fix(coverage): add test attributes in transition module (#1649)

This PR is to fix an issue that coverage report is empty when using
transition module.
This is due to the absence of the `_lcov_merger` and
`_collect_cc_coverage` attributes.
Coverage reports will be created adding these attributes.

Fixes #1600

Author: tanan

CHANGELOG.md

@@ -55,6 +55,9 @@ A brief description of the categories of changes:
   instead with a location to the patch that could be used to silence the warning.
   Copy the patch to your workspace and add it to the list if patches for the wheel
   file if you decide to do so.
+* (coverage): coverage reports are now created when the version-aware
+  rules are used.
+  ([#1600](https://github.com/bazelbuild/rules_python/issues/1600))
 
 ### Added
 

python/config_settings/transition.bzl

@@ -142,16 +142,33 @@ _COMMON_ATTRS = {
     ),
 }
 
+_PY_TEST_ATTRS = {
+    # Magic attribute to help C++ coverage work. There's no
+    # docs about this; see TestActionBuilder.java
+    "_collect_cc_coverage": attr.label(
+        default = "@bazel_tools//tools/test:collect_cc_coverage",
+        executable = True,
+        cfg = "exec",
+    ),
+    # Magic attribute to make coverage work. There's no
+    # docs about this; see TestActionBuilder.java
+    "_lcov_merger": attr.label(
+        default = configuration_field(fragment = "coverage", name = "output_generator"),
+        executable = True,
+        cfg = "exec",
+    ),
+}
+
 _transition_py_binary = rule(
     _transition_py_impl,
-    attrs = _COMMON_ATTRS,
+    attrs = _COMMON_ATTRS | _PY_TEST_ATTRS,
     cfg = _transition_python_version,
     executable = True,
 )
 
 _transition_py_test = rule(
     _transition_py_impl,
-    attrs = _COMMON_ATTRS,
+    attrs = _COMMON_ATTRS | _PY_TEST_ATTRS,
     cfg = _transition_python_version,
     test = True,
 )

python/private/common/py_binary_rule_bazel.bzl

@@ -22,11 +22,15 @@ load(
 )
 
 _PY_TEST_ATTRS = {
+    # Magic attribute to help C++ coverage work. There's no
+    # docs about this; see TestActionBuilder.java
     "_collect_cc_coverage": attr.label(
         default = "@bazel_tools//tools/test:collect_cc_coverage",
         executable = True,
         cfg = "exec",
     ),
+    # Magic attribute to make coverage work. There's no
+    # docs about this; see TestActionBuilder.java
     "_lcov_merger": attr.label(
         default = configuration_field(fragment = "coverage", name = "output_generator"),
         executable = True,