Fix offline manifest race condition

[cuu508]

django-compressor avoids rendering the same node multiple times by checking for a key in offline_manifest:

    if key in offline_manifest:
        continue

    [... render template ...]

    offline_manifest[key] = result

Multiple _compress_template calls can run concurrently, creating a race condition: each checks for a key in offline manifest, then each proceeds to render template. Finally they try to save the same file at the same time, causing #1082.

My proposed fix is to atomically

• check if the manifest key exists
• if it doesn't exist, set it to a placeholder value (None)

So, in nutshell, the first "if" part becomes:

    with offline_manifest_lock:
        if key in offline_manifest:
            continue

        offline_manifest[key] = None

I'm not sure where to store the lock, I've put it at the module level currently. Perhaps there's a way to avoid the lock entirely.

[carltongibson]

Hi @cuu508 — thanks for this.

Perhaps there's a way to avoid the lock entirely.

It looks OK to me. Maybe there's another approach but this seems simple enough to go with.

You're officially Hero of the Month here 🥇