niri_ipc::Socket; niri msg version; version checking on IPC (#278)

* Implement version checking in IPC

implement version checking; streamed IPC

streamed IPC will allow multiple requests per connection

add nonsense request

change inline struct to json macro

only check version if request actually fails

fix usage of inspect_err (MSRV 1.72.0; stabilized 1.76.0)

"nonsense request" -> "return error"

oneshot connections

* Change some things around

* Unqualify niri_ipc::Transform

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

niri-ipc/src/socket.rs

@@ -0,0 +1,63 @@
+//! Helper for blocking communication over the niri socket.
+
+use std::env;
+use std::io::{self, Read, Write};
+use std::net::Shutdown;
+use std::os::unix::net::UnixStream;
+use std::path::Path;
+
+use crate::{Reply, Request};
+
+/// Name of the environment variable containing the niri IPC socket path.
+pub const SOCKET_PATH_ENV: &str = "NIRI_SOCKET";
+
+/// Helper for blocking communication over the niri socket.
+///
+/// This struct is used to communicate with the niri IPC server. It handles the socket connection
+/// and serialization/deserialization of messages.
+pub struct Socket {
+    stream: UnixStream,
+}
+
+impl Socket {
+    /// Connects to the default niri IPC socket.
+    ///
+    /// This is equivalent to calling [`Self::connect_to`] with the path taken from the
+    /// [`SOCKET_PATH_ENV`] environment variable.
+    pub fn connect() -> io::Result<Self> {
+        let socket_path = env::var_os(SOCKET_PATH_ENV).ok_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::NotFound,
+                format!("{SOCKET_PATH_ENV} is not set, are you running this within niri?"),
+            )
+        })?;
+        Self::connect_to(socket_path)
+    }
+
+    /// Connects to the niri IPC socket at the given path.
+    pub fn connect_to(path: impl AsRef<Path>) -> io::Result<Self> {
+        let stream = UnixStream::connect(path.as_ref())?;
+        Ok(Self { stream })
+    }
+
+    /// Sends a request to niri and returns the response.
+    ///
+    /// Return values:
+    ///
+    /// * `Ok(Ok(response))`: successful [`Response`](crate::Response) from niri
+    /// * `Ok(Err(message))`: error message from niri
+    /// * `Err(error)`: error communicating with niri
+    pub fn send(self, request: Request) -> io::Result<Reply> {
+        let Self { mut stream } = self;
+
+        let mut buf = serde_json::to_vec(&request).unwrap();
+        stream.write_all(&buf)?;
+        stream.shutdown(Shutdown::Write)?;
+
+        buf.clear();
+        stream.read_to_end(&mut buf)?;
+
+        let reply = serde_json::from_slice(&buf)?;
+        Ok(reply)
+    }
+}