Squid internals

The cbdata allocator (src/cbdata.c)
What is it ?
A problem with so many callbacks is to issue a callback for some data structure which has previously been freed.
The callback data (cbdata) interface provides a unified allocator and registry to verify the validity of memory and to deallocate memory when noone is using it
Example

CBDATA_INIT_TYPE_FREECB(type, freefunc);/* Create a new cbdata pool for C type 'type' */
foo = cbdataAlloc(type);		/* Allocate of type 'type' */
...
cbdataLock(foo);			/* Lock it to keep it valid */
...
some_blocking_operation_completes()
if (cbdataValid(foo))			/* If the pointer is actually valid */
    callback_func(..., foo);		/* .. call the callback, safe :) */
cbdataUnlock(foo);			/* Unlock it, dropping the refcount */
...
cbdataFree(foo);			/* Free the data - freefunc is now called */