[HttpCache] occasionnal 503 error on locking request

I have this problem for a long time now: requests using HttpCache sometimes get caught in the waiting loop. After a bit of monitoring I'm sure the request creating the lock doesn't take more than 2s so parallel requests shouldn't return a 503 code.

I couldn't reproduce the problem on my computer, maybe because it's fairly rare (say 0.01% of requests). Couldn't this be due to a missing clearstatcache in the isLocked function?