💩 zb: Hack to make connection handshake work inside Flatpak

xdg-dbus-proxy can't handle pipelining[1], hence we need to handle
`NEGOTIATE_UNIX_FD` command's response before sending out `BEGIN` command
and `Hello` method call message.

We should remote this as soon as flatpak is fixed and fix is available in
major distros.

[1] flatpak/xdg-dbus-proxy#21

zbus/src/connection/handshake/client.rs

@@ -179,7 +179,22 @@ impl Client {
 
         let can_pass_fd = self.common.socket_mut().read_mut().can_pass_unix_fd();
         if can_pass_fd {
-            commands.push(Command::NegotiateUnixFD);
+            // xdg-dbus-proxy can't handle pipelining, hence this special handling.
+            // FIXME: Remote this as soon as flatpak is fixed and fix is available in major distros.
+            if is_flatpak().await {
+                self.common.write_command(Command::NegotiateUnixFD).await?;
+                match self.common.read_command().await? {
+                    Command::AgreeUnixFD => self.common.set_cap_unix_fd(true),
+                    Command::Error(e) => warn!("UNIX file descriptor passing rejected: {e}"),
+                    cmd => {
+                        return Err(Error::Handshake(format!(
+                            "Unexpected command from server: {cmd}"
+                        )))
+                    }
+                }
+            } else {
+                commands.push(Command::NegotiateUnixFD);
+            }
         };
         commands.push(Command::Begin);
         let hello_method = if self.bus {
@@ -290,3 +305,16 @@ async fn receive_hello_response(
         m => Err(Error::Handshake(format!("Unexpected messgage `{m:?}`"))),
     }
 }
+
+async fn is_flatpak() -> bool {
+    #[cfg(unix)]
+    {
+        crate::abstractions::file::metadata("/.flatpak-info")
+            .await
+            .is_ok()
+    }
+    #[cfg(not(unix))]
+    {
+        false
+    }
+}