Test for bug in cache for ext_globalMethodSignatureForSelector, in libextobj.

gistfile1.c

typedef struct {
    Class class;
    SEL sel;
} SelStruct;  // not sure about the name SelStruct, but works for now

NSString *NSStringFromSelStruct(SelStruct s) {
    return [NSString stringWithFormat:@"-[%@ %@]", s.class, NSStringFromSelector(s.sel)];
}

- (void)testCache {
    // test simplistic cache collisions for ext_blah
    static const size_t selectorCacheLength = 1 << 8;
    static const uintptr_t selectorCacheMask = (selectorCacheLength - 1);
    static SelStruct selectorHash[selectorCacheLength];

    unsigned classCount = 0;
    Class *classes = ext_copyClassList(&classCount);
    if (!classes)
        return;

    uint collisions = 0;
    uint methodNumber = 0;
    uint firstCollisionAtMethodNumber = 0;

    for (uint i = 0; i < classCount; ++i) {
        unsigned methodCount = 0;

        // just getting instance methods, that's enough
        Method *methods = class_copyMethodList(classes[i], &methodCount);

        for (uint j = 0; j < methodCount; ++j) {
            methodNumber++;
            Method method = methods[j];

            SelStruct thisSelStruct = (SelStruct) {.class = classes[i], .sel = method_getName(method)};
            uintptr_t hash = (uintptr_t)(void *)thisSelStruct.sel & selectorCacheMask;
            SelStruct hashedSelStruct = selectorHash[hash];
            if (hashedSelStruct.sel && hashedSelStruct.sel != thisSelStruct.sel) {
                collisions++;

                if (!firstCollisionAtMethodNumber) {
                    NSLog(@"first collision with %@ and %@", NSStringFromSelStruct(hashedSelStruct),
                          NSStringFromSelStruct(thisSelStruct));
                    firstCollisionAtMethodNumber = methodNumber;
                }
                // uncomment this for the deluge
                // NSLog(@"collision with %@ and %@", NSStringFromSelector(hashedSel), NSStringFromSelector(sel));

            } else {
                selectorHash[hash] = thisSelStruct;
            }
        }
        free(methods);
    }
    NSLog(@"tested %d classes, %d methods, %d collisions, first collision @ method %d",
          classCount, methodNumber, collisions, firstCollisionAtMethodNumber);

    // use your flavor of assertation here
    //XCTAssert(!collisions, @"hash collision for simple selector cache");
    
    free(classes);
}