Called when we have set custom validation. We do this in two cases:
        first, when cert validation is entirely disabled; and second, when
        using a custom trust DB.

def _custom_validate(self, verify, trust_bundle):
        """
        Called when we have set custom validation. We do this in two cases:
        first, when cert validation is entirely disabled; and second, when
        using a custom trust DB.
        """
        # If we disabled cert validation, just say: cool.
        if not verify:
            return

        # We want data in memory, so load it up.
        if os.path.isfile(trust_bundle):
            with open(trust_bundle, 'rb') as f:
                trust_bundle = f.read()

        cert_array = None
        trust = Security.SecTrustRef()

        try:
            # Get a CFArray that contains the certs we want.
            cert_array = _cert_array_from_pem(trust_bundle)

            # Ok, now the hard part. We want to get the SecTrustRef that ST has
            # created for this connection, shove our CAs into it, tell ST to
            # ignore everything else it knows, and then ask if it can build a
            # chain. This is a buuuunch of code.
            result = Security.SSLCopyPeerTrust(
                self.context, ctypes.byref(trust)
            )
            _assert_no_error(result)
            if not trust:
                raise ssl.SSLError("Failed to copy trust reference")

            result = Security.SecTrustSetAnchorCertificates(trust, cert_array)
            _assert_no_error(result)

            result = Security.SecTrustSetAnchorCertificatesOnly(trust, True)
            _assert_no_error(result)

            trust_result = Security.SecTrustResultType()
            result = Security.SecTrustEvaluate(
                trust, ctypes.byref(trust_result)
            )
            _assert_no_error(result)
        finally:
            if trust:
                CoreFoundation.CFRelease(trust)

            if cert_array is not None:
                CoreFoundation.CFRelease(cert_array)

        # Ok, now we can look at what the result was.
        successes = (
            SecurityConst.kSecTrustResultUnspecified,
            SecurityConst.kSecTrustResultProceed
        )
        if trust_result.value not in successes:
            raise ssl.SSLError(
                "certificate verify failed, error code: %d" %
                trust_result.value
            )