fix: make all fields of Session private (#2285)

As `Session` needs a bit of work, it will make things easier to move
around if we can start by reducing the extent of its public API. This
makes all the fields private, though adds three `pub(crate)` getters.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2285).
* #2287
* #2286
* __->__ #2285

codex-rs/core/src/codex.rs

@@ -4,6 +4,7 @@
 use std::borrow::Cow;
 use std::collections::HashMap;
 use std::collections::HashSet;
+use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::Mutex;
@@ -199,23 +200,33 @@ impl Codex {
     }
 }
 
+/// Mutable state of the agent
+#[derive(Default)]
+struct State {
+    approved_commands: HashSet<Vec<String>>,
+    current_task: Option<AgentTask>,
+    pending_approvals: HashMap<String, oneshot::Sender<ReviewDecision>>,
+    pending_input: Vec<ResponseInputItem>,
+    history: ConversationHistory,
+}
+
 /// Context for an initialized model agent
 ///
 /// A session has at most 1 running task at a time, and can be interrupted by user input.
 pub(crate) struct Session {
     client: ModelClient,
-    pub(crate) tx_event: Sender<Event>,
+    tx_event: Sender<Event>,
 
     /// The session's current working directory. All relative paths provided by
     /// the model as well as sandbox policies are resolved against this path
     /// instead of `std::env::current_dir()`.
-    pub(crate) cwd: PathBuf,
+    cwd: PathBuf,
     base_instructions: Option<String>,
     user_instructions: Option<String>,
-    pub(crate) approval_policy: AskForApproval,
+    approval_policy: AskForApproval,
     sandbox_policy: SandboxPolicy,
     shell_environment_policy: ShellEnvironmentPolicy,
-    pub(crate) writable_roots: Mutex<Vec<PathBuf>>,
+    writable_roots: Vec<PathBuf>,
     disable_response_storage: bool,
     tools_config: ToolsConfig,
 
@@ -236,24 +247,24 @@ pub(crate) struct Session {
 }
 
 impl Session {
+    pub(crate) fn get_writable_roots(&self) -> &[PathBuf] {
+        &self.writable_roots
+    }
+
+    pub(crate) fn get_approval_policy(&self) -> AskForApproval {
+        self.approval_policy
+    }
+
+    pub(crate) fn get_cwd(&self) -> &Path {
+        &self.cwd
+    }
+
     fn resolve_path(&self, path: Option<String>) -> PathBuf {
         path.as_ref()
             .map(PathBuf::from)
             .map_or_else(|| self.cwd.clone(), |p| self.cwd.join(p))
     }
-}
 
-/// Mutable state of the agent
-#[derive(Default)]
-struct State {
-    approved_commands: HashSet<Vec<String>>,
-    current_task: Option<AgentTask>,
-    pending_approvals: HashMap<String, oneshot::Sender<ReviewDecision>>,
-    pending_input: Vec<ResponseInputItem>,
-    history: ConversationHistory,
-}
-
-impl Session {
     pub fn set_task(&self, task: AgentTask) {
         let mut state = self.state.lock().unwrap();
         if let Some(current_task) = state.current_task.take() {
@@ -659,6 +670,7 @@ impl AgentTask {
             handle,
         }
     }
+
     fn compact(
         sess: Arc<Session>,
         sub_id: String,
@@ -816,7 +828,7 @@ async fn submission_loop(
                     },
                 };
 
-                let writable_roots = Mutex::new(get_writable_roots(&cwd));
+                let writable_roots = get_writable_roots(&cwd);
 
                 // Error messages to dispatch after SessionConfigured is sent.
                 let mut mcp_connection_errors = Vec::<Event>::new();