[Core] Allow overriding of identifier

Needed for e2e testing of OpenAssetIO/OpenAssetIO#1202. The upcoming
hybrid plugin system will work by locating multiple plugins that all
advertise the same unique identifier and combining them.

In preparation for e2e testing of this, make BAL's plugin identifier
configurable by an environment variable, so that it can be set to the
same identifier as another plugin on the system.

We cannot use other types of settings such as the JSON database, since
the plugin identifier is queried before the settings are provided.

Signed-off-by: David Feltell <david.feltell@foundry.com>

RELEASE_NOTES.md

@@ -1,6 +1,14 @@
 Release Notes
 =============
 
+v1.0.0-alpha.x
+---------------
+
+### New features
+
+- Added support for `OPENASSETIO_BAL_IDENTIFIER` environment variable,
+  for overriding the identifier advertised by the BAL plugin/manager.
+
 v1.0.0-alpha.16
 ---------------
 

plugin/openassetio_manager_bal/BasicAssetLibraryInterface.py

@@ -53,6 +53,9 @@
     "BasicAssetLibraryInterface",
 ]
 
+
+DEFAULT_IDENTIFIER = "org.openassetio.examples.manager.bal"
+ENV_VAR_IDENTIFIER_OVERRIDE = "OPENASSETIO_BAL_IDENTIFIER"
 SETTINGS_KEY_LIBRARY_PATH = "library_path"
 SETTINGS_KEY_SIMULATED_QUERY_LATENCY = "simulated_query_latency_ms"
 SETTINGS_KEY_ENTITY_REFERENCE_URL_SCHEME = "entity_reference_url_scheme"
@@ -105,7 +108,7 @@ def __init__(self):
         self.__library = {}
 
     def identifier(self):
-        return "org.openassetio.examples.manager.bal"
+        return os.environ.get(ENV_VAR_IDENTIFIER_OVERRIDE, DEFAULT_IDENTIFIER)
 
     def displayName(self):
         # Deliberately includes unicode chars to test string handling

plugin/openassetio_manager_bal/__init__.py

@@ -42,6 +42,7 @@
 need to be on `$PYTHONPATH` directly, the plugin system takes care
 of extending Python's runtime paths accordingly.
 """
+import os
 
 # pylint: disable=import-outside-toplevel, invalid-name
 #
@@ -51,6 +52,8 @@
 # PythonPluginSystemManagerPlugin's implementation.
 from openassetio.pluginSystem import PythonPluginSystemManagerPlugin
 
+from .BasicAssetLibraryInterface import ENV_VAR_IDENTIFIER_OVERRIDE, DEFAULT_IDENTIFIER
+
 
 class BasicAssetLibraryPlugin(PythonPluginSystemManagerPlugin):
     """
@@ -61,7 +64,7 @@ class BasicAssetLibraryPlugin(PythonPluginSystemManagerPlugin):
 
     @staticmethod
     def identifier():
-        return "org.openassetio.examples.manager.bal"
+        return os.getenv(ENV_VAR_IDENTIFIER_OVERRIDE, DEFAULT_IDENTIFIER)
 
     @classmethod
     def interface(cls):

tests/test_bal.py

@@ -74,6 +74,25 @@ def test_exposes_plugin_attribute_with_correct_type(self):
 
         assert issubclass(openassetio_manager_bal.plugin, PythonPluginSystemManagerPlugin)
 
+    def test_when_not_overridden_by_env_var_then_identifier_matches_default(self):
+        import openassetio_manager_bal  # pylint: disable=import-outside-toplevel
+
+        assert (
+            openassetio_manager_bal.plugin.identifier() == "org.openassetio.examples.manager.bal"
+        )
+        assert (
+            openassetio_manager_bal.plugin.interface().identifier()
+            == "org.openassetio.examples.manager.bal"
+        )
+
+    def test_when_overridden_by_env_var_then_identifier_matches_env_var(self, monkeypatch):
+        import openassetio_manager_bal  # pylint: disable=import-outside-toplevel
+
+        monkeypatch.setenv("OPENASSETIO_BAL_IDENTIFIER", "foo")
+
+        assert openassetio_manager_bal.plugin.identifier() == "foo"
+        assert openassetio_manager_bal.plugin.interface().identifier() == "foo"
+
 
 @pytest.fixture
 def bal_business_logic_suite(bal_base_dir):