cmd/go: display test binary output if invoked with -help

Fixes #39997

Change-Id: I87ea616bac809b96fcd40f3bbdbbf1c603b9d00e
Reviewed-on: https://go-review.googlesource.com/c/go/+/240878
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -487,6 +487,8 @@ var (
 	pkgArgs  []string
 	pkgs     []*load.Package
 
+	testHelp bool // -help option passed to test via -args
+
 	testKillTimeout = 100 * 365 * 24 * time.Hour // backup alarm; defaults to about a century if no timeout is set
 	testCacheExpire time.Time                    // ignore cached test results before this time
 
@@ -532,7 +534,7 @@ func testNeedBinary() bool {
 
 // testShowPass reports whether the output for a passing test should be shown.
 func testShowPass() bool {
-	return testV || (testList != "")
+	return testV || (testList != "") || testHelp
 }
 
 var defaultVetFlags = []string{

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

@@ -333,6 +333,23 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 		injectedFlags = append(injectedFlags, "-test.outputdir="+testOutputDir)
 	}
 
+	// If the user is explicitly passing -help or -h, show output
+	// of the test binary so that the help output is displayed
+	// even though the test will exit with success.
+	// This loop is imperfect: it will do the wrong thing for a case
+	// like -args -test.outputdir -help. Such cases are probably rare,
+	// and getting this wrong doesn't do too much harm.
+helpLoop:
+	for _, arg := range explicitArgs {
+		switch arg {
+		case "--":
+			break helpLoop
+		case "-h", "-help", "--help":
+			testHelp = true
+			break helpLoop
+		}
+	}
+
 	// Ensure that -race and -covermode are compatible.
 	if testCoverMode == "" {
 		testCoverMode = "set"

src/cmd/go/testdata/script/test_flags.txt

@@ -57,6 +57,10 @@ stderr -count=1 'invalid value "walrus" for flag -covermode: valid modes are .*$
 stderr '^usage: go test .*$'
 stderr '^Run ''go help test'' and ''go help testflag'' for details.$'
 
+# Passing -help to the test binary should show flag help.
+go test ./x -args -help
+stdout 'usage_message'
+
 # -covermode, -coverpkg, and -coverprofile should imply -cover
 go test -covermode=set ./x
 stdout '\s+coverage:\s+'
@@ -98,6 +102,8 @@ import (
 	"testing"
 )
 
+var _ = flag.String("usage_message", "", "dummy flag to check usage message")
+
 func TestLogTimeout(t *testing.T) {
 	t.Log(flag.Lookup("test.timeout").Value)
 }