Warn user at startup when another DataLab instance is already running

[PierreRaybaut]

Description

DataLab does not fully support running multiple instances simultaneously. Known side effects include:

• Preferences in the .ini configuration file being overwritten by the last instance to close
• XML-RPC auto-discovery only finding the most recently started instance
• Potential Web API connection file conflicts

Rather than attempting full multi-instance support (which would require significant architectural changes), we should warn the user at startup when another instance is detected.

Proposed implementation

• Use a PID-based lock file (datalab.lock in the config directory) to detect running instances
• On startup, if another instance is detected, show a warning dialog with the option to continue or abort
• Clean up the lock file on normal exit; handle stale lock files from crashes automatically

[PierreRaybaut]

Additionnal notes and suggestions for implementation

Context

DataLab does not fully support running multiple instances simultaneously. Known side effects include:

• Preferences overwritten: The .ini configuration file is shared by all instances. Every setting change triggers a full file rewrite (delete + write). The last instance to close overwrites all preferences.
• XML-RPC discovery broken: The auto-assigned XML-RPC port is stored in the shared .ini file. Only the most recently started instance is discoverable via get_datalab_xmlrpc_port().
• Web API connection file conflict: The webapi_connection.json file is a single file that gets overwritten by the last instance starting a Web API server.

Rather than attempting full multi-instance support (which would require significant architectural changes to the configuration system), we should warn the user at startup when another instance is detected, and let them choose whether to continue.

Note: This replaces the previous issue about multi-instance configuration handling, which is closed as "won't fix".

Proposed implementation

1. Instance detection via PID lock file

Create a new module datalab/utils/instancecheck.py providing:

• is_another_instance_running() -> int | None: Reads a lock file at Conf.get_path("datalab.lock") (e.g., ~/.DataLab_v1/datalab.lock on Windows). If the file exists, reads the stored PID and checks if the process is still alive. Returns the PID if alive, None otherwise. Automatically removes stale lock files from crashed instances.

• create_lock_file() -> None: Writes the current PID (os.getpid()) to the lock file.

• remove_lock_file() -> None: Removes the lock file if it exists and contains the current PID (safety check to avoid removing another instance's lock).

PID liveness check: Use os.kill(pid, 0) on Unix and ctypes.windll.kernel32.OpenProcess on Windows — no external dependency required.

2. Startup warning dialog

In datalab/app.py, inside the run() function, after the Qt application is created (so dialogs can be shown) but before the main window is instantiated:

1. Call is_another_instance_running().

2. If another instance is detected (returns a PID), show a QMessageBox.warning with Yes/No buttons:

   "Another instance of DataLab (PID {pid}) appears to be running.
   Running multiple instances simultaneously may cause side effects:
   preferences being overwritten, remote control discovery issues, etc.
   Do you want to continue anyway?"

3. If the user chooses No → exit cleanly (sys.exit(0)).

4. If the user chooses Yes or no other instance is detected → create the lock file and proceed normally.

3. Cleanup on exit

• Normal exit: Call remove_lock_file() in DLMainWindow.close_properly(), just before return True.
• Safety net: Register atexit.register(remove_lock_file) after creating the lock file, to cover cases where close_properly() isn't reached (e.g., SIGTERM).
• Abnormal termination (crash, kill -9): Handled automatically — on next startup, is_another_instance_running() detects that the PID is dead and cleans up the stale lock file.

4. Testing

Add unit tests verifying:

• Lock file with alive PID → detection returns the PID
• Lock file with dead PID → stale file is cleaned up, returns None
• create_lock_file() / remove_lock_file() round-trip
• remove_lock_file() only removes if PID matches (no cross-instance removal)

Files to modify

File	Change
datalab/utils/instancecheck.py	New — lock file management and instance detection
datalab/app.py	Add instance check + warning dialog before create()
datalab/gui/main.py	Call remove_lock_file() in close_properly()
datalab/tests/	Add unit tests for instance detection
datalab/locale/	Update translations for the warning message