The function was returning NULL under some loads but not others. I had the stack traces. The failing path went through cache_lookup, then alloc, then the write path. I re-read the alloc function. Looked right. I re-read the lookup. Looked right. I added a print and ran the repro and the print didn't fire. I added another one earlier. That one fired but the output didn't tell me anything. The two stack traces were basically the same. I scrolled up. I scrolled down. I opened the file I'd already opened six times and looked at the same code and nothing looked different than the last time.
