ios - willSendRequestForAuthenticationChallenge メソッドは再帰的と呼ばれます

Question

iOS 10 を使用しています。以下のように自己署名証明書を評価しています。

-(void) connection:(NSURLConnection *)connection willSendRequestForAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
    NSURLProtectionSpace *protectionSpace = [challenge protectionSpace];

    if ([protectionSpace authenticationMethod] == NSURLAuthenticationMethodServerTrust) {

        SecTrustRef trust = [protectionSpace serverTrust];

        SecPolicyRef policyOverride = SecPolicyCreateSSL(true, (CFStringRef)@"HOSTNAME");
        SecTrustSetPolicies(trust, policyOverride);

        CFMutableArrayRef certificates = CFArrayCreateMutable(kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks);

        /* Copy the certificates from the original trust object */
        CFIndex count = SecTrustGetCertificateCount(trust);
        CFIndex i=0;
        for (i = 0; i < count; i++) {
            SecCertificateRef item = SecTrustGetCertificateAtIndex(trust, i);
            CFArrayAppendValue(certificates, item);
        }

        /* Create a new trust object */
        SecTrustRef newtrust = NULL;
        if (SecTrustCreateWithCertificates(certificates, policyOverride, &newtrust) != errSecSuccess) {
            /* Probably a good spot to log something. */
            NSLog(@"Error in SecTrustCreateWithCertificates");
            [connection cancel];
            return;
        }

        CFRelease(policyOverride);

        /* Re-evaluate the trust policy. */
        SecTrustResultType secresult = kSecTrustResultInvalid;
        if (SecTrustEvaluate(trust, &secresult) != errSecSuccess) {

            /* Trust evaluation failed. */
            [connection cancel];

            // Perform other cleanup here, as needed.
            return;
        }

        switch (secresult) {
                //case kSecTrustResultInvalid:
                //case kSecTrustResultRecoverableTrustFailure:
            case kSecTrustResultUnspecified: // The OS trusts this certificate implicitly.
            case kSecTrustResultProceed: // The user explicitly told the OS to trust it.
            {
                NSURLCredential *credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
                [challenge.sender useCredential:credential forAuthenticationChallenge:challenge];

                return;
            }
            default: ;
                /* It's somebody else's key. Fall through. */
                [challenge.sender performDefaultHandlingForAuthenticationChallenge:challenge];
                break;
        }
        /* The server sent a key other than the trusted key. */
        [connection cancel];
        // Perform other cleanup here, as needed.
    }
}

評価後の結果は ' kSecTrustResultUnspecified' で、同じメソッド ' willSendRequestForAuthenticationChallenge' が再帰的に呼び出されています。メソッドが再帰的に呼び出される理由がわかりません。コードの問題を教えてください。

ありがとう