Large multigets with binary protocol may hang client

(from http://lists.tangent.org/pipermail/libmemcached/2009-August/000918.html)

There's a problem with how memcached_purge attempts to flush the output buffer and read in responses which leads to hangs on read(2) calls. Fundamentally the issue is that memcached_purge expects to find a response for each request, when in general it has no way of knowing whether to a response will be forthcoming for quiet commands. This occurs at least in the situations where MEMCACHED_BEHAVIOR_BUFFER_REQUESTS is set or when doing binary multigets (which buffer getq commands before sending a noop command).

Consider the multiget case: assume a multiget for 1,000 keys from a particular server; that only 100 getq commands fit in the 8K output buffer;
and that 10 of those 100 are cache misses. Only 90 responses are sent, and on memcached_purge's 91st call to memcached_read_one_response, the read() call blocks (forever, unless a timeout has been set).