fix(servo): lease persistent profile data

crates/ely_servo_host/src/bin/ely_servo_sidecar/live.rs

@@ -1,7 +1,8 @@
 use std::{
     collections::HashMap,
-    fs,
+    fs::{self, File, OpenOptions, TryLockError},
     io::{self, BufRead},
+    path::Path,
 };
 
 use ely_domain::{ProfileId, TabId, UrlText};
@@ -24,9 +25,12 @@ use super::{
     },
 };
 
+const PROFILE_DATA_LEASE_FILE: &str = ".ely-servo-sidecar.lock";
+
 pub(super) fn run(args: LiveArgs) -> Result<(), LiveSidecarError> {
     let LiveArgs { profile_data_dir, rendering_context, iosurface_mach_service } = args;
     fs::create_dir_all(&profile_data_dir)?;
+    let _profile_data_lease = acquire_profile_data_lease(&profile_data_dir)?;
     let rendering_context_kind = match rendering_context {
         SidecarRenderingContext::Software => RenderingContextKind::Software,
         SidecarRenderingContext::Hardware => RenderingContextKind::Hardware,
@@ -84,6 +88,22 @@ pub(super) fn run(args: LiveArgs) -> Result<(), LiveSidecarError> {
     Ok(())
 }
 
+fn acquire_profile_data_lease(profile_data_dir: &Path) -> Result<File, LiveSidecarError> {
+    let lease = OpenOptions::new()
+        .create(true)
+        .read(true)
+        .write(true)
+        .truncate(false)
+        .open(profile_data_dir.join(PROFILE_DATA_LEASE_FILE))?;
+    match lease.try_lock() {
+        Ok(()) => Ok(lease),
+        Err(TryLockError::WouldBlock) => Err(LiveSidecarError::ProfileDataDirectoryInUse {
+            path: profile_data_dir.to_path_buf(),
+        }),
+        Err(TryLockError::Error(error)) => Err(error.into()),
+    }
+}
+
 fn handle_request(
     host: &mut SoftwareServoHost,
     sessions: &mut HashMap<String, LiveSession>,

crates/ely_servo_host/src/bin/ely_servo_sidecar/live_protocol.rs

@@ -1,4 +1,4 @@
-use std::io;
+use std::{io, path::PathBuf};
 
 use ely_servo_host::{
     ConsumedPermission, IOSurfaceHandle, RenderedFrame, ServoHostError, WebViewSnapshot,
@@ -305,6 +305,9 @@ pub(super) enum LiveSidecarError {
     #[error("sidecar process is already bound to profile {expected}; received {actual}")]
     ProfileMismatch { expected: String, actual: String },
 
+    #[error("servo profile data directory is already in use: {path}")]
+    ProfileDataDirectoryInUse { path: PathBuf },
+
     #[error("{input} requires both x and y coordinates")]
     IncompletePoint { input: &'static str },
 

crates/ely_servo_host/tests/sidecar.rs

@@ -2,7 +2,9 @@
 
 use std::{
     error::Error,
-    io, thread,
+    io,
+    process::{Command, Stdio},
+    thread,
     time::{Duration, Instant},
 };
 
@@ -64,6 +66,38 @@ fn live_sidecar_streams_rgba_and_flushes_profile_storage_on_shutdown() -> Result
     Ok(())
 }
 
+#[test]
+fn live_sidecar_leases_profile_data_directory_for_process_lifetime() -> Result<(), Box<dyn Error>> {
+    let root = TestDirectory::new()?;
+    let leased_dir = root.path().join("leased");
+    let independent_dir = root.path().join("independent");
+    let mut owner = Sidecar::spawn(&leased_dir)?;
+    assert!(leased_dir.join(".ely-servo-sidecar.lock").is_file());
+
+    let rejected = Command::new(env!("CARGO_BIN_EXE_ely_servo_sidecar"))
+        .arg("live")
+        .arg("--profile-data-dir")
+        .arg(&leased_dir)
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::piped())
+        .output()?;
+    let stderr = String::from_utf8(rejected.stderr)?;
+    assert!(!rejected.status.success(), "second sidecar unexpectedly acquired the profile lease");
+    assert!(
+        stderr.contains("ProfileDataDirectoryInUse"),
+        "second sidecar reported an unexpected error: {stderr}"
+    );
+
+    let mut independent = Sidecar::spawn(&independent_dir)?;
+    independent.shutdown()?;
+    owner.shutdown()?;
+
+    let mut replacement = Sidecar::spawn(&leased_dir)?;
+    replacement.shutdown()?;
+    Ok(())
+}
+
 #[test]
 fn servo_originated_history_url_does_not_trigger_a_second_navigation() -> Result<(), Box<dyn Error>>
 {