fix: unknown 'patch' names should produce warnings

Author: nedbat

coverage/patch.py

@@ -13,15 +13,19 @@
     from coverage import Coverage
     from coverage.config import CoverageConfig
 
+_old_os_exit = os._exit
+
 def apply_patches(cov: Coverage, config: CoverageConfig) -> None:
     """Apply invasive patches requested by `[run] patch=`."""
 
-    if "os._exit" in config.patch:
-        _old_os_exit = os._exit
-        def _coverage_os_exit_patch(status: int) -> NoReturn:
-            try:
-                cov.save()
-            except: # pylint: disable=bare-except
-                pass
-            _old_os_exit(status)
-        os._exit = _coverage_os_exit_patch
+    for patch in set(config.patch):
+        if patch == "os._exit":
+            def _coverage_os_exit_patch(status: int) -> NoReturn:
+                try:
+                    cov.save()
+                except: # pylint: disable=bare-except
+                    pass
+                _old_os_exit(status)
+            os._exit = _coverage_os_exit_patch
+        else:
+            cov._warn(f"Unknown patch {patch!r}, ignored", slug="unknown-patch")

doc/cmd.rst

@@ -294,6 +294,9 @@ sys.monitoring doesn't yet support dynamic contexts, using default core (no-sysm
   This isn't supported by coverage.py yet.  A default core will be used
   instead.
 
+Unknown patch 'XYZ', ignored (unknown-patch)
+  You specified an incorrect patch name in the :ref:`config_run_patch` setting.
+
 Individual warnings can be disabled with the :ref:`disable_warnings
 <config_run_disable_warnings>` configuration setting.  It is a list of the
 short parenthetical nicknames in the warning messages.  For example, to silence

tests/test_config.py

@@ -19,7 +19,7 @@
 from coverage.types import FilePathClasses, FilePathType
 
 from tests.coveragetest import CoverageTest, UsingModulesMixin
-
+from tests.helpers import assert_coverage_warnings
 
 class ConfigTest(CoverageTest):
     """Tests of the different sources of configuration settings."""
@@ -469,6 +469,19 @@ def test_unknown_option_toml(self) -> None:
         with pytest.warns(CoverageWarning, match=msg):
             _ = coverage.Coverage()
 
+    def test_unknown_patch(self) -> None:
+        self.make_file("foo.py", "a = 1")
+        self.make_file(".coveragerc", """\
+            [run]
+            patch =
+                os._exit
+                xyzzy
+            """)
+        with pytest.warns(CoverageWarning) as warns:
+            cov = coverage.Coverage()
+            self.start_import_stop(cov, "foo")
+        assert_coverage_warnings(warns, "Unknown patch 'xyzzy', ignored (unknown-patch)")
+
     def test_misplaced_option(self) -> None:
         self.make_file(".coveragerc", """\
             [report]