fix: use ContextVar for client config overrides and propagate context to thread pool

[halfbaked]

Problem

CLIENT_CONFIG_OVERRIDES_KEY (introduced for per-request client config overrides via middleware) does not work. Two bugs prevent the override from being applied:

Bug 1: Async/sync mismatch in create_clickhouse_client

create_clickhouse_client() is a sync function that calls ctx.get_state(CLIENT_CONFIG_OVERRIDES_KEY) without await:

session_config_overrides = ctx.get_state(CLIENT_CONFIG_OVERRIDES_KEY)

Context.get_state is async def, so calling it without await returns a coroutine object, not the dict. The override is silently ignored. The unit tests pass because MagicMock.get_state.return_value returns synchronously, masking the bug.

Bug 2: QUERY_EXECUTOR.submit doesn't propagate contextvars

future = QUERY_EXECUTOR.submit(execute_query, query)

ThreadPoolExecutor.submit does not copy the current context into worker threads. Even if Bug 1 were fixed, ContextVar values set by middleware in the async context are invisible to execute_query running in the thread pool.

Fix

1. Replace ctx.get_state/set_state with a module-level ContextVar (_client_config_overrides_var). This is synchronous and works naturally with contextvars propagation.

2. Wrap ThreadPoolExecutor in _ContextPropagatingExecutor that uses contextvars.copy_context().run() to propagate the calling context into worker threads.

How middleware should use it

from mcp_clickhouse.mcp_server import _client_config_overrides_var

class MyMiddleware(Middleware):
    async def on_call_tool(self, context, call_next):
        token = _client_config_overrides_var.set({"username": "other_user", "password": "..."})
        try:
            return await call_next(context)
        finally:
            _client_config_overrides_var.reset(token)

Testing

• Updated unit tests to use ContextVar directly instead of mocked ctx.get_state
• Added test for username/password override (the primary use case for per-user credential passthrough)
• Existing integration tests unchanged

Related

• Docker image on Docker Hub is outdated (missing middleware and config override features) #161 — Docker image is outdated and missing middleware/override features

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