SDL_FindHashInTable() is not read-only

[icculus]

This pattern is in SDL_properties.c in several places:

    if (SDL_LockRWLockForReading(SDL_properties_lock) == 0) {
        SDL_FindInHashTable(SDL_properties, (const void *)(uintptr_t)props, (const void **)&properties);
        SDL_UnlockRWLock(SDL_properties_lock);
    }

The problem is that SDL_FindHashInTable() will modify the hash...

            /* Matched! Move to the front of list for faster lookup next time.
               (stackable tables have to remain in the same order, though!) */
            if ((!table->stackable) && (prev != NULL)) {
                SDL_assert(prev->next == i);
                prev->next = i->next;
                i->next = table->table[hash];
                table->table[hash] = i;
            }

...and if two things have a read lock and come through here at the same time, it'll corrupt the hashtable.

So we should decide if we want to either move SDL_properties_lock to a normal mutex, or remove the optimization in SDL_FindInHashTable. I guess it's a question of which is more likely: multiple lookups of the same thing, or multiple threads looking stuff up in parallel.

If we remove the optimization, we should also move SDL_Properties::lock over to a RWlock.

[icculus]

(I'm inclined to say remove the optimization, so multiple threads can read from a hashtable simultaneously without using a lock at all when the hash is otherwise static...and also because this is a surprising implementation detail of the hashtable one wouldn't guess from the interface.)

[icculus]

Talked myself into the right solution eventually. :)