fix: allow signed url version v4 without signed credentials (googleap…

…is#356)

* Fix: signed_url_v4 to accept credentials without private key. Preserve checks for efficiency in case of neither access_token nor service_account_email are provided.
Fix: tests v4 with token to take into account not Signing credential class.

* fix typo: 2 new lines before new test class

* fix doc: Improve docstring to explain the use of the access_token AND the service_account_email, or the signer email.

* fix: test coverage with the new IF branch

* lint

Co-authored-by: Tres Seaver <tseaver@palladion.com>
Co-authored-by: Frank Natividad <frankyn@users.noreply.github.com>
Co-authored-by: Frank Natividad <franknatividad@google.com>

google/cloud/storage/_signing.py

@@ -457,9 +457,12 @@ def generate_signed_url_v4(
                    google-cloud-python/issues/922
     .. _reference: https://cloud.google.com/storage/docs/reference-headers
 
+
     :type credentials: :class:`google.auth.credentials.Signing`
     :param credentials: Credentials object with an associated private key to
-                        sign text.
+                        sign text. That credentials must provide signer_email
+                        only if service_account_email and access_token are not
+                        passed.
 
     :type resource: str
     :param resource: A pointer to a specific resource
@@ -533,7 +536,6 @@ def generate_signed_url_v4(
     :returns: A signed URL you can use to access the resource
               until expiration.
     """
-    ensure_signed_credentials(credentials)
     expiration_seconds = get_expiration_seconds_v4(expiration)
 
     if _request_timestamp is None:
@@ -542,7 +544,11 @@ def generate_signed_url_v4(
         request_timestamp = _request_timestamp
         datestamp = _request_timestamp[:8]
 
-    client_email = credentials.signer_email
+    client_email = service_account_email
+    if not access_token or not service_account_email:
+        ensure_signed_credentials(credentials)
+        client_email = credentials.signer_email
+
     credential_scope = "{}/auto/storage/goog4_request".format(datestamp)
     credential = "{}/{}".format(client_email, credential_scope)
 

tests/unit/test__signing.py

@@ -653,7 +653,22 @@ def test_w_custom_query_parameters_w_string_value(self):
     def test_w_custom_query_parameters_w_none_value(self):
         self._generate_helper(query_parameters={"qux": None})
 
-    def test_with_access_token(self):
+    def test_with_access_token_and_service_account_email(self):
+        resource = "/name/path"
+        credentials = _make_credentials()
+        email = mock.sentinel.service_account_email
+        with mock.patch(
+            "google.cloud.storage._signing._sign_message", return_value=b"DEADBEEF"
+        ):
+            self._call_fut(
+                credentials,
+                resource=resource,
+                expiration=datetime.timedelta(days=5),
+                service_account_email=email,
+                access_token="token",
+            )
+
+    def test_with_access_token_and_service_account_email_and_signer_email(self):
         resource = "/name/path"
         signer_email = "service@example.com"
         credentials = _make_credentials(signer_email=signer_email)
@@ -668,6 +683,39 @@ def test_with_access_token(self):
                 access_token="token",
             )
 
+    def test_with_signer_email(self):
+        resource = "/name/path"
+        signer_email = "service@example.com"
+        credentials = _make_credentials(signer_email=signer_email)
+        credentials.sign_bytes.return_value = b"DEADBEEF"
+        self._call_fut(
+            credentials, resource=resource, expiration=datetime.timedelta(days=5),
+        )
+
+    def test_with_service_account_email_and_signer_email(self):
+        resource = "/name/path"
+        signer_email = "service@example.com"
+        credentials = _make_credentials(signer_email=signer_email)
+        credentials.sign_bytes.return_value = b"DEADBEEF"
+        self._call_fut(
+            credentials,
+            resource=resource,
+            expiration=datetime.timedelta(days=5),
+            service_account_email=signer_email,
+        )
+
+    def test_with_token_and_signer_email(self):
+        resource = "/name/path"
+        signer_email = "service@example.com"
+        credentials = _make_credentials(signer_email=signer_email)
+        credentials.sign_bytes.return_value = b"DEADBEEF"
+        self._call_fut(
+            credentials,
+            resource=resource,
+            expiration=datetime.timedelta(days=5),
+            access_token="token",
+        )
+
 
 class Test_sign_message(unittest.TestCase):
     @staticmethod