chore: unify history loading (#2736)

We have two ways of loading conversation with a previous history. Fork
conversation and the experimental resume that we had before. In this PR,
I am unifying their code path. The path is getting the history items and
recording them in a brand new conversation. This PR also constraint the
rollout recorder responsibilities to be only recording to the disk and
loading from the disk.

The PR also fixes a current bug when we have two forking in a row:
History 1:
<Environment Context>
UserMessage_1
UserMessage_2
UserMessage_3

**Fork with n = 1 (only remove one element)**
History 2:
<Environment Context>
UserMessage_1
UserMessage_2
<Environment Context>

**Fork with n = 1 (only remove one element)**
History 2:
<Environment Context>
UserMessage_1
UserMessage_2
**<Environment Context>**

This shouldn't happen but because we were appending the `<Environment
Context>` after each spawning and it's considered as _user message_.
Now, we don't add this message if restoring and old conversation.

codex-rs/core/src/conversation_manager.rs

@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::path::PathBuf;
 use std::sync::Arc;
 
 use codex_login::AuthManager;
@@ -16,8 +17,15 @@ use crate::error::Result as CodexResult;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
 use crate::protocol::SessionConfiguredEvent;
+use crate::rollout::RolloutRecorder;
 use codex_protocol::models::ResponseItem;
 
+#[derive(Debug, Clone, PartialEq)]
+pub enum InitialHistory {
+    New,
+    Resumed(Vec<ResponseItem>),
+}
+
 /// Represents a newly created Codex conversation, including the first event
 /// (which is [`EventMsg::SessionConfigured`]).
 pub struct NewConversation {
@@ -57,14 +65,21 @@ impl ConversationManager {
         config: Config,
         auth_manager: Arc<AuthManager>,
     ) -> CodexResult<NewConversation> {
-        let CodexSpawnOk {
-            codex,
-            session_id: conversation_id,
-        } = {
-            let initial_history = None;
-            Codex::spawn(config, auth_manager, initial_history).await?
-        };
-        self.finalize_spawn(codex, conversation_id).await
+        // TO BE REFACTORED: use the config experimental_resume field until we have a mainstream way.
+        if let Some(resume_path) = config.experimental_resume.as_ref() {
+            let initial_history = RolloutRecorder::get_rollout_history(resume_path).await?;
+            let CodexSpawnOk {
+                codex,
+                session_id: conversation_id,
+            } = Codex::spawn(config, auth_manager, initial_history).await?;
+            self.finalize_spawn(codex, conversation_id).await
+        } else {
+            let CodexSpawnOk {
+                codex,
+                session_id: conversation_id,
+            } = { Codex::spawn(config, auth_manager, InitialHistory::New).await? };
+            self.finalize_spawn(codex, conversation_id).await
+        }
     }
 
     async fn finalize_spawn(
@@ -110,6 +125,20 @@ impl ConversationManager {
             .ok_or_else(|| CodexErr::ConversationNotFound(conversation_id))
     }
 
+    pub async fn resume_conversation_from_rollout(
+        &self,
+        config: Config,
+        rollout_path: PathBuf,
+        auth_manager: Arc<AuthManager>,
+    ) -> CodexResult<NewConversation> {
+        let initial_history = RolloutRecorder::get_rollout_history(&rollout_path).await?;
+        let CodexSpawnOk {
+            codex,
+            session_id: conversation_id,
+        } = Codex::spawn(config, auth_manager, initial_history).await?;
+        self.finalize_spawn(codex, conversation_id).await
+    }
+
     pub async fn remove_conversation(&self, conversation_id: Uuid) {
         self.conversations.write().await.remove(&conversation_id);
     }
@@ -125,7 +154,7 @@ impl ConversationManager {
         config: Config,
     ) -> CodexResult<NewConversation> {
         // Compute the prefix up to the cut point.
-        let truncated_history =
+        let history =
             truncate_after_dropping_last_messages(conversation_history, num_messages_to_drop);
 
         // Spawn a new conversation with the computed initial history.
@@ -133,7 +162,7 @@ impl ConversationManager {
         let CodexSpawnOk {
             codex,
             session_id: conversation_id,
-        } = Codex::spawn(config, auth_manager, Some(truncated_history)).await?;
+        } = Codex::spawn(config, auth_manager, history).await?;
 
         self.finalize_spawn(codex, conversation_id).await
     }
@@ -141,9 +170,9 @@ impl ConversationManager {
 
 /// Return a prefix of `items` obtained by dropping the last `n` user messages
 /// and all items that follow them.
-fn truncate_after_dropping_last_messages(items: Vec<ResponseItem>, n: usize) -> Vec<ResponseItem> {
-    if n == 0 || items.is_empty() {
-        return items;
+fn truncate_after_dropping_last_messages(items: Vec<ResponseItem>, n: usize) -> InitialHistory {
+    if n == 0 {
+        return InitialHistory::Resumed(items);
     }
 
     // Walk backwards counting only `user` Message items, find cut index.
@@ -161,11 +190,11 @@ fn truncate_after_dropping_last_messages(items: Vec<ResponseItem>, n: usize) ->
             }
         }
     }
-    if count < n {
-        // If fewer than n messages exist, drop everything.
-        Vec::new()
+    if cut_index == 0 {
+        // No prefix remains after dropping; start a new conversation.
+        InitialHistory::New
     } else {
-        items.into_iter().take(cut_index).collect()
+        InitialHistory::Resumed(items.into_iter().take(cut_index).collect())
     }
 }
 
@@ -223,10 +252,10 @@ mod tests {
         let truncated = truncate_after_dropping_last_messages(items.clone(), 1);
         assert_eq!(
             truncated,
-            vec![items[0].clone(), items[1].clone(), items[2].clone()]
+            InitialHistory::Resumed(vec![items[0].clone(), items[1].clone(), items[2].clone(),])
         );
 
         let truncated2 = truncate_after_dropping_last_messages(items, 2);
-        assert!(truncated2.is_empty());
+        assert_eq!(truncated2, InitialHistory::New);
     }
 }