memoryd: the crate, memoryd.toml, and listening on a socket

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/memoryd/tests/socket.rs

@@ -0,0 +1,44 @@
+//! `listen`: the directory at 0700, the socket at 0600, a stale socket replaced, and no mode
+//! changed through a symbolic link (M5a spec, section 7). Do not edit.
+
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::fs::PermissionsExt;
+use std::os::unix::net::UnixStream;
+
+use memoryd::socket::listen;
+use tmp::TempDir;
+
+fn mode(path: &std::path::Path) -> u32 {
+    std::fs::metadata(path).unwrap().permissions().mode() & 0o777
+}
+
+#[test]
+fn directory_and_socket_are_private_and_a_stale_socket_is_replaced() {
+    let dir = TempDir::new("socket");
+    let run = dir.path().join("run/loop-memory");
+    std::fs::create_dir_all(&run).unwrap();
+    std::fs::set_permissions(&run, std::fs::Permissions::from_mode(0o755)).unwrap();
+    let socket = run.join("memory.sock");
+    let first = listen(&socket).unwrap();
+    assert_eq!((mode(&run), mode(&socket)), (0o700, 0o600));
+    drop(first);
+    let second = listen(&socket).expect("a stale socket is removed");
+    assert!(UnixStream::connect(&socket).is_ok());
+    drop(second);
+}
+
+#[test]
+fn no_directory_and_symbolic_links_are_refused() {
+    let dir = TempDir::new("socket-link");
+    let target = dir.path().join("shared");
+    std::fs::create_dir_all(&target).unwrap();
+    std::fs::set_permissions(&target, std::fs::Permissions::from_mode(0o755)).unwrap();
+    std::os::unix::fs::symlink(&target, dir.path().join("link")).unwrap();
+    let err = listen(&dir.path().join("link/memory.sock")).unwrap_err();
+    assert!(err.contains("symbolic link"), "{err}");
+    assert_eq!(mode(&target), 0o755, "the target's mode is untouched");
+    let err = listen(std::path::Path::new("/memory.sock")).unwrap_err();
+    assert!(err.contains("directory of its own"), "{err}");
+}