Save test coverage data to bazel-testlogs. (#1387)

This lets `bazel coverage //some:test` produce a Go coverage data file
in `bazel-testlogs/some/test/coverage.dat`. This file can be processed
by normal Go coverage tooling to produce a coverage report.

The change to `go/private/rules/test.bzl` needs some explanation --
Bazel only looks for coverage data if the test target has an
`InstrumentedFilesProvider`, but this provider can currently only be
created using "legacy" provider syntax. Old and new provider syntaxes
can be combined by putting new-style providers in a `providers` field
of the old-style struct.

If the provider is found and at least one source file is present, Bazel
will set the `COVERAGE_OUTPUT_FILE` environment variable during tests
and will save that file to the build events + test outputs.

go/private/rules/test.bzl

@@ -136,13 +136,30 @@ def _go_test_impl(ctx):
 
   runfiles = ctx.runfiles(files = [executable])
   runfiles = runfiles.merge(test_archive.runfiles)
-  return [
-      DefaultInfo(
-          files = depset([executable]),
-          runfiles = runfiles,
-          executable = executable,
+
+  # Bazel only looks for coverage data if the test target has an
+  # InstrumentedFilesProvider, but this provider can currently only be
+  # created using "legacy" provider syntax. Old and new provider syntaxes
+  # can be combined by putting new-style providers in a providers field
+  # of the old-style struct.
+  # If the provider is found and at least one source file is present, Bazel
+  # will set the COVERAGE_OUTPUT_FILE environment variable during tests
+  # and will save that file to the build events + test outputs.
+  return struct(
+      providers = [
+          DefaultInfo(
+              files = depset([executable]),
+              runfiles = runfiles,
+              executable = executable,
+          ),
+      ],
+      instrumented_files = struct(
+          extensions = ['go'],
+          source_attributes = ['srcs'],
+          dependency_attributes = ['deps', 'embed'],
       ),
-]
+  )
+
 
 go_test = go_rule(
     _go_test_impl,

go/tools/builders/generate_test_main.go

@@ -187,6 +187,12 @@ func main() {
 		testing.RegisterCover(coverage)
 	}
 
+	if coverageDat, ok := os.LookupEnv("COVERAGE_OUTPUT_FILE"); ok {
+		if testing.CoverMode() != "" {
+			flag.Lookup("test.coverprofile").Value.Set(coverageDat)
+		}
+	}
+
 	m := testing.MainStart(testdeps.TestDeps{}, testsInShard(), benchmarks, examples)
 	{{if not .TestMain}}
 	os.Exit(m.Run())

tests/legacy/coverage/BUILD.bazel

@@ -23,6 +23,10 @@ bazel_test(
 if ! grep -q '^coverage: 50.0% of statements' "bazel-testlogs/$RULES_GO_OUTPUT/go_default_test/test.log"; then
   echo "error: no coverage output found in test log file" >&2
   exit 1
+fi
+if ! grep -q 'io_bazel_rules_go/tests/legacy/coverage/lib.go:' "bazel-testlogs/$RULES_GO_OUTPUT/go_default_test/coverage.dat"; then
+  echo "error: no coverage data found in bazel-testlogs"
+  exit 1
 fi
     """,
     command = "coverage",