Move file watcher out of core (#21290)

## Why

The app-server watcher relocation leaves the generic filesystem watcher
as the last watcher-specific implementation still living inside
`codex-core`. Moving that code to a small crate keeps `codex-core`
focused on thread execution and lets app-server depend on the watcher
without reaching back into core for filesystem watching primitives.

This PR is stacked on #21287.

## What changed

- Added a new `codex-file-watcher` crate containing the existing watcher
implementation and its unit tests.
- Updated app-server `fs_watch`, `skills_watcher`, and listener state to
import watcher types from `codex-file-watcher`.
- Removed the `file_watcher` module and `notify` dependency from
`codex-core`.
- Updated Cargo workspace metadata and `Cargo.lock` for the new internal
crate.

## Validation

- `cargo check -p codex-file-watcher -p codex-core -p codex-app-server`
- `cargo test -p codex-file-watcher`
- `cargo test -p codex-app-server
skills_changed_notification_is_emitted_after_skill_change`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just fix -p codex-file-watcher`
- `just fix -p codex-core`
- `just fix -p codex-app-server`

codex-rs/Cargo.lock

@@ -1900,6 +1900,7 @@ dependencies = [
  "codex-features",
  "codex-feedback",
  "codex-file-search",
+ "codex-file-watcher",
  "codex-git-utils",
  "codex-hooks",
  "codex-login",
@@ -2551,7 +2552,6 @@ dependencies = [
  "insta",
  "libc",
  "maplit",
- "notify",
  "once_cell",
  "openssl-sys",
  "opentelemetry",
@@ -2894,6 +2894,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "codex-file-watcher"
+version = "0.0.0"
+dependencies = [
+ "notify",
+ "pretty_assertions",
+ "tempfile",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "codex-git-utils"
 version = "0.0.0"

codex-rs/Cargo.toml

@@ -49,6 +49,7 @@ members = [
     "external-agent-sessions",
     "keyring-store",
     "file-search",
+    "file-watcher",
     "linux-sandbox",
     "lmstudio",
     "login",
@@ -166,6 +167,7 @@ codex-features = { path = "features" }
 codex-feedback = { path = "feedback" }
 codex-install-context = { path = "install-context" }
 codex-file-search = { path = "file-search" }
+codex-file-watcher = { path = "file-watcher" }
 codex-git-utils = { path = "git-utils" }
 codex-hooks = { path = "hooks" }
 codex-keyring-store = { path = "keyring-store" }

codex-rs/app-server/Cargo.toml

@@ -41,6 +41,7 @@ codex-external-agent-migration = { workspace = true }
 codex-external-agent-sessions = { workspace = true }
 codex-features = { workspace = true }
 codex-git-utils = { workspace = true }
+codex-file-watcher = { workspace = true }
 codex-hooks = { workspace = true }
 codex-otel = { workspace = true }
 codex-plugin = { workspace = true }

codex-rs/app-server/src/fs_watch.rs

@@ -8,12 +8,12 @@ use codex_app_server_protocol::FsWatchParams;
 use codex_app_server_protocol::FsWatchResponse;
 use codex_app_server_protocol::JSONRPCErrorError;
 use codex_app_server_protocol::ServerNotification;
-use codex_core::file_watcher::FileWatcher;
-use codex_core::file_watcher::FileWatcherEvent;
-use codex_core::file_watcher::FileWatcherSubscriber;
-use codex_core::file_watcher::Receiver;
-use codex_core::file_watcher::WatchPath;
-use codex_core::file_watcher::WatchRegistration;
+use codex_file_watcher::FileWatcher;
+use codex_file_watcher::FileWatcherEvent;
+use codex_file_watcher::FileWatcherSubscriber;
+use codex_file_watcher::Receiver;
+use codex_file_watcher::WatchPath;
+use codex_file_watcher::WatchRegistration;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::collections::hash_map::Entry;

codex-rs/app-server/src/skills_watcher.rs

@@ -6,14 +6,14 @@ use codex_app_server_protocol::ServerNotification;
 use codex_app_server_protocol::SkillsChangedNotification;
 use codex_core::ThreadManager;
 use codex_core::config::Config;
-use codex_core::file_watcher::FileWatcher;
-use codex_core::file_watcher::FileWatcherSubscriber;
-use codex_core::file_watcher::Receiver;
-use codex_core::file_watcher::ThrottledWatchReceiver;
-use codex_core::file_watcher::WatchPath;
-use codex_core::file_watcher::WatchRegistration;
 use codex_core::skills::SkillsLoadInput;
 use codex_core::skills::SkillsManager;
+use codex_file_watcher::FileWatcher;
+use codex_file_watcher::FileWatcherSubscriber;
+use codex_file_watcher::Receiver;
+use codex_file_watcher::ThrottledWatchReceiver;
+use codex_file_watcher::WatchPath;
+use codex_file_watcher::WatchRegistration;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 use tracing::warn;
 

codex-rs/app-server/src/thread_state.rs

@@ -7,7 +7,7 @@ use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnError;
 use codex_core::CodexThread;
 use codex_core::ThreadConfigSnapshot;
-use codex_core::file_watcher::WatchRegistration;
+use codex_file_watcher::WatchRegistration;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::RolloutItem;

codex-rs/core/Cargo.toml

@@ -84,7 +84,6 @@ iana-time-zone = { workspace = true }
 image = { workspace = true, features = ["jpeg", "png", "webp"] }
 indexmap = { workspace = true }
 libc = { workspace = true }
-notify = { workspace = true }
 once_cell = { workspace = true }
 rand = { workspace = true }
 regex-lite = { workspace = true }

codex-rs/core/src/lib.rs

@@ -35,7 +35,6 @@ mod environment_selection;
 pub mod exec;
 pub mod exec_env;
 mod exec_policy;
-pub mod file_watcher;
 mod flags;
 #[cfg(test)]
 mod git_info_tests;
@@ -194,7 +193,6 @@ pub use exec_policy::ExecPolicyError;
 pub use exec_policy::check_execpolicy_for_warnings;
 pub use exec_policy::format_exec_policy_error_with_source;
 pub use exec_policy::load_exec_policy;
-pub use file_watcher::FileWatcherEvent;
 pub use installation_id::resolve_installation_id;
 pub use turn_metadata::build_turn_metadata_header;
 pub mod compact;

codex-rs/file-watcher/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "file-watcher",
+    crate_name = "codex_file_watcher",
+)

codex-rs/file-watcher/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "codex-file-watcher"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+name = "codex_file_watcher"
+path = "src/lib.rs"
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+notify = { workspace = true }
+tokio = { workspace = true, features = ["macros", "rt", "sync", "time"] }
+tracing = { workspace = true, features = ["log"] }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tempfile = { workspace = true }