https://issues.apache.org/bugzilla/show_bug.cgi?id=49393
Summary: mod_cache doesn't remove previous entry when stale
entry is replaced
Product: Apache httpd-2
Version: 2.3-HEAD
Platform: PC
OS/Version: Mac OS X 10.5
Status: NEW
Severity: normal
Priority: P2
Component: mod_cache
AssignedTo: [email protected]
ReportedBy: [email protected]
When we have a cached-and-stale entry, and the entry is updated with a 200 OK,
mod_cache attempts to remove the old cached entity:
else if (!r->header_only) {
/* Oh, well. Toss it. */
cache->provider->remove_entity(cache->stale_handle);
/* Treat the request as if it wasn't conditional. */
cache->stale_handle = NULL;
/*
* Restore the original request headers as they may be needed
* by further output filters like the byterange filter to make
* the correct decisions.
*/
r->headers_in = cache->stale_headers;
}
remove_entity() just NULLs out the cache object, it does not remove the stale
file on disk. This creates a race condition where for a brief time, the headers
from the new entity exist alongside the body of the stale entry.
To work around this, remove_entity() should be replaced with remove_url().
To properly fix this on trunk, we need to relook at the format for the data
file. The data file needs to be linked to from the headers file, and the
headers file needs to be written after the data file is written. This removes
the race completely, and allows us to update the cache atomically. This is not
something that is likely to be backportable to v2.2 though, thus the workaround
first.
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