fix(fly): warm rclone dir cache after activate to avoid first-readdir race

`rclone mount --daemon` returns immediately after fork; FUSE's first
readdir on the mountpoint root then races the daemon's first remote
listing fetch. The agent's first `ls` could observe an empty directory
even though the mount is live. Add a single throw-away `ls` inside
`_verify_mount_active` after `mountpoint -q` passes — its only purpose
is the side effect of priming rclone's directory cache so subsequent
listings see the bucket contents immediately.

Verified end-to-end: previously the first listing would briefly show
empty for ~half a second; with the warmup, the very first agent-side
listing returns the populated mount.

Author: vaurdan

src/agents/extensions/sandbox/flyio/mounts.py

@@ -150,6 +150,13 @@ async def _verify_mount_active(session: BaseSandboxSession, mount_path: Path) ->
             context={"path": str(mount_path)},
         )
 
+    # Force rclone to materialize the root directory listing before we hand
+    # control back to the caller. Without this, the next ``readdir`` from the
+    # agent races the daemon's first listing fetch and can briefly observe an
+    # empty directory. The exit code is irrelevant here — we just want the
+    # side effect of priming rclone's dir cache.
+    await session.exec("sh", "-lc", f"ls {quoted} >/dev/null 2>&1", shell=False, timeout=15)
+
 
 async def _default_user_ids(session: BaseSandboxSession) -> tuple[str, str] | None:
     result = await session.exec("sh", "-lc", "id -u; id -g", shell=False, timeout=30)

tests/extensions/test_sandbox_fly.py

@@ -1694,12 +1694,17 @@ async def test_ensure_fuse_support_raises_when_post_install_fusermount_still_mis
 
 @pytest.mark.asyncio
 async def test_verify_mount_active_passes_when_mountpoint_reports_mounted() -> None:
-    session = _FakeMountSession([_mounted()])
+    session = _FakeMountSession([_mounted(), _ok()])
 
     await _verify_mount_active(session, Path("/workspace/tigris"))
 
+    # Two calls: the mountpoint probe followed by a directory-listing warm-up
+    # that forces rclone to populate its root readdir cache before the caller
+    # uses the mount.
+    assert len(session.exec_calls) == 2
     assert "mountpoint -q /workspace/tigris" in session.exec_calls[0]
     assert _MOUNTED in session.exec_calls[0]
+    assert "ls /workspace/tigris >/dev/null 2>&1" in session.exec_calls[1]
 
 
 @pytest.mark.asyncio
@@ -1711,6 +1716,22 @@ async def test_verify_mount_active_raises_when_path_is_not_a_mount() -> None:
     assert excinfo.value.context.get("path") == "/workspace/tigris"
 
 
+@pytest.mark.asyncio
+async def test_verify_mount_active_warmup_runs_after_mountpoint_check() -> None:
+    """The post-mountpoint listing warmup runs even if the directory is empty.
+
+    Confirms the warmup exec is fired regardless of what ``ls`` returns —
+    we only care about the side effect of priming rclone's dir cache.
+    """
+
+    session = _FakeMountSession([_mounted(), _ok()])
+
+    await _verify_mount_active(session, Path("/workspace/some/nested/mount"))
+
+    assert "mountpoint -q /workspace/some/nested/mount" in session.exec_calls[0]
+    assert "ls /workspace/some/nested/mount" in session.exec_calls[1]
+
+
 @pytest.mark.asyncio
 async def test_rclone_pattern_appends_allow_other_and_user_ids() -> None:
     session = _FakeMountSession([_ok(stdout=b"1001\n1001\n")])