forbid invalid plugin prefixes in plugin loading

src/flake8/plugins/finder.py

@@ -1,7 +1,9 @@
 """Functions related to finding and loading plugins."""
 import configparser
 import inspect
+import itertools
 import logging
+import re
 import sys
 from typing import Any
 from typing import Dict
@@ -20,6 +22,8 @@ from flake8.exceptions import FailedToLoadPlugin
 
 LOG = logging.getLogger(__name__)
 
+VALID_CODE = re.compile("^[A-Z]{1,3}[0-9]{0,3}$", re.ASCII)
+
 FLAKE8_GROUPS = frozenset(("flake8.extension", "flake8.report"))
 
 BANNED_PLUGINS = {
@@ -328,6 +332,13 @@ def _classify_plugins(
         else:
             raise NotImplementedError(f"what plugin type? {loaded}")
 
+    for loaded in itertools.chain(tree, logical_line, physical_line):
+        if not VALID_CODE.match(loaded.entry_name):
+            raise ExecutionError(
+                f"plugin code for `{loaded.display_name}` does not match "
+                f"{VALID_CODE.pattern}"
+            )
+
     return Plugins(
         checkers=Checkers(
             tree=tree,

tests/unit/plugins/finder_test.py

@@ -29,6 +29,37 @@ def _loaded(plugin=None, obj=None, parameters=None):
     return finder.LoadedPlugin(plugin, obj, parameters)
 
 
+@pytest.mark.parametrize(
+    "s",
+    (
+        "E",
+        "E1",
+        "E123",
+        "ABC",
+        "ABC1",
+        "ABC123",
+    ),
+)
+def test_valid_plugin_prefixes(s):
+    assert finder.VALID_CODE.match(s)
+
+
+@pytest.mark.parametrize(
+    "s",
+    (
+        "",
+        "A1234",
+        "ABCD",
+        "abc",
+        "a-b",
+        "☃",
+        "A𝟗",
+    ),
+)
+def test_invalid_plugin_prefixes(s):
+    assert finder.VALID_CODE.match(s) is None
+
+
 def test_loaded_plugin_entry_name_vs_display_name():
     loaded = _loaded(_plugin(package="package-name", ep=_ep(name="Q")))
     assert loaded.entry_name == "Q"
@@ -761,6 +792,35 @@ def test_classify_plugins_enable_a_disabled_plugin():
     )
 
 
+def test_classify_plugins_does_not_error_on_reporter_prefix():
+    # these are ok, don't check their name
+    plugin = _plugin(ep=_ep(name="report-er", group="flake8.report"))
+    loaded = _loaded(plugin=plugin)
+
+    opts = finder.PluginOptions.blank()
+    classified = finder._classify_plugins([loaded], opts)
+
+    assert classified == finder.Plugins(
+        checkers=finder.Checkers([], [], []),
+        reporters={"report-er": loaded},
+        disabled=[],
+    )
+
+
+def test_classify_plugins_errors_on_incorrect_checker_name():
+    plugin = _plugin(ep=_ep(name="INVALID", group="flake8.extension"))
+    loaded = _loaded(plugin=plugin, parameters={"tree": True})
+
+    with pytest.raises(ExecutionError) as excinfo:
+        finder._classify_plugins([loaded], finder.PluginOptions.blank())
+
+    (msg,) = excinfo.value.args
+    assert msg == (
+        "plugin code for `local[INVALID]` "
+        "does not match ^[A-Z]{1,3}[0-9]{0,3}$"
+    )
+
+
 @pytest.mark.usefixtures("reset_sys")
 def test_load_plugins():
     plugin = _plugin(ep=_ep(value="aplugin:ExtensionTestPlugin2"))