fix(build): prevent Redis connections during Docker collectstatic build step

[mendral-app]

Summary

• Two separate call sites attempt live Redis connections when Django initialises during python manage.py collectstatic in the Docker image build layer, producing redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused. tracebacks on every build (both linux/amd64 and linux/arm64). The errors are caught so builds don't fail, but the noise is unnecessary.

Root cause

Call site 1 — posthog/apps.py:106: PostHogConfig.ready() calls queue_sync_hog_function_templates() unconditionally for all non-test invocations. That function immediately does r.setnx(lock_key, 1) to acquire a distributed Redis lock before queuing a Celery task. No Redis is available during docker build.

Call site 2 — products/notebooks/backend/kernel_runtime.py:1564: KernelRuntimeService was instantiated at module level, which registers an atexit hook that calls shutdown_all() → acquires a Redis lock on process exit. The module is imported transitively via NotebooksConfig.ready() → api/notebook.py → kernel_runtime.py. The atexit fires when collectstatic finishes.

Fix

1. Guard queue_sync_hog_function_templates() with the STATIC_COLLECTION env var already set by the Dockerfile for this step.
2. Replace the module-level KernelRuntimeService() singleton with a lazy getter — the instance (and its atexit/signal hooks) is only created on first real use, not at import time.

Fixes: https://app.mendral.com/insights/01KMD080H98VRD7A9GER5RHWDY

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Note

Created by Mendral. Tag @mendral-app with feedback or questions.

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[webjunkie]

@mendral-app you already changed the same call site?

<<<<<<< mendral/fix-redis-connection-during-collectstatic
        # Skip during Docker image builds (collectstatic) — no Redis available at build time
        if not settings.TEST and not os.environ.get("STATIC_COLLECTION"):
=======
        # Skip during collectstatic (STATIC_COLLECTION=1 in Dockerfile) — no Redis available at build time
        if not settings.TEST and not settings.STATIC_COLLECTION:
>>>>>>> master
            queue_sync_hog_function_templates()

[mendral-app]

Good catch @webjunkie! Yes, master already added settings.STATIC_COLLECTION for that call site, so my change to os.environ.get("STATIC_COLLECTION") was redundant and conflicting. I've rebased the branch onto master and dropped my apps.py change — the PR now only contains the kernel_runtime.py lazy initialization fix (which master doesn't have yet).