cmd/go: warn on -race with -covermode=set.

Fixes #20435.

Change-Id: I15576f36b26d01642c1187325baea82d3077e578
Reviewed-on: https://go-review.googlesource.com/43777Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -4120,3 +4120,21 @@ func TestCgoFlagContainsSpace(t *testing.T) {
 	tg.cd(tg.path("src/cgo"))
 	tg.run("run", "main.go")
 }
+
+// Issue #20435.
+func TestGoTestRaceCoverModeFailures(t *testing.T) {
+	if !canRace {
+		t.Skip("skipping because race detector not supported")
+	}
+
+	tg := testgo(t)
+	tg.parallel()
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+
+	tg.run("test", "testrace")
+
+	tg.runFail("test", "-race", "-covermode=set", "testrace")
+	tg.grepStderr(`-covermode must be "atomic", not "set", when -race is enabled`, "-race -covermode=set was allowed")
+	tg.grepBothNot("PASS", "something passed")
+}

src/cmd/go/internal/test/testflag.go

@@ -193,6 +193,10 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 		}
 	}
 
+	if cfg.BuildRace && testCoverMode != "atomic" {
+		base.Fatalf(`-covermode must be "atomic", not %q, when -race is enabled`, testCoverMode)
+	}
+
 	// Tell the test what directory we're running in, so it can write the profiles there.
 	if testProfile && outputDir == "" {
 		dir, err := os.Getwd()