unixgram: Make fd non-blocking

When creating unixgram backend from existing fd we need to make it
non-blocking so the user does not have to care about this implementation
detail. Disabling SIGPIPE is needed on the existing fd for the same
reason.

Fix by moving these steps from open() to new().

Authored-by: Nir Soffer <nirsof@gmail.com>
Signed-off-by: Nir Soffer <nirsof@gmail.com>
Signed-off-by: Jake Correnti <jakecorrenti+github@proton.me>

Author: jakecorrenti

src/devices/src/virtio/net/unixgram.rs

@@ -19,34 +19,6 @@ pub struct Unixgram {
 impl Unixgram {
     /// Create the backend with a pre-established connection to the userspace network proxy.
     pub fn new(fd: RawFd) -> Self {
-        Self { fd }
-    }
-
-    /// Create the backend opening a connection to the userspace network proxy.
-    pub fn open(path: PathBuf, send_vfkit_magic: bool) -> Result<Self, ConnectError> {
-        let fd = socket(
-            AddressFamily::Unix,
-            SockType::Datagram,
-            SockFlag::empty(),
-            None,
-        )
-        .map_err(ConnectError::CreateSocket)?;
-        let peer_addr = UnixAddr::new(&path).map_err(ConnectError::InvalidAddress)?;
-        let local_addr = UnixAddr::new(&PathBuf::from(format!("{}-krun.sock", path.display())))
-            .map_err(ConnectError::InvalidAddress)?;
-        if let Some(path) = local_addr.path() {
-            _ = unlink(path);
-        }
-        bind(fd, &local_addr).map_err(ConnectError::Binding)?;
-
-        // Connect so we don't need to use the peer address again. This also
-        // allows the server to remove the socket after the connection.
-        connect(fd, &peer_addr).map_err(ConnectError::Binding)?;
-
-        if send_vfkit_magic {
-            send(fd, &VFKIT_MAGIC, MsgFlags::empty()).map_err(ConnectError::SendingMagic)?;
-        }
-
         // macOS forces us to do this here instead of just using SockFlag::SOCK_NONBLOCK above.
         match fcntl(fd, FcntlArg::F_GETFL) {
             Ok(flags) => match OFlag::from_bits(flags) {
@@ -75,6 +47,35 @@ impl Unixgram {
             };
         }
 
+        Self { fd }
+    }
+
+    /// Create the backend opening a connection to the userspace network proxy.
+    pub fn open(path: PathBuf, send_vfkit_magic: bool) -> Result<Self, ConnectError> {
+        // We cannot create a non-blocking socket on macOS here. This is done later in new().
+        let fd = socket(
+            AddressFamily::Unix,
+            SockType::Datagram,
+            SockFlag::empty(),
+            None,
+        )
+        .map_err(ConnectError::CreateSocket)?;
+        let peer_addr = UnixAddr::new(&path).map_err(ConnectError::InvalidAddress)?;
+        let local_addr = UnixAddr::new(&PathBuf::from(format!("{}-krun.sock", path.display())))
+            .map_err(ConnectError::InvalidAddress)?;
+        if let Some(path) = local_addr.path() {
+            _ = unlink(path);
+        }
+        bind(fd, &local_addr).map_err(ConnectError::Binding)?;
+
+        // Connect so we don't need to use the peer address again. This also
+        // allows the server to remove the socket after the connection.
+        connect(fd, &peer_addr).map_err(ConnectError::Binding)?;
+
+        if send_vfkit_magic {
+            send(fd, &VFKIT_MAGIC, MsgFlags::empty()).map_err(ConnectError::SendingMagic)?;
+        }
+
         if let Err(e) = setsockopt(fd, sockopt::SndBuf, &(7 * 1024 * 1024)) {
             log::warn!("Failed to increase SO_SNDBUF (performance may be decreased): {e}");
         }