fix: Do not exit MCP if the parent process is alive

crates/fff-mcp/Cargo.toml

@@ -30,3 +30,13 @@ tokio = { version = "1", features = ["full"] }
 tracing = { workspace = true }
 git2 = { workspace = true }
 clap = { version = "4", features = ["derive", "env"] }
+
+[target.'cfg(windows)'.dependencies]
+windows-sys = { version = "0.60", features = [
+    "Win32_Foundation",
+    "Win32_System_Threading",
+    "Win32_System_Diagnostics_ToolHelp",
+] }
+
+[dev-dependencies]
+tempfile = "3.8"

crates/fff-mcp/src/main.rs

@@ -1,9 +1,12 @@
 mod cursor;
 mod healthcheck;
 mod output;
+mod parent;
 mod server;
 mod update_check;
 
+use std::time::{Duration, SystemTime};
+
 use clap::Parser;
 use fff::file_picker::FilePicker;
 use fff::frecency::FrecencyTracker;
@@ -92,7 +95,7 @@ pub const MCP_INSTRUCTIONS: &str = concat!(
     "  !generated/ - exclude generated code",
 );
 
-/// FFF MCP Server -- a high performance & accuracy file finder for AI code assistants.
+/// FFF MCP Server - a high performance & accuracy file finder for AI code assistants.
 #[derive(Parser)]
 #[command(name = "fff-mcp", version = concat!(env!("CARGO_PKG_VERSION"), " (", env!("FFF_GIT_HASH"), ")"))]
 pub(crate) struct Args {
@@ -131,6 +134,7 @@ pub(crate) struct Args {
 
     /// Disable the content index built after the initial scan.
     /// This makes grep calls slower but consumes less RAM (recommended to not turn off)
+    #[arg(long = "no-content-indexing")]
     no_content_indexing: bool,
 
     /// Explicitly enable content indexing even when `--no-warmup` is set.
@@ -158,11 +162,12 @@ pub(crate) struct Args {
     #[arg(long = "healthcheck")]
     pub(crate) healthcheck: bool,
 
-    /// Exit after this many seconds of inactivity. 0 = never exit.
+    /// Timeout of inactivity after which fff mcp will be exited. Even if the parent process
+    /// is alive we don't want to occupy resources on index and file watches if fff is unused
     #[arg(
         long = "idle-timeout-secs",
         env = "FFF_MCP_IDLE_TIMEOUT_SECS",
-        default_value_t = 900
+        default_value_t = 60 * 60
     )]
     idle_timeout_secs: u64,
 }
@@ -317,9 +322,20 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         }
     };
 
-    if idle_timeout_secs > 0 {
+    let parent_watcher = parent::ParentWatcher::new();
+    match &parent_watcher {
+        Some(watcher) => tracing::info!(
+            "Watching parent process (pid {}); will exit when it dies",
+            watcher.parent_pid()
+        ),
+        None => tracing::warn!(
+            "Parent process liveness detection unavailable; idle timeout will exit unconditionally"
+        ),
+    }
+
+    if idle_timeout_secs > 0 || parent_watcher.is_some() {
         last_activity.store(
-            std::time::SystemTime::now()
+            SystemTime::now()
                 .duration_since(std::time::UNIX_EPOCH)
                 .map(|d| d.as_secs())
                 .unwrap_or(0),
@@ -328,22 +344,36 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
         let last_activity_for_watchdog = last_activity.clone();
         tokio::spawn(async move {
-            let tick = std::time::Duration::from_secs(60);
+            let tick = watchdog_interval();
             loop {
                 tokio::time::sleep(tick).await;
+
+                if let Some(ref watcher) = parent_watcher {
+                    if !watcher.parent_alive() {
+                        tracing::info!(
+                            "Parent process (pid {}) exited, shutting down",
+                            watcher.parent_pid()
+                        );
+                        flush_logs_and_exit().await;
+                    }
+                    // Parent is alive: it owns our lifecycle, never exit on idle
+                    // Clients like Codex do not restart MCP servers @see #703
+                    continue;
+                }
+
+                if idle_timeout_secs == 0 {
+                    continue;
+                }
+
                 let now = std::time::SystemTime::now()
                     .duration_since(std::time::UNIX_EPOCH)
                     .map(|d| d.as_secs())
                     .unwrap_or(0);
 
                 let last = last_activity_for_watchdog.load(std::sync::atomic::Ordering::Relaxed);
                 if now.saturating_sub(last) >= idle_timeout_secs {
-                    tracing::info!(
-                        "Exiting after {}s of inactivity (idle_timeout_secs={})",
-                        now.saturating_sub(last),
-                        idle_timeout_secs
-                    );
-                    std::process::exit(0);
+                    tracing::info!(?idle_timeout_secs, "Exiting due to inactivity",);
+                    flush_logs_and_exit().await;
                 }
             }
         });
@@ -370,3 +400,20 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     Ok(())
 }
+
+// Tracing appender is non blocking, to get full log give it some time before hard exit
+async fn flush_logs_and_exit() -> ! {
+    tokio::time::sleep(std::time::Duration::from_millis(250)).await;
+    std::process::exit(0);
+}
+
+fn watchdog_interval() -> Duration {
+    if cfg!(debug_assertions)
+        && let Some(milliseconds) = std::env::var("FFF_MCP_TEST_WATCHDOG_INTERVAL_MS")
+            .ok()
+            .and_then(|value| value.parse().ok())
+    {
+        return Duration::from_millis(milliseconds);
+    }
+    Duration::from_secs(60)
+}

crates/fff-mcp/src/parent.rs

@@ -0,0 +1,99 @@
+#[cfg(unix)]
+mod imp {
+    pub struct ParentWatcher {
+        ppid: u32,
+    }
+
+    impl ParentWatcher {
+        pub fn new() -> Option<Self> {
+            let ppid = std::os::unix::process::parent_id();
+            // ppid <= 1 means we were spawned by init and can't detect death
+            (ppid > 1).then_some(Self { ppid })
+        }
+
+        pub fn parent_pid(&self) -> u32 {
+            self.ppid
+        }
+
+        // When the parent dies the kernel reparents us, so getppid() changes.
+        // Race-free and immune to PID reuse, unlike kill(ppid, 0).
+        pub fn parent_alive(&self) -> bool {
+            std::os::unix::process::parent_id() == self.ppid
+        }
+    }
+}
+
+#[cfg(windows)]
+mod imp {
+    use windows_sys::Win32::Foundation::{CloseHandle, HANDLE, INVALID_HANDLE_VALUE, WAIT_TIMEOUT};
+    use windows_sys::Win32::System::Diagnostics::ToolHelp::{
+        CreateToolhelp32Snapshot, PROCESSENTRY32, Process32First, Process32Next, TH32CS_SNAPPROCESS,
+    };
+    use windows_sys::Win32::System::Threading::{
+        GetCurrentProcessId, OpenProcess, PROCESS_SYNCHRONIZE, WaitForSingleObject,
+    };
+
+    pub struct ParentWatcher {
+        handle: HANDLE,
+        ppid: u32,
+    }
+
+    // HANDLE is a raw pointer; it is only ever used via WaitForSingleObject
+    // which is thread-safe, so moving/sharing the watcher across threads is fine.
+    unsafe impl Send for ParentWatcher {}
+    unsafe impl Sync for ParentWatcher {}
+
+    impl ParentWatcher {
+        pub fn new() -> Option<Self> {
+            let ppid = parent_pid_of_current()?;
+            let handle = unsafe { OpenProcess(PROCESS_SYNCHRONIZE, 0, ppid) };
+            if handle.is_null() {
+                return None;
+            }
+            // Holding the handle pins the PID, preventing reuse for the process lifetime
+            Some(Self { handle, ppid })
+        }
+
+        pub fn parent_pid(&self) -> u32 {
+            self.ppid
+        }
+
+        pub fn parent_alive(&self) -> bool {
+            unsafe { WaitForSingleObject(self.handle, 0) == WAIT_TIMEOUT }
+        }
+    }
+
+    impl Drop for ParentWatcher {
+        fn drop(&mut self) {
+            unsafe { CloseHandle(self.handle) };
+        }
+    }
+
+    fn parent_pid_of_current() -> Option<u32> {
+        unsafe {
+            let snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
+            if snapshot == INVALID_HANDLE_VALUE {
+                return None;
+            }
+            let mut entry: PROCESSENTRY32 = std::mem::zeroed();
+            entry.dwSize = std::mem::size_of::<PROCESSENTRY32>() as u32;
+            let current = GetCurrentProcessId();
+            let mut found = None;
+            if Process32First(snapshot, &mut entry) != 0 {
+                loop {
+                    if entry.th32ProcessID == current {
+                        found = Some(entry.th32ParentProcessID);
+                        break;
+                    }
+                    if Process32Next(snapshot, &mut entry) == 0 {
+                        break;
+                    }
+                }
+            }
+            CloseHandle(snapshot);
+            found
+        }
+    }
+}
+
+pub use imp::ParentWatcher;