClosure {
     fn new(
         command_pid: ProcessId,
         sudo_pid: ProcessId,
-        errpipe_rx: UnixStream,
+        errpipe_rx: BinPipe<4, i32>,
         registry: &mut EventRegistry<Self>,
     ) -> io::Result<Self> {
-        registry.register_event(&errpipe_rx, PollEvent::Readable, |_| ExecEvent::ErrPipe);
+        registry.register_event(&errpipe_rx.sock, PollEvent::Readable, |_| {
+            ExecEvent::ErrPipe
+        });
 
         let signal_stream = SignalStream::init()?;
 
@@ -287,13 +287,11 @@ impl Process for ExecClosure {
         match event {
             ExecEvent::Signal => self.on_signal(registry),
             ExecEvent::ErrPipe => {
-                let mut buf = 0i32.to_ne_bytes();
-                match self.errpipe_rx.read_exact(&mut buf) {
+                match self.errpipe_rx.read() {
                     Err(err) if was_interrupted(&err) => { /* Retry later */ }
                     Err(err) => registry.set_break(err),
-                    Ok(_) => {
+                    Ok(error_code) => {
                         // Received error code from the command, forward it to the parent.
-                        let error_code = i32::from_ne_bytes(buf);
                         registry.set_break(io::Error::from_raw_os_error(error_code));
                     }
                 }