cmd/go: allow arguments to 'go test' and 'go vet' to duplicate or override flags from GOFLAGS

This is a minimal fix for Go 1.14, but this parsing logic is much too
complex and seems like it will cause more trouble going forward.

I intend to mail a followup change to refactor this logic for 1.15.

Updates #32471

Change-Id: I00ed07dcf3a23c9cd4ffa8cf764921fb5c18bcd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/210940
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -88,7 +88,8 @@ func init() {
 //	go test fmt -custom-flag-for-fmt-test
 //	go test -x math
 func testFlags(usage func(), args []string) (packageNames, passToTest []string) {
-	args = str.StringList(cmdflag.FindGOFLAGS(testFlagDefn), args)
+	goflags := cmdflag.FindGOFLAGS(testFlagDefn)
+	args = str.StringList(goflags, args)
 	inPkg := false
 	var explicitArgs []string
 	for i := 0; i < len(args); i++ {
@@ -127,6 +128,9 @@ func testFlags(usage func(), args []string) (packageNames, passToTest []string)
 			passToTest = append(passToTest, args[i])
 			continue
 		}
+		if i < len(goflags) {
+			f.Present = false // Not actually present on the command line.
+		}
 		if f.Value != nil {
 			if err := f.Value.Set(value); err != nil {
 				base.Fatalf("invalid flag argument for -%s: %v", f.Name, err)

src/cmd/go/internal/vet/vetflag.go

@@ -126,7 +126,8 @@ func vetFlags(usage func(), args []string) (passToVet, packageNames []string) {
 	})
 
 	// Process args.
-	args = str.StringList(cmdflag.FindGOFLAGS(vetFlagDefn), args)
+	goflags := cmdflag.FindGOFLAGS(vetFlagDefn)
+	args = str.StringList(goflags, args)
 	for i := 0; i < len(args); i++ {
 		if !strings.HasPrefix(args[i], "-") {
 			return args[:i], args[i:]
@@ -139,6 +140,9 @@ func vetFlags(usage func(), args []string) (passToVet, packageNames []string) {
 			base.SetExitStatus(2)
 			base.Exit()
 		}
+		if i < len(goflags) {
+			f.Present = false // Not actually present on the command line.
+		}
 		if f.Value != nil {
 			if err := f.Value.Set(value); err != nil {
 				base.Fatalf("invalid flag argument for -%s: %v", f.Name, err)

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

@@ -49,3 +49,11 @@ stderr '^go: invalid boolean value \"asdf\" for flag -e \(from (\$GOFLAGS|%GOFLA
 go env
 stdout GOFLAGS
 
+# Flags listed in GOFLAGS should be safe to duplicate on the command line.
+env GOFLAGS=-tags=magic
+go list -tags=magic
+go test -tags=magic -c -o $devnull
+go vet -tags=magic
+
+-- foo_test.go --
+package foo