feat: add helper func to for default encrypted cert (#514)

* feat: helper func to for default encrpted cert

google/auth/transport/mtls.py

@@ -58,3 +58,45 @@ def callback():
         return cert_bytes, key_bytes
 
     return callback
+
+
+def default_client_encrypted_cert_source(cert_path, key_path):
+    """Get a callback which returns the default encrpyted client SSL credentials.
+
+    Args:
+        cert_path (str): The cert file path. The default client certificate will
+            be written to this file when the returned callback is called.
+        key_path (str): The key file path. The default encrypted client key will
+            be written to this file when the returned callback is called.
+
+    Returns:
+        Callable[[], [str, str, bytes]]: A callback which generates the default
+            client certificate, encrpyted private key and passphrase. It writes
+            the certificate and private key into the cert_path and key_path, and
+            returns the cert_path, key_path and passphrase bytes.
+
+    Raises:
+        google.auth.exceptions.DefaultClientCertSourceError: If any problem
+            occurs when loading or saving the client certificate and key.
+    """
+    if not has_default_client_cert_source():
+        raise exceptions.MutualTLSChannelError(
+            "Default client encrypted cert source doesn't exist"
+        )
+
+    def callback():
+        try:
+            _, cert_bytes, key_bytes, passphrase_bytes = _mtls_helper.get_client_ssl_credentials(
+                generate_encrypted_key=True
+            )
+            with open(cert_path, "wb") as cert_file:
+                cert_file.write(cert_bytes)
+            with open(key_path, "wb") as key_file:
+                key_file.write(key_bytes)
+        except (exceptions.ClientCertError, OSError) as caught_exc:
+            new_exc = exceptions.MutualTLSChannelError(caught_exc)
+            six.raise_from(new_exc, caught_exc)
+
+        return cert_path, key_path, passphrase_bytes
+
+    return callback

tests/transport/test_mtls.py

@@ -53,3 +53,31 @@ def test_default_client_cert_source(
     callback = mtls.default_client_cert_source()
     with pytest.raises(exceptions.MutualTLSChannelError):
         callback()
+
+
+@mock.patch(
+    "google.auth.transport._mtls_helper.get_client_ssl_credentials", autospec=True
+)
+@mock.patch("google.auth.transport.mtls.has_default_client_cert_source", autospec=True)
+def test_default_client_encrypted_cert_source(
+    has_default_client_cert_source, get_client_ssl_credentials
+):
+    # Test default client cert source doesn't exist.
+    has_default_client_cert_source.return_value = False
+    with pytest.raises(exceptions.MutualTLSChannelError):
+        mtls.default_client_encrypted_cert_source("cert_path", "key_path")
+
+    # The following tests will assume default client cert source exists.
+    has_default_client_cert_source.return_value = True
+
+    # Test good callback.
+    get_client_ssl_credentials.return_value = (True, b"cert", b"key", b"passphrase")
+    callback = mtls.default_client_encrypted_cert_source("cert_path", "key_path")
+    with mock.patch("{}.open".format(__name__), return_value=mock.MagicMock()):
+        assert callback() == ("cert_path", "key_path", b"passphrase")
+
+    # Test bad callback which throws exception.
+    get_client_ssl_credentials.side_effect = exceptions.ClientCertError()
+    callback = mtls.default_client_encrypted_cert_source("cert_path", "key_path")
+    with pytest.raises(exceptions.MutualTLSChannelError):
+        callback()