Squid's extensive use of callback functions makes it very susceptible to memory access errors. To address this all callback functions make use of a construct called "cbdata". This allows functions doing callbacks to verify that the caller is still valid before making the callback.
Note: cbdata is intended for callback data and is tailored specifically to make callbacks less dangerous leaving as few windows of errors as possible. It is not suitable or intended as a generic referencecounted memory allocator.
CBDATA_TYPE(datatype);
Macro that defines a new cbdata datatype. Similar to a variable or struct definition. Scope is always local to the file/block where it is defined and all calls to cbdataAlloc for this type must be within the same scope as the CBDATA_TYPE declaration. Allocated entries may be referenced or freed anywhere with no restrictions on scope.
/* Module header file */
external CBDATA_GLOBAL_TYPE(datatype);
/* Module main C file */
CBDATA_GLOBAL_TYPE(datatype);
Defines a global cbdata type that can be referenced anywhere in the code.
CBDATA_INIT_TYPE(datatype);
/* or */
CBDATA_INIT_TYPE_FREECB(datatype, FREE *freehandler);
Initializes the cbdatatype. Must be called prior to the first use of cbdataAlloc() for the type.
The freehandler is called when the last known reference to a allocated entry goes away.
pointer = cbdataAlloc(datatype);
Allocates a new entry of a registered cbdata type.
cbdataFree(pointer);
Frees a entry allocated by cbdataAlloc().
Note: If there are active references to the entry then the entry will be freed with the last reference is removed. However, cbdataReferenceValid() will return false for those references.
reference = cbdataReference(pointer);
Creates a new reference to a cbdata entry. Used when you need to store a reference in another structure. The reference can later be verified for validity by cbdataReferenceValid().
Note: The reference variable is a pointer to the entry, in all aspects identical to the original pointer. But semantically it is quite different. It is best if the reference is thought of and handled as a "void *".
cbdataReferenceDone(reference);
Removes a reference created by cbdataReference().
Note: The reference variable will be automatically cleared to NULL.
if (cbdataReferenceValid(reference)) {
...
}
cbdataReferenceValid() returns false if a reference is stale (refers to a entry freed by cbdataFree).
void *pointer;
bool cbdataReferenceValidDone(reference, &pointer);
Removes a reference created by cbdataReference() and checks it for validity. A temporary pointer to the referenced data (if valid) is returned in the &pointer argument.
Meant to be used on the last dereference, usually to make a callback.
void *cbdata;
...
if (cbdataReferenceValidDone(reference, &cbdata)) != NULL)
callback(..., cbdata);
Note: The reference variable will be automatically cleared to NULL.
Here you can find some examples on how to use cbdata, and why
For a asyncronous operation with callback functions, the normal sequence of events in programs NOT using cbdata is as follows:
/* initialization */
type_of_data our_data;
...
our_data = malloc(...);
...
/* Initiate a asyncronous operation, with our_data as callback_data */
fooOperationStart(bar, callback_func, our_data);
...
/* The asyncronous operation completes and makes the callback */
callback_func(callback_data, ....);
/* Some time later we clean up our data */
free(our_data);
However, things become more interesting if we want or need
to free the callback_data, or otherwise cancel the callback,
before the operation completes. In constructs like this you
can quite easily end up with having the memory referenced
pointed to by callback_data freed before the callback is invoked
causing a program failure or memory corruption:
/* initialization */
type_of_data our_data;
...
our_data = malloc(...);
...
/* Initiate a asyncronous operation, with our_data as callback_data */
fooOperationStart(bar, callback_func, our_data);
...
/* ouch, something bad happened elsewhere.. try to cleanup
* but the programmer forgot there is a callback pending from
* fooOperationsStart() (an easy thing to forget when writing code
* to deal with errors, especially if there may be many different
* pending operation)
*/
free(our_data);
...
/* The asyncronous operation completes and makes the callback */
callback_func(callback_data, ....);
/* CRASH, the memory pointer to by callback_data is no longer valid
* at the time of the callback
*/
The callback data allocator lets us do this in a uniform and safe manner. The callback data allocator is used to allocate, track and free memory pool objects used during callback operations. Allocated memory is locked while the asyncronous operation executes elsewhere, and is freed when the operation completes. The normal sequence of events is:
/* initialization */
type_of_data our_data;
...
our_data = cbdataAlloc(type_of_data);
...
/* Initiate a asyncronous operation, with our_data as callback_data */
fooOperationStart(..., callback_func, our_data);
...
/* foo */
void *local_pointer = cbdataReference(callback_data);
....
/* The asyncronous operation completes and makes the callback */
void *cbdata;
if (cbdataReferenceValidDone(local_pointer, &cbdata))
callback_func(...., cbdata);
...
cbdataFree(our_data);
With this scheme, nothing bad happens if cbdataFree gets called
before fooOperantionComplete(...).
/* initialization */
type_of_data our_data;
...
our_data = cbdataAlloc(type_of_data);
...
/* Initiate a asyncronous operation, with our_data as callback_data */
fooOperationStart(..., callback_func, our_data);
...
/* foo */
void *local_pointer = cbdataReference(callback_data);
....
/* something bad happened elsewhere.. cleanup */
cbdataFree(our_data);
...
/* The asyncronous operation completes and tries to make the callback */
void *cbdata;
if (cbdataReferenceValidDone(local_pointer, &cbdata))
/* won't be called, as the data is no longer valid */
callback_func(...., cbdata);
In this case, when cbdataFree is called before
cbdataReferenceValidDone, the callback_data gets marked as invalid.
When the callback_data is invalid before executing the callback
function, cbdataReferenceValidDone will return 0 and
callback_func is never executed.
To add new module specific data types to the allocator one uses the macros CBDATA_TYPE and CBDATA_INIT_TYPE. These creates a local cbdata definition (file or block scope). Any cbdataAlloc calls must be made within this scope. However, cbdataFree might be called from anywhere.
/* First the cbdata type needs to be defined in the module. This
* is usually done at file scope, but it can also be local to a
* function or block..
*/
CBDATA_TYPE(type_of_data);
/* Then in the code somewhere before the first allocation
* (can be called multiple times with only a minimal overhead)
*/
CBDATA_INIT_TYPE(type_of_data);
/* Or if a free function is associated with the data type. This
* function is responsible for cleaning up any dependencies etc
* referenced by the structure and is called on cbdataFree or
* when the last reference is deleted by cbdataReferenceDone /
* cbdataReferenceValidDone
*/
CBDATA_INIT_TYPE_FREECB(type_of_data, free_function);
To add new global data types that can be allocated from anywhere within the code one have to add them to the cbdata_type enum in enums.h, and a corresponding CREATE_CBDATA call in cbdata.c:cbdataInit(). Or alternatively add a CBDATA_GLOBAL_TYPE definition to globals.h as shown below and use CBDATA_INIT_TYPE at the appropriate location(s) as described above.
extern CBDATA_GLOBAL_TYPE(type_of_data); /* CBDATA_UNDEF */