Add support for disabling an existing entry_point by setting it to null

README.md

@@ -463,6 +463,53 @@ This follows the general rule defined in [duplicate definitions](#duplicate-defi
 Entry_point names should start with `hab.` and use `.` between each following word
 following the group specification on https://packaging.python.org/en/latest/specifications/entry-points/#data-model.
 
+##### Overriding Entry Points
+
+When using multiple site config files you may end up needing to disable a entry point
+in specific conditions. For example you want to enable
+[hab-gui's cli integration](https://github.com/blurstudio/hab-gui) by default,
+but need to disable it for linux farm nodes that have no gui enabled. If you set
+a entry point's value to `null` hab will ignore it and not attempt to load it.
+
+For example if you have your `HAB_PATH` set to `c:\hab\host.json;\\server\share\studio.json`.
+- The `host.json` site file is stored on each workstation and adds distros that
+are not able to be run from over the network.
+- The `studio.json` site file that is shared on the network for ease of deployment.
+It is used by all hab users on all platforms to define most of the studio defaults
+including adding the hab-gui cli as well as the URI configs.
+
+```json5
+// studio.json
+{
+   "append": {
+      "entry_points": {
+         "cli": {
+            "gui": "hab_gui.cli:gui"
+         }
+      }
+   }
+}
+```
+
+For workstations that don't support a gui you modify the workstations `host.json`
+site file, adding a override of the value from `"hab_gui.cli:gui"` to `null`.
+```json5
+// host.json
+{
+   "append": {
+      "entry_points": {
+         "hab.cli": {
+            "gui": null
+         }
+      }
+   }
+}
+```
+Alternatively, you could create a second host site file named `c:\hab\host_no_gui.json`
+put the gui disabling config in that file and on the host's you want to disable
+the gui prepend to `HAB_PATHS=c:\hab\host_no_gui.json;c:\hab\host.json;\\server\share\studio.json`.
+
+
 ### Python version
 
 Hab uses shell script files instead of an entry_point executable. This allows it

hab/site.py

@@ -86,7 +86,9 @@ def dump(self, verbosity=0, color=None):
         ret = "\n".join(ret)
         return utils.dump_title("Dump of Site", f"{site_ret}\n{ret}", color=color)
 
-    def entry_points_for_group(self, group, default=None, entry_points=None):
+    def entry_points_for_group(
+        self, group, default=None, entry_points=None, omit_none=True
+    ):
         """Returns a list of importlib_metadata.EntryPoint objects enabled by
         this site config. To import and resolve the defined object call `ep.load()`.
 
@@ -98,6 +100,9 @@ def entry_points_for_group(self, group, default=None, entry_points=None):
                 example: `{"gui": "hab_gui.cli:gui"}`
             entry_points (dict, optional): Use this dictionary instead of the one
                 defined on this Site object.
+            omit_none (bool, optional): If an entry_point's value is set to null/None
+                then don't include an EntryPoint object for it in the return. This
+                allows a second site file to disable a entry_point already set.
         """
         # Delay this import to when required. It's faster than pkg_resources but
         # no need to pay the import price for it if you are not using it.
@@ -119,6 +124,9 @@ def entry_points_for_group(self, group, default=None, entry_points=None):
         # using it's `entry_points` function. We want the current site configuration
         # to define the entry points being loaded not the installed pip packages.
         for name, value in ep_defs.items():
+            if omit_none and value is None:
+                continue
+
             ep = EntryPoint(name=name, group=group, value=value)
             ret.append(ep)
         return ret

tests/site/site_entry_point_c.json

@@ -0,0 +1,9 @@
+{
+    "append": {
+        "entry_points": {
+            "hab.cli": {
+                "test-gui": null
+            }
+        }
+    }
+}

tests/test_site.py

@@ -482,32 +482,68 @@ def test_default(self, config_root):
         assert ep.value == "case:func"
 
     @pytest.mark.parametrize(
-        "site_files,import_name,fname",
+        # Note: The default for omit_none is True
+        "site_files,import_name,fname,omit_none",
         (
-            (["site/site_entry_point_a.json"], "hab_test_entry_points", "gui"),
+            (["site/site_entry_point_a.json"], "hab_test_entry_points", "gui", True),
             (
                 ["site/site_entry_point_b.json", "site/site_entry_point_a.json"],
                 "hab_test_entry_points",
                 "gui_alt",
+                True,
             ),
             (
                 ["site/site_entry_point_a.json", "site/site_entry_point_b.json"],
                 "hab_test_entry_points",
                 "gui",
+                True,
+            ),
+            # Tests handling an entry_point value of None
+            (
+                # None value is ignored due to order
+                ["site/site_entry_point_a.json", "site/site_entry_point_c.json"],
+                "hab_test_entry_points",
+                "gui",
+                True,
+            ),
+            (
+                # None value is used, but ignored due to omit_none setting
+                ["site/site_entry_point_c.json", "site/site_entry_point_a.json"],
+                None,
+                None,
+                True,
+            ),
+            (
+                # None value is used, but still returned due to omit_none setting
+                ["site/site_entry_point_c.json", "site/site_entry_point_a.json"],
+                None,
+                None,
+                False,
             ),
         ),
     )
-    def test_site_cli(self, config_root, site_files, import_name, fname):
+    def test_site_cli(self, config_root, site_files, import_name, fname, omit_none):
         """Test a site defining an entry point for `hab.cli`, possibly multiple times."""
         site = Site([config_root / f for f in site_files])
-        entry_points = site.entry_points_for_group("hab.cli")
+        entry_points = site.entry_points_for_group("hab.cli", omit_none=omit_none)
+
+        if import_name is None and omit_none is True:
+            assert len(entry_points) == 0
+            # Nothing else to test if the value is null
+            return
+
         assert len(entry_points) == 1
 
         # Test that the `test-gui` `hab.cli` entry point is handled correctly
         ep = entry_points[0]
         assert ep.name == "test-gui"
         assert ep.group == "hab.cli"
-        assert ep.value == f"{import_name}:{fname}"
+        if omit_none is False:
+            assert ep.value is None
+            # Noting else to test, we can't load a value of None.
+            return
+        else:
+            assert ep.value == f"{import_name}:{fname}"
 
         # Load the module's function
         funct = ep.load()