## Why
Remote environments may still be resolving when Codex creates a session
or turn. Waiting for the existing all-or-nothing environment snapshot
can hold startup until the selected environment is usable.
Behind the default-off `deferred_executor` feature, let callers take a
useful snapshot immediately: completed environments remain available
normally, while unfinished environments are reported without blocking
startup. With the feature disabled, snapshots preserve the existing
blocking behavior.
Depends on #28674.
## What changed
- Store one ordered list of selected environments in
`ThreadEnvironments`. Each selection owns one shared resolution that
produces its complete `TurnEnvironment`.
- Start new resolutions in the background with `remote_handle()`,
allowing snapshots and the future wait tool to share the same result
while cancellation follows the retained handles.
- Make `snapshot()` a read-only operation: nonblocking snapshots collect
completed resolutions and retain handles for unfinished ones, while
blocking snapshots await every resolution.
- Replace completed failed resolutions from the current manager entry
and log when failed environments are omitted.
- Return attached and starting environments as a point-in-time view, and
count starting environments when deciding whether a snapshot is
local-only.
- Keep existing consumers attached-only. `to_selections()` derives from
attached environments, so child threads do not inherit an environment
that is still starting.
## Test plan
- `just test -p codex-core environment_selection`
- `just test -p codex-core
deferred_executor_reaches_model_before_remote_environment_is_ready`
## Landing note
Keep `deferred_executor` disabled for slow-starting executors until
configurable `environment/add` connection timeouts and caller support
land. When enabled, an environment that attaches after session startup
may remain absent from environment-derived model context, tools,
instructions, skills, and related state until follow-up refresh work
lands.
sayan-oai
45a133bae0