19. Callback Data Database

Squid's extensive use of callback functions makes it very susceptible to memory access errors. For a blocking operation with callback functions, the normal sequence of events is as follows:

        callback_data = malloc(...);
        ...
        fooOperationStart(bar, callback_func, callback_data);
        ...
        fooOperationComplete(...);
        callback_func(callback_data, ....);
        ...
        free(callback_data);

The callback data database lets us do this in a uniform and safe manner. Every callback_data pointer must be added to the database. It is then locked while the blocking operation executes elsewhere, and is freed when the operation completes. The normal sequence of events is:

        callback_data = malloc(...);
        cbdataAdd(callback_data);
        ...
        cbdataLock(callback_data);
        fooOperationStart(bar, callback_func, callback_data);
        ...
        fooOperationComplete(...);
        if (cbdataValid(callback_data)) {
                callback_func(callback_data, ....);
        cbdataUnlock(callback_data);
        cbdataFree(callback_data);

With this scheme, nothing bad happens if cbdataFree gets called before cbdataUnlock:

        callback_data = malloc(...);
        cbdataAdd(callback_data);
        ...
        cbdataLock(callback_data);
        fooOperationStart(bar, callback_func, callback_data);
        ...
        cbdataFree(callback_data);
        ...
        fooOperationComplete(...);
        if (cbdataValid(callback_data)) {
                callback_func(callback_data, ....);
        cbdataUnlock(callback_data);