DATASETTE_LOAD_PLUGINS environment variable for loading specific plugins

Closes #2164

* Load only specified plugins for DATASETTE_LOAD_PLUGINS=datasette-one,datasette-two
* Load no plugins if DATASETTE_LOAD_PLUGINS=''
* Automated tests in a Bash script for DATASETTE_LOAD_PLUGINS

.github/workflows/test.yml

@@ -50,3 +50,7 @@ jobs:
       run: |
         # This fails on syntax errors, or a diff was applied
         blacken-docs -l 60 docs/*.rst
+    - name: Test DATASETTE_LOAD_PLUGINS
+      run: |
+        pip install datasette-init datasette-json-html
+        tests/test-datasette-load-plugins.sh

datasette/plugins.py

@@ -1,4 +1,5 @@
 import importlib
+import os
 import pluggy
 import pkg_resources
 import sys
@@ -22,10 +23,30 @@ DEFAULT_PLUGINS = (
 pm = pluggy.PluginManager("datasette")
 pm.add_hookspecs(hookspecs)
 
-if not hasattr(sys, "_called_from_test"):
+DATASETTE_LOAD_PLUGINS = os.environ.get("DATASETTE_LOAD_PLUGINS", None)
+
+if not hasattr(sys, "_called_from_test") and DATASETTE_LOAD_PLUGINS is None:
     # Only load plugins if not running tests
     pm.load_setuptools_entrypoints("datasette")
 
+# Load any plugins specified in DATASETTE_LOAD_PLUGINS")
+if DATASETTE_LOAD_PLUGINS is not None:
+    for package_name in [
+        name for name in DATASETTE_LOAD_PLUGINS.split(",") if name.strip()
+    ]:
+        try:
+            distribution = pkg_resources.get_distribution(package_name)
+            entry_map = distribution.get_entry_map()
+            if "datasette" in entry_map:
+                for plugin_name, entry_point in entry_map["datasette"].items():
+                    mod = entry_point.load()
+                    pm.register(mod, name=entry_point.name)
+                    # Ensure name can be found in plugin_to_distinfo later:
+                    pm._plugin_distinfo.append((mod, distribution))
+        except pkg_resources.DistributionNotFound:
+            sys.stderr.write("Plugin {} could not be found\n".format(package_name))
+
+
 # Load default plugins
 for plugin in DEFAULT_PLUGINS:
     mod = importlib.import_module(plugin)

docs/plugins.rst

@@ -81,6 +81,60 @@ You can use the name of a package on PyPI or any of the other valid arguments to
     datasette publish cloudrun mydb.db \
         --install=https://url-to-my-package.zip
 
+
+.. _plugins_datasette_load_plugins:
+
+Controlling which plugins are loaded
+------------------------------------
+
+Datasette defaults to loading every plugin that is installed in the same virtual environment as Datasette itself.
+
+You can set the ``DATASETTE_LOAD_PLUGINS`` environment variable to a comma-separated list of plugin names to load a controlled subset of plugins instead.
+
+For example, to load just the ``datasette-vega`` and ``datasette-cluster-map`` plugins, set ``DATASETTE_LOAD_PLUGINS`` to ``datasette-vega,datasette-cluster-map``:
+
+.. code-block:: bash
+
+    export DATASETTE_LOAD_PLUGINS='datasette-vega,datasette-cluster-map'
+    datasette mydb.db
+
+Or:
+
+.. code-block:: bash
+
+    DATASETTE_LOAD_PLUGINS='datasette-vega,datasette-cluster-map' \
+      datasette mydb.db
+
+To disable the loading of all additional plugins, set ``DATASETTE_LOAD_PLUGINS`` to an empty string:
+
+.. code-block:: bash
+
+    export DATASETTE_LOAD_PLUGINS=''
+    datasette mydb.db
+
+A quick way to test this setting is to use it with the ``datasette plugins`` command:
+
+.. code-block:: bash
+
+    DATASETTE_LOAD_PLUGINS='datasette-vega' datasette plugins
+
+This should output the following:
+
+.. code-block:: json
+
+    [
+        {
+            "name": "datasette-vega",
+            "static": true,
+            "templates": false,
+            "version": "0.6.2",
+            "hooks": [
+                "extra_css_urls",
+                "extra_js_urls"
+            ]
+        }
+    ]
+
 .. _plugins_installed:
 
 Seeing what plugins are installed

tests/test-datasette-load-plugins.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+# This should only run in environemnts where both
+# datasette-init and datasette-json-html are installed
+
+PLUGINS=$(datasette plugins)
+echo "$PLUGINS" | jq 'any(.[]; .name == "datasette-json-html")' | \
+  grep -q true || ( \
+    echo "Test failed: datasette-json-html not found" && \
+    exit 1 \
+  )
+# With the DATASETTE_LOAD_PLUGINS we should not see that
+PLUGINS2=$(DATASETTE_LOAD_PLUGINS=datasette-init datasette plugins)
+echo "$PLUGINS2" | jq 'any(.[]; .name == "datasette-json-html")' | \
+  grep -q false || ( \
+    echo "Test failed: datasette-json-html should not have been loaded" && \
+    exit 1 \
+  )
+echo "$PLUGINS2" | jq 'any(.[]; .name == "datasette-init")' | \
+  grep -q true || ( \
+    echo "Test failed: datasette-init should have been loaded" && \
+    exit 1 \
+  )
+# With DATASETTE_LOAD_PLUGINS='' we should see no plugins
+PLUGINS3=$(DATASETTE_LOAD_PLUGINS='' datasette plugins)
+echo "$PLUGINS3"| \
+  grep -q '\[\]' || ( \
+    echo "Test failed: datasette plugins should have returned []" && \
+    exit 1 \
+  )