Only set py_runtime.coverage_tool for Bazel 6 and higher. (#1061)

Only set `py_runtime.coverage_tool` for Bazel 6 and higher.

Avoid setting it in earlier version by checking `native.bazel_version`
in the repository rule and disabling it if less than Bazel 6 is
detected. A warning is also printed if coverage was requested, but
the Bazel version check is ignoring it.

Fixes #1056

python/repositories.bzl

@@ -243,6 +243,21 @@ def _python_repository_impl(rctx):
             "share/**",
         ]
 
+    if rctx.attr.coverage_tool:
+        if "windows" in rctx.os.name:
+            coverage_tool = None
+        else:
+            coverage_tool = '"{}"'.format(rctx.attr.coverage_tool)
+
+        coverage_attr_text = """\
+    coverage_tool = select({{
+        ":coverage_enabled": {coverage_tool},
+        "//conditions:default": None
+    }}),
+""".format(coverage_tool = coverage_tool)
+    else:
+        coverage_attr_text = "    # coverage_tool attribute not supported by this Bazel version"
+
     build_content = """\
 # Generated by python/repositories.bzl
 
@@ -308,10 +323,7 @@ config_setting(
 py_runtime(
     name = "py3_runtime",
     files = [":files"],
-    coverage_tool = select({{
-        ":coverage_enabled": {coverage_tool},
-        "//conditions:default": None,
-    }}),
+{coverage_attr}
     interpreter = "{python_path}",
     python_version = "PY3",
 )
@@ -327,7 +339,7 @@ py_runtime_pair(
         python_path = python_bin,
         python_version = python_short_version,
         python_version_nodot = python_short_version.replace(".", ""),
-        coverage_tool = rctx.attr.coverage_tool if rctx.attr.coverage_tool == None or "windows" in rctx.os.name else "\"{}\"".format(rctx.attr.coverage_tool),
+        coverage_attr = coverage_attr_text,
     )
     rctx.delete("python")
     rctx.symlink(python_bin, "python")
@@ -459,6 +471,8 @@ def python_register_toolchains(
         distutils_content: see the distutils_content attribute in the python_repository repository rule.
         register_toolchains: Whether or not to register the downloaded toolchains.
         register_coverage_tool: Whether or not to register the downloaded coverage tool to the toolchains.
+            NOTE: Coverage support using the toolchain is only supported in Bazel 6 and higher.
+
         set_python_version_constraint: When set to true, target_compatible_with for the toolchains will include a version constraint.
         tool_versions: a dict containing a mapping of version with SHASUM and platform info. If not supplied, the defaults
             in python/versions.bzl will be used.
@@ -472,6 +486,19 @@ def python_register_toolchains(
 
     toolchain_repo_name = "{name}_toolchains".format(name = name)
 
+    bazel_major = int(native.bazel_version.split(".")[0])
+    if bazel_major < 6:
+        if register_coverage_tool:
+            # buildifier: disable=print
+            print((
+                "WARNING: ignoring register_coverage_tool=True when " +
+                "registering @{name}: Bazel 6+ required, got {version}"
+            ).format(
+                name = name,
+                version = native.bazel_version,
+            ))
+        register_coverage_tool = False
+
     for platform in PLATFORMS.keys():
         sha256 = tool_versions[python_version]["sha256"].get(platform, None)
         if not sha256: