cmd/go: improve handling of os.DevNull on Windows

The "go test" and "go build" commands have special-case behavior when
passed "-o /dev/null". These checks are case-sensitive and assume that
os.DevNull is an absolute path. Windows filesystems are case-insensitive
and os.DevNull is NUL, which is not an absolute path.

CL 145220 changed filepath.IsAbs to report "NUL" as absolute to work
around this issue; that change is being rolled back and a better fix here
is to compare the value of -o against os.DevNull before attempting to
merge it with a base path. Make that fix.

On Windows, accept any capitilization of "NUL" as the null device.

This change doesn't cover every possible name for the null device, such
as "-o //./NUL", but this test is for efficiency rather than correctness.
Accepting just the most common name is fine.

For #56217.

Change-Id: I60b59b671789fc456074d3c8bc755a74ea8d5765
Reviewed-on: https://go-review.googlesource.com/c/go/+/449117
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/base/path.go

@@ -7,6 +7,7 @@ package base
 import (
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 )
@@ -54,3 +55,17 @@ func IsTestFile(file string) bool {
 	// We don't cover tests, only the code they test.
 	return strings.HasSuffix(file, "_test.go")
 }
+
+// IsNull reports whether the path is a common name for the null device.
+// It returns true for /dev/null on Unix, or NUL (case-insensitive) on Windows.
+func IsNull(path string) bool {
+	if path == os.DevNull {
+		return true
+	}
+	if runtime.GOOS == "windows" {
+		if strings.EqualFold(path, "NUL") {
+			return true
+		}
+	}
+	return false
+}

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

@@ -1010,13 +1010,16 @@ func builderTest(b *work.Builder, ctx context.Context, pkgOpts load.PackageOpts,
 	if testC || testNeedBinary() {
 		// -c or profiling flag: create action to copy binary to ./test.out.
 		target := filepath.Join(base.Cwd(), testBinary+cfg.ExeSuffix)
+		isNull := false
 		if testO != "" {
 			target = testO
-			if !filepath.IsAbs(target) {
+			if base.IsNull(target) {
+				isNull = true
+			} else if !filepath.IsAbs(target) {
 				target = filepath.Join(base.Cwd(), target)
 			}
 		}
-		if target == os.DevNull {
+		if isNull {
 			runAction = buildAction
 		} else {
 			pmain.Target = target

src/cmd/go/internal/work/build.go

@@ -482,7 +482,7 @@ func runBuild(ctx context.Context, cmd *base.Command, args []string) {
 	pkgs = omitTestOnly(pkgsFilter(pkgs))
 
 	// Special case -o /dev/null by not writing at all.
-	if cfg.BuildO == os.DevNull {
+	if base.IsNull(cfg.BuildO) {
 		cfg.BuildO = ""
 	}