Code mode on v8 (#15276)

Moves Code Mode to a new crate with no dependencies on codex. This
create encodes the code mode semantics that we want for lifetime,
mounting, tool calling.

The model-facing surface is mostly unchanged. `exec` still runs raw
JavaScript, `wait` still resumes or terminates a `cell_id`, nested tools
are still available through `tools.*`, and helpers like `text`, `image`,
`store`, `load`, `notify`, `yield_control`, and `exit` still exist.

The major change is underneath that surface:

- Old code mode was an external Node runtime.
- New code mode is an in-process V8 runtime embedded directly in Rust.
- Old code mode managed cells inside a long-lived Node runner process.
- New code mode manages cells in Rust, with one V8 runtime thread per
active `exec`.
- Old code mode used JSON protocol messages over child stdin/stdout plus
Node worker-thread messages.
- New code mode uses Rust channels and direct V8 callbacks/events.

This PR also fixes the two migration regressions that fell out of that
substrate change:

- `wait { terminate: true }` now waits for the V8 runtime to actually
stop before reporting termination.
- synchronous top-level `exit()` now succeeds again instead of surfacing
as a script error.

---

- `core/src/tools/code_mode/*` is now mostly an adapter layer for the
public `exec` / `wait` tools.
- `code-mode/src/service.rs` owns cell sessions and async control flow
in Rust.
- `code-mode/src/runtime/*.rs` owns the embedded V8 isolate and
JavaScript execution.
- each `exec` spawns a dedicated runtime thread plus a Rust
session-control task.
- helper globals are installed directly into the V8 context instead of
being injected through a source prelude.
- helper modules like `tools.js` and `@openai/code_mode` are synthesized
through V8 module resolution callbacks in Rust.

---

Also added a benchmark for showing the speed of init and use of a code
mode env:
```
$ cargo bench -p codex-code-mode --bench exec_overhead -- --samples 30 --warm-iterations 25 --tool-counts 0,32,128
Finished [`bench` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 0.18s
     Running benches/exec_overhead.rs (target/release/deps/exec_overhead-008c440d800545ae)
exec_overhead: samples=30, warm_iterations=25, tool_counts=[0, 32, 128]
scenario       tools samples    warmups      iters      mean/exec       p95/exec       rssΔ p50       rssΔ max
cold_exec          0      30          0          1         1.13ms         1.20ms        8.05MiB        8.06MiB
warm_exec          0      30          1         25       473.43us       512.49us      912.00KiB        1.33MiB
cold_exec         32      30          0          1         1.03ms         1.15ms        8.08MiB        8.11MiB
warm_exec         32      30          1         25       509.73us       545.76us      960.00KiB        1.30MiB
cold_exec        128      30          0          1         1.14ms         1.19ms        8.30MiB        8.34MiB
warm_exec        128      30          1         25       575.08us       591.03us      736.00KiB      864.00KiB
memory uses a fresh-process max RSS delta for each scenario
```

---------

Co-authored-by: Codex <noreply@openai.com>

Author: cconger

codex-rs/Cargo.lock

@@ -13,6 +13,7 @@ members = [
     "feedback",
     "features",
     "codex-backend-openapi-models",
+    "code-mode",
     "cloud-requirements",
     "cloud-tasks",
     "cloud-tasks-client",
@@ -91,6 +92,7 @@ app_test_support = { path = "app-server/tests/common" }
 codex-ansi-escape = { path = "ansi-escape" }
 codex-api = { path = "codex-api" }
 codex-artifacts = { path = "artifacts" }
+codex-code-mode = { path = "code-mode" }
 codex-package-manager = { path = "package-manager" }
 codex-app-server = { path = "app-server" }
 codex-app-server-client = { path = "app-server-client" }
@@ -374,7 +376,7 @@ ignored = [
     "openssl-sys",
     "codex-utils-readiness",
     "codex-secrets",
-    "codex-v8-poc"
+    "codex-v8-poc",
 ]
 
 [profile.release]

codex-rs/Cargo.toml

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "code-mode",
+    crate_name = "codex_code_mode",
+)

codex-rs/code-mode/BUILD.bazel

@@ -0,0 +1,25 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-code-mode"
+version.workspace = true
+
+[lib]
+doctest = false
+name = "codex_code_mode"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+async-trait = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+tokio = { workspace = true, features = ["macros", "rt", "sync", "time"] }
+tokio-util = { workspace = true, features = ["rt"] }
+tracing = { workspace = true }
+v8 = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }