Add jwt.OnDemandCredentials (#142)

diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 59769de..22c5bc5 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -22,6 +22,7 @@
 
 from google.auth import _helpers
 from google.auth import crypt
+from google.auth import exceptions
 from google.auth import jwt
 
 
@@ -196,7 +197,7 @@
     assert payload['user'] == 'billy bob'
 
 
-class TestCredentials:
+class TestCredentials(object):
     SERVICE_ACCOUNT_EMAIL = 'service-account@example.com'
     SUBJECT = 'subject'
     AUDIENCE = 'audience'
@@ -343,3 +344,135 @@
         self.credentials.before_request(
             None, 'GET', 'http://example.com?a=1#3', {})
         assert self.credentials.valid
+
+
+class TestOnDemandCredentials(object):
+    SERVICE_ACCOUNT_EMAIL = 'service-account@example.com'
+    SUBJECT = 'subject'
+    ADDITIONAL_CLAIMS = {'meta': 'data'}
+    credentials = None
+
+    @pytest.fixture(autouse=True)
+    def credentials_fixture(self, signer):
+        self.credentials = jwt.OnDemandCredentials(
+            signer, self.SERVICE_ACCOUNT_EMAIL, self.SERVICE_ACCOUNT_EMAIL,
+            max_cache_size=2)
+
+    def test_from_service_account_info(self):
+        with open(SERVICE_ACCOUNT_JSON_FILE, 'r') as fh:
+            info = json.load(fh)
+
+        credentials = jwt.OnDemandCredentials.from_service_account_info(info)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == info['client_email']
+
+    def test_from_service_account_info_args(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.OnDemandCredentials.from_service_account_info(
+            info, subject=self.SUBJECT,
+            additional_claims=self.ADDITIONAL_CLAIMS)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == self.SUBJECT
+        assert credentials._additional_claims == self.ADDITIONAL_CLAIMS
+
+    def test_from_service_account_file(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.OnDemandCredentials.from_service_account_file(
+            SERVICE_ACCOUNT_JSON_FILE)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == info['client_email']
+
+    def test_from_service_account_file_args(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.OnDemandCredentials.from_service_account_file(
+            SERVICE_ACCOUNT_JSON_FILE, subject=self.SUBJECT,
+            additional_claims=self.ADDITIONAL_CLAIMS)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == self.SUBJECT
+        assert credentials._additional_claims == self.ADDITIONAL_CLAIMS
+
+    def test_from_signing_credentials(self):
+        jwt_from_signing = self.credentials.from_signing_credentials(
+            self.credentials)
+        jwt_from_info = jwt.OnDemandCredentials.from_service_account_info(
+            SERVICE_ACCOUNT_INFO)
+
+        assert isinstance(jwt_from_signing, jwt.OnDemandCredentials)
+        assert jwt_from_signing._signer.key_id == jwt_from_info._signer.key_id
+        assert jwt_from_signing._issuer == jwt_from_info._issuer
+        assert jwt_from_signing._subject == jwt_from_info._subject
+
+    def test_default_state(self):
+        # Credentials are *always* valid.
+        assert self.credentials.valid
+        # Credentials *never* expire.
+        assert not self.credentials.expired
+
+    def test_with_claims(self):
+        new_claims = {'meep': 'moop'}
+        new_credentials = self.credentials.with_claims(
+            additional_claims=new_claims)
+
+        assert new_credentials._signer == self.credentials._signer
+        assert new_credentials._issuer == self.credentials._issuer
+        assert new_credentials._subject == self.credentials._subject
+        assert new_credentials._additional_claims == new_claims
+
+    def test_sign_bytes(self):
+        to_sign = b'123'
+        signature = self.credentials.sign_bytes(to_sign)
+        assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
+
+    def test_signer(self):
+        assert isinstance(self.credentials.signer, crypt.RSASigner)
+
+    def test_signer_email(self):
+        assert (self.credentials.signer_email ==
+                SERVICE_ACCOUNT_INFO['client_email'])
+
+    def _verify_token(self, token):
+        payload = jwt.decode(token, PUBLIC_CERT_BYTES)
+        assert payload['iss'] == self.SERVICE_ACCOUNT_EMAIL
+        return payload
+
+    def test_refresh(self):
+        with pytest.raises(exceptions.RefreshError):
+            self.credentials.refresh(None)
+
+    def test_before_request(self):
+        headers = {}
+
+        self.credentials.before_request(
+            None, 'GET', 'http://example.com?a=1#3', headers)
+
+        _, token = headers['authorization'].split(' ')
+        payload = self._verify_token(token)
+
+        assert payload['aud'] == 'http://example.com'
+
+        # Making another request should re-use the same token.
+        self.credentials.before_request(
+            None, 'GET', 'http://example.com?b=2', headers)
+
+        _, new_token = headers['authorization'].split(' ')
+
+        assert new_token == token
+
+    def test_expired_token(self):
+        self.credentials._cache['audience'] = (
+            mock.sentinel.token, datetime.datetime.min)
+
+        token = self.credentials._get_jwt_for_audience('audience')
+
+        assert token != mock.sentinel.token