chore: improve rollout session init errors (#7336)

Title: Improve rollout session initialization error messages

Issue: https://github.com/openai/codex/issues/7283

What: add targeted mapping for rollout/session initialization errors so
users get actionable messages when Codex cannot access session files.

Why: session creation previously returned a generic internal error,
hiding permissions/FS issues and making support harder.

How:
- Added rollout::error::map_session_init_error to translate the more
common io::Error kinds into user-facing hints (permission, missing dir,
file blocking, corruption). Others are passed through directly with
`CodexErr::Fatal`.
- Reused the mapper in Codex session creation to preserve root causes
instead of returning InternalAgentDied.

codex-rs/core/src/rollout/error.rs

@@ -0,0 +1,49 @@
+use std::io::ErrorKind;
+use std::path::Path;
+
+use crate::error::CodexErr;
+use crate::rollout::SESSIONS_SUBDIR;
+
+pub(crate) fn map_session_init_error(err: &anyhow::Error, codex_home: &Path) -> CodexErr {
+    if let Some(mapped) = err
+        .chain()
+        .filter_map(|cause| cause.downcast_ref::<std::io::Error>())
+        .find_map(|io_err| map_rollout_io_error(io_err, codex_home))
+    {
+        return mapped;
+    }
+
+    CodexErr::Fatal(format!("Failed to initialize session: {err:#}"))
+}
+
+fn map_rollout_io_error(io_err: &std::io::Error, codex_home: &Path) -> Option<CodexErr> {
+    let sessions_dir = codex_home.join(SESSIONS_SUBDIR);
+    let hint = match io_err.kind() {
+        ErrorKind::PermissionDenied => format!(
+            "Codex cannot access session files at {} (permission denied). If sessions were created using sudo, fix ownership: sudo chown -R $(whoami) {}",
+            sessions_dir.display(),
+            codex_home.display()
+        ),
+        ErrorKind::NotFound => format!(
+            "Session storage missing at {}. Create the directory or choose a different Codex home.",
+            sessions_dir.display()
+        ),
+        ErrorKind::AlreadyExists => format!(
+            "Session storage path {} is blocked by an existing file. Remove or rename it so Codex can create sessions.",
+            sessions_dir.display()
+        ),
+        ErrorKind::InvalidData | ErrorKind::InvalidInput => format!(
+            "Session data under {} looks corrupt or unreadable. Clearing the sessions directory may help (this will remove saved conversations).",
+            sessions_dir.display()
+        ),
+        ErrorKind::IsADirectory | ErrorKind::NotADirectory => format!(
+            "Session storage path {} has an unexpected type. Ensure it is a directory Codex can use for session files.",
+            sessions_dir.display()
+        ),
+        _ => return None,
+    };
+
+    Some(CodexErr::Fatal(format!(
+        "{hint} (underlying error: {io_err})"
+    )))
+}

codex-rs/core/src/rollout/mod.rs

@@ -7,11 +7,13 @@ pub const ARCHIVED_SESSIONS_SUBDIR: &str = "archived_sessions";
 pub const INTERACTIVE_SESSION_SOURCES: &[SessionSource] =
     &[SessionSource::Cli, SessionSource::VSCode];
 
+pub(crate) mod error;
 pub mod list;
 pub(crate) mod policy;
 pub mod recorder;
 
 pub use codex_protocol::protocol::SessionMeta;
+pub(crate) use error::map_session_init_error;
 pub use list::find_conversation_path_by_id_str;
 pub use recorder::RolloutRecorder;
 pub use recorder::RolloutRecorderParams;