sqlite3.OperationalError: database is locked when using --testmon-nocollect with pytest-xdist

[jogendra-india]

Bug Description

When using --testmon --testmon-nocollect with pytest-xdist (-n auto or -n <num>), the test run fails intermittently with:

sqlite3.OperationalError: database is locked

This is especially common on CI systems with multiple parallel workers.

Root Cause

The TestmonXdistSync plugin is only registered inside the if should_collect: block in register_plugins():

def register_plugins(config, should_select, should_collect, cov_plugin):
    if should_select or should_collect:
        config.pluginmanager.register(TestmonSelect(...))

    if should_collect:  # BUG: TestmonXdistSync only registered here!
        config.pluginmanager.register(TestmonCollect(...))
        if config.pluginmanager.hasplugin("xdist"):
            config.pluginmanager.register(TestmonXdistSync())

When --testmon-nocollect is used:

1. should_collect = False
2. TestmonXdistSync is not registered
3. The pytest_configure_node hook doesn't run
4. Workers don't receive testmon_exec_id from the controller
5. Each worker calls TestmonData.for_local_run() instead of TestmonData.for_worker()
6. for_local_run() → initiate_execution() → fetch_or_create_environment()
7. fetch_or_create_environment() uses BEGIN IMMEDIATE TRANSACTION (write lock)
8. Multiple workers race for the write lock → database locked error

Steps to Reproduce

# Create a simple test file
echo 'def test_one(): pass' > test_sample.py

# First run to create .testmondata
pytest --testmon test_sample.py

# Second run with nocollect + xdist - this causes database lock
pytest --testmon --testmon-nocollect -n 4 test_sample.py

Expected Behavior

The --testmon-nocollect mode should work seamlessly with pytest-xdist. Workers should receive the exec_id from the controller and open the database in read-only mode.

Proposed Fix

1. Register TestmonXdistSync whenever testmon is active (not just when collecting):

if should_select or should_collect:
    config.pluginmanager.register(TestmonSelect(...))
    
    # Register xdist sync even in nocollect mode
    if config.pluginmanager.hasplugin("xdist"):
        config.pluginmanager.register(TestmonXdistSync(should_collect=should_collect))

2. Skip database sync in pytest_xdist_node_collection_finished when in nocollect mode:

class TestmonXdistSync:
    def __init__(self, should_collect=True):
        self._should_collect = should_collect
    
    def pytest_xdist_node_collection_finished(self, node, ids):
        self.await_nodes += -1
        if self.await_nodes == 0 and self._should_collect:
            node.config.testmon_data.sync_db_fs_tests(retain=set(ids))

Environment

• pytest-testmon version: 2.2.0
• pytest version: 7.x / 8.x
• pytest-xdist version: 3.x
• Python version: 3.10+
• OS: Linux (GitHub Actions runners), but reproducible on any OS

Impact

This bug affects all users who:

• Use --testmon-nocollect for CI pipelines (read-only mode)
• Use pytest-xdist for parallel test execution
• Run on systems with multiple workers