feat: add support for login with ChatGPT (#1212)

This does not implement the full Login with ChatGPT experience, but it
should unblock people.

**What works**

* The `codex` multitool now has a `login` subcommand, so you can run
`codex login`, which should write `CODEX_HOME/auth.json` if you complete
the flow successfully. The TUI will now read the `OPENAI_API_KEY` from
`auth.json`.
* The TUI should refresh the token if it has expired and the necessary
information is in `auth.json`.
* There is a `LoginScreen` in the TUI that tells you to run `codex
login` if both (1) your model provider expects to use `OPENAI_API_KEY`
as its env var, and (2) `OPENAI_API_KEY` is not set.

**What does not work**

* The `LoginScreen` does not support the login flow from within the TUI.
Instead, it tells you to quit, run `codex login`, and then run `codex`
again.
* `codex exec` does read from `auth.json` yet, nor does it direct the
user to go through the login flow if `OPENAI_API_KEY` is not be found.
* The `maybeRedeemCredits()` function from `get-api-key.tsx` has not
been ported from TypeScript to `login_with_chatgpt.py` yet:


https://github.com/openai/codex/blob/a67a67f3258fc21e147b6786a143fe3e15e6d5ba/codex-cli/src/utils/get-api-key.tsx#L84-L89

**Implementation**

Currently, the OAuth flow requires running a local webserver on
`127.0.0.1:1455`. It seemed wasteful to incur the additional binary cost
of a webserver dependency in the Rust CLI just to support login, so
instead we implement this logic in Python, as Python has a `http.server`
module as part of its standard library. Specifically, we bundle the
contents of a single Python file as a string in the Rust CLI and then
use it to spawn a subprocess as `python3 -c
{{SOURCE_FOR_PYTHON_SERVER}}`.

As such, the most significant files in this PR are:

```
codex-rs/login/src/login_with_chatgpt.py
codex-rs/login/src/lib.rs
```

Now that the CLI may load `OPENAI_API_KEY` from the environment _or_
`CODEX_HOME/auth.json`, we need a new abstraction for reading/writing
this variable, so we introduce:

```
codex-rs/core/src/openai_api_key.rs
```

Note that `std::env::set_var()` is [rightfully] `unsafe` in Rust 2024,
so we use a LazyLock<RwLock<Option<String>>> to store `OPENAI_API_KEY`
so it is read in a thread-safe manner.

Ultimately, it should be possible to go through the entire login flow
from the TUI. This PR introduces a placeholder `LoginScreen` UI for that
right now, though the new `codex login` subcommand introduced in this PR
should be a viable workaround until the UI is ready.

**Testing**

Because the login flow is currently implemented in a standalone Python
file, you can test it without building any Rust code as follows:

```
rm -rf /tmp/codex_home && mkdir /tmp/codex_home
CODEX_HOME=/tmp/codex_home python3 codex-rs/login/src/login_with_chatgpt.py
```

For reference:

* the original TypeScript implementation was introduced in
https://github.com/openai/codex/pull/963
* support for redeeming credits was later added in
https://github.com/openai/codex/pull/974

codex-rs/cli/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod debug_sandbox;
 mod exit_status;
+pub mod login;
 pub mod proto;
 
 use clap::Parser;

codex-rs/cli/src/login.rs

@@ -0,0 +1,35 @@
+use codex_common::CliConfigOverrides;
+use codex_core::config::Config;
+use codex_core::config::ConfigOverrides;
+use codex_login::login_with_chatgpt;
+
+pub async fn run_login_with_chatgpt(cli_config_overrides: CliConfigOverrides) -> ! {
+    let cli_overrides = match cli_config_overrides.parse_overrides() {
+        Ok(v) => v,
+        Err(e) => {
+            eprintln!("Error parsing -c overrides: {e}");
+            std::process::exit(1);
+        }
+    };
+
+    let config_overrides = ConfigOverrides::default();
+    let config = match Config::load_with_cli_overrides(cli_overrides, config_overrides) {
+        Ok(config) => config,
+        Err(e) => {
+            eprintln!("Error loading configuration: {e}");
+            std::process::exit(1);
+        }
+    };
+
+    let capture_output = false;
+    match login_with_chatgpt(&config.codex_home, capture_output).await {
+        Ok(_) => {
+            eprintln!("Successfully logged in");
+            std::process::exit(0);
+        }
+        Err(e) => {
+            eprintln!("Error logging in: {e}");
+            std::process::exit(1);
+        }
+    }
+}

codex-rs/cli/src/main.rs

@@ -1,6 +1,7 @@
 use clap::Parser;
 use codex_cli::LandlockCommand;
 use codex_cli::SeatbeltCommand;
+use codex_cli::login::run_login_with_chatgpt;
 use codex_cli::proto;
 use codex_common::CliConfigOverrides;
 use codex_exec::Cli as ExecCli;
@@ -36,6 +37,9 @@ enum Subcommand {
     #[clap(visible_alias = "e")]
     Exec(ExecCli),
 
+    /// Login with ChatGPT.
+    Login(LoginCommand),
+
     /// Experimental: run Codex as an MCP server.
     Mcp,
 
@@ -63,7 +67,10 @@ enum DebugCommand {
 }
 
 #[derive(Debug, Parser)]
-struct ReplProto {}
+struct LoginCommand {
+    #[clap(skip)]
+    config_overrides: CliConfigOverrides,
+}
 
 fn main() -> anyhow::Result<()> {
     codex_linux_sandbox::run_with_sandbox(|codex_linux_sandbox_exe| async move {
@@ -88,6 +95,10 @@ async fn cli_main(codex_linux_sandbox_exe: Option<PathBuf>) -> anyhow::Result<()
         Some(Subcommand::Mcp) => {
             codex_mcp_server::run_main(codex_linux_sandbox_exe).await?;
         }
+        Some(Subcommand::Login(mut login_cli)) => {
+            prepend_config_flags(&mut login_cli.config_overrides, cli.config_overrides);
+            run_login_with_chatgpt(login_cli.config_overrides).await;
+        }
         Some(Subcommand::Proto(mut proto_cli)) => {
             prepend_config_flags(&mut proto_cli.config_overrides, cli.config_overrides);
             proto::run_main(proto_cli).await?;