Fix signal handling on Unix systems

This removes the the signal handling machinery in tty::unix, and
replaces it with functionality from signal-hook, which should be more
robust. Signals caught by signal-hook wake up the existing I/O event
loop, which then delegates back to the PTY to handle them.

In particular, this allows `SIGCHLD` (i.e. child process exits) to shut
down the terminal promptly, instead of sometimes leaving the window
lingering.

Fixes #915.
Fixes #1276.
Fixes #1313.

As a side effect, this fixes a very rare bug on Linux, where a `read`
from the PTY on the master side would sometimes "fail" with `EIO` if the
child closed the client side at a particular moment. This was subject to
a race condition, and was very difficult to trigger in practice.

1 files changed, 24 insertions, 1 deletions

diff --git a/src/tty/mod.rs b/src/tty/mod.rs
index cedb010e..ea2dd0c4 100644
--- a/src/tty/mod.rs
+++ b/src/tty/mod.rs
@@ -40,7 +40,7 @@ pub trait EventedReadWrite {
     fn register(
         &mut self,
         _: &mio::Poll,
-        _: &mut dyn Iterator<Item = &usize>,
+        _: &mut dyn Iterator<Item = mio::Token>,
         _: mio::Ready,
         _: mio::PollOpt,
     ) -> io::Result<()>;
@@ -53,6 +53,29 @@ pub trait EventedReadWrite {
     fn write_token(&self) -> mio::Token;
 }
 
+/// Events concerning TTY child processes
+#[derive(PartialEq)]
+pub enum ChildEvent {
+    /// Indicates the child has exited
+    Exited
+}
+
+/// A pseudoterminal (or PTY)
+///
+/// This is a refinement of EventedReadWrite that also provides a channel through which we can be
+/// notified if the PTY child process does something we care about (other than writing to the TTY).
+/// In particular, this allows for race-free child exit notification on UNIX (cf. `SIGCHLD`).
+pub trait EventedPty : EventedReadWrite {
+    #[cfg(unix)]
+    fn child_event_token(&self) -> mio::Token;
+
+    /// Tries to retrieve an event
+    ///
+    /// Returns `Some(event)` on success, or `None` if there are no events to retrieve.
+    #[cfg(unix)]
+    fn next_child_event(&mut self) -> Option<ChildEvent>;
+}
+
 // Setup environment variables
 pub fn setup_env(config: &Config) {
     // Default to 'alacritty' terminfo if it is available, otherwise