bxcore, loopd, inferproxy: a lock file keeps each to one instance

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/bxcore/tests/instance.rs

@@ -0,0 +1,137 @@
+//! `bxcore::instance`: the lock that keeps a daemon to one instance (M5b spec, section 9a). Two
+//! opens of the same file in one process are two holders, as two processes are. Do not edit.
+
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::fs::PermissionsExt;
+use std::path::{Path, PathBuf};
+
+use bxcore::instance::{LockError, RUNBOOK, lock};
+use tmp::TempDir;
+
+/// Gives a directory back its owner's permissions when dropped, so the `TempDir` declared before
+/// it can be removed even when an assertion fails (tip T44).
+struct Restore(PathBuf);
+
+impl Drop for Restore {
+    fn drop(&mut self) {
+        let _ = std::fs::set_permissions(&self.0, std::fs::Permissions::from_mode(0o700));
+    }
+}
+
+fn mode(path: &Path) -> u32 {
+    std::fs::metadata(path).unwrap().permissions().mode() & 0o777
+}
+
+#[test]
+fn a_second_lock_is_held_while_the_first_lives() {
+    let dir = TempDir::new("lock-held");
+    let path = dir.path().join("run").join("loopd.lock");
+    let first = lock(&path).expect("the first lock");
+    for _ in 0..3 {
+        match lock(&path) {
+            Err(LockError::Held) => {}
+            other => panic!("a second lock while the first lives: {other:?}"),
+        }
+    }
+    drop(first);
+}
+
+#[test]
+fn the_lock_is_free_again_once_the_first_is_dropped() {
+    let dir = TempDir::new("lock-free");
+    let path = dir.path().join("run").join("loopd.lock");
+    let first = lock(&path).expect("the first lock");
+    drop(first);
+    let second = lock(&path).expect("the lock after the first was dropped");
+    assert!(
+        matches!(lock(&path), Err(LockError::Held)),
+        "the second holds it now"
+    );
+    drop(second);
+    assert!(path.exists(), "the lock file is left in place");
+}
+
+#[test]
+fn a_lock_file_left_behind_is_taken() {
+    // A killed daemon leaves its lock file, unlocked; the next start takes it and keeps its bytes.
+    let dir = TempDir::new("lock-left");
+    let path = dir.write("run/memoryd.lock", "left\n");
+    let held = lock(&path).expect("a lock file nobody holds");
+    assert_eq!(std::fs::read_to_string(&path).unwrap(), "left\n");
+    drop(held);
+}
+
+#[test]
+fn the_lock_file_is_created_with_mode_0600_and_its_directory_0700() {
+    let dir = TempDir::new("lock-mode");
+    let run = dir.path().join("home").join("run");
+    let path = run.join("brokerd.lock");
+    let held = lock(&path).expect("a lock in a directory that does not exist yet");
+    assert_eq!(mode(&path), 0o600, "the lock file");
+    assert_eq!(mode(&run), 0o700, "the directory it made");
+    assert_eq!(mode(&dir.path().join("home")), 0o700, "and the one above");
+    assert_eq!(
+        std::fs::metadata(&path).unwrap().len(),
+        0,
+        "nothing is written"
+    );
+    drop(held);
+}
+
+#[test]
+fn a_directory_that_cannot_be_written_is_an_io_error() {
+    let dir = TempDir::new("lock-ro");
+    let run = dir.path().join("run");
+    std::fs::create_dir(&run).unwrap();
+    let _restore = Restore(run.clone());
+    std::fs::set_permissions(&run, std::fs::Permissions::from_mode(0o500)).unwrap();
+    match lock(&run.join("gatewayd.lock")) {
+        Err(LockError::Io(e)) => {
+            assert_eq!(e.kind(), std::io::ErrorKind::PermissionDenied, "{e}");
+        }
+        other => panic!("a lock in a read-only directory: {other:?}"),
+    }
+    assert!(!run.join("gatewayd.lock").exists());
+}
+
+#[test]
+fn a_path_through_a_file_is_an_io_error() {
+    let dir = TempDir::new("lock-file");
+    let file = dir.write("run", "not a directory\n");
+    match lock(&file.join("loopd.lock")) {
+        Err(LockError::Io(_)) => {}
+        other => panic!("a lock under a file: {other:?}"),
+    }
+}
+
+#[test]
+fn the_lines_a_daemon_prints() {
+    let path = Path::new("/home/bx/run/loopd.lock");
+    assert_eq!(
+        LockError::Held.line("loopd", path),
+        "loopd: another loopd is running (it holds /home/bx/run/loopd.lock); \
+         see docs/runbook.md#already-running"
+    );
+    let io = LockError::Io(std::io::Error::other("the disk is on fire"));
+    assert_eq!(
+        io.line("inferproxy", Path::new("/run/bx/infer.sock.lock")),
+        "inferproxy: cannot take the lock /run/bx/infer.sock.lock: the disk is on fire; \
+         see docs/runbook.md#already-running"
+    );
+    assert_eq!(RUNBOOK, "see docs/runbook.md#already-running");
+}
+
+#[test]
+fn the_error_displays_and_has_a_source() {
+    use std::error::Error;
+    assert_eq!(
+        LockError::Held.to_string(),
+        "another instance holds the lock"
+    );
+    assert!(LockError::Held.source().is_none());
+    let io = LockError::Io(std::io::Error::other("no room"));
+    assert_eq!(io.to_string(), "no room");
+    assert!(io.source().is_some());
+}