[Internal] Add unit tests for external-browser authentication (#863)

## What changes are proposed in this pull request?

This PR is a noop that adds unit tests for the "external-browser"
credential strategy.

## How is this tested?

This PR adds the unit test. Though, these are a little brittle and
likely deserve using a stronger typing model in the future.

Author: renaudhartert-db

databricks/sdk/credentials_provider.py

@@ -188,19 +188,19 @@ def token() -> Token:
 def external_browser(cfg: 'Config') -> Optional[CredentialsProvider]:
     if cfg.auth_type != 'external-browser':
         return None
+
     client_id, client_secret = None, None
     if cfg.client_id:
         client_id = cfg.client_id
         client_secret = cfg.client_secret
     elif cfg.azure_client_id:
         client_id = cfg.azure_client
         client_secret = cfg.azure_client_secret
-
     if not client_id:
         client_id = 'databricks-cli'
 
-    # Load cached credentials from disk if they exist.
-    # Note that these are local to the Python SDK and not reused by other SDKs.
+    # Load cached credentials from disk if they exist. Note that these are
+    # local to the Python SDK and not reused by other SDKs.
     oidc_endpoints = cfg.oidc_endpoints
     redirect_url = 'http://localhost:8020'
     token_cache = TokenCache(host=cfg.host,
@@ -210,12 +210,13 @@ def external_browser(cfg: 'Config') -> Optional[CredentialsProvider]:
                              redirect_url=redirect_url)
     credentials = token_cache.load()
     if credentials:
-        # Force a refresh in case the loaded credentials are expired.
-        # If the refresh fails, rather than throw exception we will initiate a new OAuth login flow.
         try:
+            # Pro-actively refresh the loaded credentials. This is done
+            # to detect if the token is expired and needs to be refreshed
+            # by going through the OAuth login flow.
             credentials.token()
             return credentials(cfg)
-        # TODO: we should ideally use more specific exceptions.
+        # TODO: We should ideally use more specific exceptions.
         except Exception as e:
             logger.warning(f'Failed to refresh cached token: {e}. Initiating new OAuth login flow')
 
@@ -226,6 +227,7 @@ def external_browser(cfg: 'Config') -> Optional[CredentialsProvider]:
     consent = oauth_client.initiate_consent()
     if not consent:
         return None
+
     credentials = consent.launch_external_browser()
     token_cache.save(credentials)
     return credentials(cfg)

tests/test_credentials_provider.py

@@ -0,0 +1,145 @@
+from unittest.mock import Mock
+
+from databricks.sdk.credentials_provider import external_browser
+
+
+def test_external_browser_refresh_success(mocker):
+    """Tests successful refresh of existing credentials."""
+
+    # Mock Config.
+    mock_cfg = Mock()
+    mock_cfg.auth_type = 'external-browser'
+    mock_cfg.host = 'test-host'
+    mock_cfg.oidc_endpoints = {'token_endpoint': 'test-token-endpoint'}
+    mock_cfg.client_id = 'test-client-id' # Or use azure_client_id
+    mock_cfg.client_secret = 'test-client-secret' # Or use azure_client_secret
+
+    # Mock TokenCache.
+    mock_token_cache = Mock()
+    mock_session_credentials = Mock()
+    mock_session_credentials.token.return_value = "valid_token" # Simulate successful refresh
+    mock_token_cache.load.return_value = mock_session_credentials
+
+    # Mock SessionCredentials.
+    want_credentials_provider = lambda c: "new_credentials"
+    mock_session_credentials.return_value = want_credentials_provider
+
+    # Inject the mock implementations.
+    mocker.patch('databricks.sdk.credentials_provider.TokenCache', return_value=mock_token_cache)
+
+    got_credentials_provider = external_browser(mock_cfg)
+
+    mock_token_cache.load.assert_called_once()
+    mock_session_credentials.token.assert_called_once() # Verify token refresh was attempted
+    assert got_credentials_provider == want_credentials_provider
+
+
+def test_external_browser_refresh_failure_new_oauth_flow(mocker):
+    """Tests failed refresh, triggering a new OAuth flow."""
+
+    # Mock Config.
+    mock_cfg = Mock()
+    mock_cfg.auth_type = 'external-browser'
+    mock_cfg.host = 'test-host'
+    mock_cfg.oidc_endpoints = {'token_endpoint': 'test-token-endpoint'}
+    mock_cfg.client_id = 'test-client-id'
+    mock_cfg.client_secret = 'test-client-secret'
+
+    # Mock TokenCache.
+    mock_token_cache = Mock()
+    mock_session_credentials = Mock()
+    mock_session_credentials.token.side_effect = Exception(
+        "Simulated refresh error") # Simulate a failed refresh
+    mock_token_cache.load.return_value = mock_session_credentials
+
+    # Mock SessionCredentials.
+    want_credentials_provider = lambda c: "new_credentials"
+    mock_session_credentials.return_value = want_credentials_provider
+
+    # Mock OAuthClient.
+    mock_oauth_client = Mock()
+    mock_consent = Mock()
+    mock_consent.launch_external_browser.return_value = mock_session_credentials
+    mock_oauth_client.initiate_consent.return_value = mock_consent
+
+    # Inject the mock implementations.
+    mocker.patch('databricks.sdk.credentials_provider.TokenCache', return_value=mock_token_cache)
+    mocker.patch('databricks.sdk.credentials_provider.OAuthClient', return_value=mock_oauth_client)
+
+    got_credentials_provider = external_browser(mock_cfg)
+
+    mock_token_cache.load.assert_called_once()
+    mock_session_credentials.token.assert_called_once() # Refresh attempt
+    mock_oauth_client.initiate_consent.assert_called_once()
+    mock_consent.launch_external_browser.assert_called_once()
+    mock_token_cache.save.assert_called_once_with(mock_session_credentials)
+    assert got_credentials_provider == want_credentials_provider
+
+
+def test_external_browser_no_cached_credentials(mocker):
+    """Tests the case where there are no cached credentials, initiating a new OAuth flow."""
+
+    # Mock Config.
+    mock_cfg = Mock()
+    mock_cfg.auth_type = 'external-browser'
+    mock_cfg.host = 'test-host'
+    mock_cfg.oidc_endpoints = {'token_endpoint': 'test-token-endpoint'}
+    mock_cfg.client_id = 'test-client-id'
+    mock_cfg.client_secret = 'test-client-secret'
+
+    # Mock TokenCache.
+    mock_token_cache = Mock()
+    mock_token_cache.load.return_value = None # No cached credentials
+
+    # Mock SessionCredentials.
+    mock_session_credentials = Mock()
+    want_credentials_provider = lambda c: "new_credentials"
+    mock_session_credentials.return_value = want_credentials_provider
+
+    # Mock OAuthClient.
+    mock_consent = Mock()
+    mock_consent.launch_external_browser.return_value = mock_session_credentials
+    mock_oauth_client = Mock()
+    mock_oauth_client.initiate_consent.return_value = mock_consent
+
+    # Inject the mock implementations.
+    mocker.patch('databricks.sdk.credentials_provider.TokenCache', return_value=mock_token_cache)
+    mocker.patch('databricks.sdk.credentials_provider.OAuthClient', return_value=mock_oauth_client)
+
+    got_credentials_provider = external_browser(mock_cfg)
+
+    mock_token_cache.load.assert_called_once()
+    mock_oauth_client.initiate_consent.assert_called_once()
+    mock_consent.launch_external_browser.assert_called_once()
+    mock_token_cache.save.assert_called_once_with(mock_session_credentials)
+    assert got_credentials_provider == want_credentials_provider
+
+
+def test_external_browser_consent_fails(mocker):
+    """Tests the case where OAuth consent initiation fails."""
+
+    # Mock Config.
+    mock_cfg = Mock()
+    mock_cfg.auth_type = 'external-browser'
+    mock_cfg.host = 'test-host'
+    mock_cfg.oidc_endpoints = {'token_endpoint': 'test-token-endpoint'}
+    mock_cfg.client_id = 'test-client-id'
+    mock_cfg.client_secret = 'test-client-secret'
+
+    # Mock TokenCache.
+    mock_token_cache = Mock()
+    mock_token_cache.load.return_value = None # No cached credentials
+
+    # Mock OAuthClient.
+    mock_oauth_client = Mock()
+    mock_oauth_client.initiate_consent.return_value = None # Simulate consent failure
+
+    # Inject the mock implementations.
+    mocker.patch('databricks.sdk.credentials_provider.TokenCache', return_value=mock_token_cache)
+    mocker.patch('databricks.sdk.credentials_provider.OAuthClient', return_value=mock_oauth_client)
+
+    got_credentials_provider = external_browser(mock_cfg)
+
+    mock_token_cache.load.assert_called_once()
+    mock_oauth_client.initiate_consent.assert_called_once()
+    assert got_credentials_provider is None