Add support for sandboxed socket path

Rather than try a single path, try all supported paths, in the preferred
order. This avoids breaking compatibility with previous KeePassXC
versions.

See: keepassxreboot/keepassxc#8018
Fixes: varjolintu#8
Supercedes: varjolintu#7

src/proxy_socket.rs

@@ -4,6 +4,7 @@ use std::io::{self, Read, Write};
 #[cfg(not(windows))]
 use std::os::unix::io::AsRawFd;
 use std::os::unix::net::UnixStream;
+use std::path::PathBuf;
 use nix::sys::socket;
 use nix::sys::socket::sockopt::SndBuf;
 use nix::sys::socket::sockopt::RcvBuf;
@@ -39,17 +40,45 @@ pub fn connect(buffer_size: usize) -> io::Result<ProxySocket<PipeClient>> {
     Ok(ProxySocket { inner: client })
 }
 
+#[cfg(not(windows))]
+/// Returns the directories where the socket could possible be located.
+///
+/// These directories should be tried in sequence, until one of them is found
+/// to contain the socket.
+fn get_socket_dirs() -> Vec<PathBuf> {
+    let mut dirs = Vec::new();
+
+    if !cfg!(target_os = "macos") {
+        if let Ok(dir) = env::var("XDG_RUNTIME_DIR")  {
+            let xdg_runtime_dir: PathBuf = dir.into();
+
+            // Sandbox-friendly path.
+            // Used in KeePassXC >= 2.7.2 and for all versions on Flatpak.
+            dirs.push(xdg_runtime_dir.join("app/org.keepassxc.KeePassXC/"));
+
+            // Legacy path.
+            // Used by KeePassXC < 2.7.2.
+            dirs.push(xdg_runtime_dir);
+        };
+    };
+
+    // Default for macOS, and final fallback for Linux.
+    dirs.push(env::temp_dir());
+
+    dirs
+}
+
 #[cfg(not(windows))]
 pub fn connect(buffer_size: usize) -> io::Result<ProxySocket<UnixStream>> {
     use std::time::Duration;
 
     let socket_name = "org.keepassxc.KeePassXC.BrowserServer";
-    let socket = if let Ok(dir) = if cfg!(target_os = "macos") {env::var("TMPDIR") } else { env::var("XDG_RUNTIME_DIR") } {
-        format!("{}/{}", dir, socket_name)
-    } else {
-        format!("/tmp/{}", socket_name)
-    };
-    let s = UnixStream::connect(socket)?;
+    let dirs = get_socket_dirs();
+    let s = dirs
+        .iter()
+        .find_map(|dir| UnixStream::connect(dir.join(socket_name)).ok())
+        .ok_or_else(|| io::Error::from(io::ErrorKind::NotFound))?;
+
     socket::setsockopt(s.as_raw_fd(), SndBuf, &buffer_size).expect("setsockopt for SndBuf failed");
     socket::setsockopt(s.as_raw_fd(), RcvBuf, &buffer_size).expect("setsockopt for RcvBuf failed");
     let timeout: Option<Duration> = Some(Duration::from_secs(1));