bazel/rules_r: bazel 0.18 compatibility for coverage

Summary:
From bazel 0.18.0, an lcov_merger tool is needed for every test rule
that collects coverage.

See bazelbuild/bazel#6293 for more details.

Reviewers: ysaito

Reviewed By: ysaito

Differential Revision: https://phabricator.grailbio.com/D20750

fbshipit-source-id: f480a5e

R/internal/tests.bzl

@@ -113,6 +113,10 @@ r_unit_test = rule(
             allow_single_file = True,
             default = "@com_grail_rules_r//R/scripts:collect_coverage.R",
         ),
+        "_lcov_merger": attr.label(
+            allow_single_file = True,
+            default = "@com_grail_rules_r//R/scripts:lcov_merger.sh",
+        ),
     },
     doc = ("Rule to keep all deps in the sandbox, and run the test " +
            "scripts of the specified package. The package itself must " +

R/scripts/BUILD

@@ -19,6 +19,7 @@ exports_files([
     "check.sh.tpl",
     "collect_coverage.R",
     "instrument.R",
+    "lcov_merger.sh",
     "library.sh.tpl",
     "test.sh.tpl",
 ])

R/scripts/collect_coverage.R

@@ -15,12 +15,8 @@
 # Collect code coverage
 
 # NOTE:
-# bazel currently always tries to run lcov with gcov to generate lcov trace
-# files, which is not applicable here. The only reason collect_coverage.sh from
-# bazel succeeds is because it can not find gcov defined in the toolchain for
-# the rule and skips the step. This is brittle and will break when bazel
-# changes collect_coverage.sh or if we start specifying cc_deps as a dependency
-# attribute in instrumented_files. More proper support for coverage in skylark
+# This might break in the future if we start specifying cc_deps as a dependency
+# attribute in instrumented_files. More proper support for coverage in starlark
 # rules is in the roadmap --
 # https://bazel.build/roadmaps/coverage.html#improve-adding-coverage-support-for-skylark-rules-p2
 

R/scripts/lcov_merger.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Since bazel 0.18.0, an lcov_merger tool is mandatory for tests that
+# collect coverage. This tool is run after the tests to generate the
+# COVERAGE_OUTPUT_FILE. Since we generate this file from within our
+# test runner directly, the lcov_merger tool is a no-op for us.
+
+exit 0