Memory leak due to use of `ngx_cycle->pool` in partial (wildcard) purge code.

[TaylanUB]

I believe this is effectively a memory leak:

ngx_cache_purge/ngx_cache_purge_module.c

Lines 1345 to 1347 in a84b0f3

	/* I don't know if it's a good idea to use the ngx_cycle pool for this,
	but the request is not available here */
	key_in_file = ngx_pcalloc(ngx_cycle->pool, sizeof(u_char) * (len + 1));

If I understand correctly, the ngx_cycle pool lives until a config reload is issued.

Indeed, on our server where I recently started using wildcard purging, I've begun observing Nginx using more and more memory, until I issue a reload, so this matches perfectly.

It appears there is an ngx_pfree() function that should probably be called on key_in_file once we're done with it. That would be after line 1365, before exiting the scope in which key_in_file is set.

I hope this module is still maintained? :-)

[TaylanUB]

I just realized that this allocation happens once for every single file in the cache. That's a lot! Explains why I can make Nginx's memory use skyrocket by sending a bunch of PURGE requests.

Got 37K files in the cache right now, so if key_in_file is 20 bytes on average, that'd be 722 kB for each PURGE request.

Apart from the fact that it needs to be freed, it would also be better if key_in_file was not allocated within that function at all (i.e. once for every file walked), and instead once at the beginning of the walk. Not sure how to do that in a thread-safe manner.

[TaylanUB]

See PR #46 for a potential solution.