Match a pre-upgrade studio.pid to the blocked port before falling back

studio/backend/run.py

@@ -875,7 +875,38 @@ def _own_studio_on_port(port: int, host: str) -> "int | None":
         # None (unverifiable) counts as ours: refusing beats a silent duplicate.
         if _pid_is_studio_backend(pid, [created]) is not False:
             return pid
-    return None
+    return _legacy_studio_on_port(port)
+
+
+def _legacy_studio_on_port(port: int) -> "int | None":
+    """A pre-upgrade server recorded only its PID, so match it to the listener.
+
+    Falling back past one leaves it running while `_write_pid_file` overwrites the
+    only record of it. When the listener is unknowable, assume it is ours.
+    """
+    record = _read_pid_record(_PID_FILE)
+    if record is None:
+        return None
+    pid, created, _address = record
+    if not _pid_alive(pid):
+        return None
+    # A current build writes a per-port file too, so its port is already known --
+    # and this port's records were just checked. Only unported records get here.
+    if any(r and r[0] == pid for r in _per_port_records()):
+        return None
+    blocker = _get_pid_on_port(port)
+    if blocker is not None and blocker[0] != pid:
+        return None
+    if _pid_is_studio_backend(pid, [created]) is False:
+        return None
+    return pid
+
+
+def _per_port_records() -> "list[tuple[int, float | None, str | None] | None]":
+    try:
+        return [_read_pid_record(p) for p in _studio_root().glob(PID_FILE_GLOB)]
+    except OSError:
+        return []
 
 
 def _abort_already_running(pid: int, port: int) -> "NoReturn":

studio/backend/tests/test_studio_pid_files.py

@@ -228,6 +228,57 @@ def test_legacy_records_do_not_match_a_training_run(monkeypatch):
     assert run._pid_is_studio_backend(9999) is False
 
 
+def test_a_legacy_server_on_the_port_is_recognised(tmp_path, monkeypatch):
+    # Pre-upgrade servers wrote only studio.pid. Falling back past one strands it
+    # and then overwrites its record.
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: (8550, "python"))
+    (tmp_path / "studio.pid").write_text("8550", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8901, "127.0.0.1") == 8550
+
+
+def test_a_legacy_record_for_a_different_listener_falls_back(tmp_path, monkeypatch):
+    # jupyter holds the port; the legacy server is elsewhere. Keep falling back.
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: (117, "jupyter-lab"))
+    (tmp_path / "studio.pid").write_text("8550", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8901, "127.0.0.1") is None
+
+
+def test_an_unknowable_listener_treats_the_legacy_record_as_ours(tmp_path, monkeypatch):
+    # No psutil: _get_pid_on_port can't say. Refusing beats a silent duplicate.
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: None)
+    (tmp_path / "studio.pid").write_text("8550", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8901, "127.0.0.1") == 8550
+
+
+def test_a_dead_legacy_record_falls_back(tmp_path, monkeypatch):
+    monkeypatch.setattr(run, "_pid_alive", lambda pid: False)
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: None)
+    (tmp_path / "studio.pid").write_text("8550", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8901, "127.0.0.1") is None
+
+
+def test_a_current_server_elsewhere_does_not_block_a_foreign_port(tmp_path, monkeypatch):
+    # Current builds write studio.pid too. Without psutil the legacy check can't
+    # see the listener, so it must not claim our 8901 server holds jupyter's 8888.
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: None)
+    (tmp_path / "studio-8901-5000.pid").write_text("5000\n\n127.0.0.1", encoding = "utf-8")
+    (tmp_path / "studio.pid").write_text("5000", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8888, "127.0.0.1") is None
+
+
+def test_a_per_port_record_is_preferred_over_the_legacy_one(tmp_path, monkeypatch):
+    monkeypatch.setattr(run, "_get_pid_on_port", lambda p: (8550, "python"))
+    (tmp_path / "studio-8901-8600.pid").write_text("8600\n\n127.0.0.1", encoding = "utf-8")
+    (tmp_path / "studio.pid").write_text("8550", encoding = "utf-8")
+
+    assert run._own_studio_on_port(8901, "127.0.0.1") == 8600
+
+
 def test_our_studio_on_another_bind_address_does_not_abort(tmp_path):
     # Our server holds ::1:8889; binding 127.0.0.1:8889 is not a conflict with us,
     # so fall through to the next port instead of refusing.