syscall: respect permission bits on file opening on Windows

On Windows, os.Chmod and syscall.Chmod toggle the FILE_ATTRIBUTES_
READONLY flag depending on the permission bits. That's a bit odd but I
guess some compromises were made at some point and this is what was
chosen to map to a Unix concept that Windows doesn't really have in the
same way. That's fine. However, the logic used in Chmod was forgotten
from os.Open and syscall.Open, which then manifested itself in various
places, most recently, go modules' read-only behavior.

This makes syscall.Open consistent with syscall.Chmod and adds a test
for the permission _behavior_ using ioutil. By testing the behavior
instead of explicitly testing for the attribute bits we care about, we
make sure this doesn't regress in unforeseen ways in the future, as well
as ensuring the test works on platforms other than Windows.

In the process, we fix some tests that never worked and relied on broken
behavior, as well as tests that were disabled on Windows due to the
broken behavior and had TODO notes.

Fixes #35033

Change-Id: I6f7cf54517cbe5f6b1678d1c24f2ab337edcc7f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/202439
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cover/cover_test.go

@@ -457,7 +457,7 @@ func TestHtmlUnformatted(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if err := ioutil.WriteFile(filepath.Join(htmlUDir, "go.mod"), []byte("module htmlunformatted\n"), 0444); err != nil {
+	if err := ioutil.WriteFile(filepath.Join(htmlUDir, "go.mod"), []byte("module htmlunformatted\n"), 0666); err != nil {
 		t.Fatal(err)
 	}
 
@@ -540,7 +540,7 @@ func TestFuncWithDuplicateLines(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if err := ioutil.WriteFile(filepath.Join(lineDupDir, "go.mod"), []byte("module linedup\n"), 0444); err != nil {
+	if err := ioutil.WriteFile(filepath.Join(lineDupDir, "go.mod"), []byte("module linedup\n"), 0666); err != nil {
 		t.Fatal(err)
 	}
 	if err := ioutil.WriteFile(lineDupGo, []byte(lineDupContents), 0444); err != nil {

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

@@ -13,9 +13,7 @@ cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 
 # However, files within those directories should still be read-only to avoid
 # accidental mutations.
-# TODO: Today, this does not seem to be effective on Windows.
-# (https://golang.org/issue/35033)
-[!windows] [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
 
 # If all 'go' commands ran with the flag, the system's 'rm' binary
 # should be able to remove the module cache if the '-rf' flags are set.
@@ -27,8 +25,11 @@ cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 
 # The directories in the module cache should by default be unwritable,
 # so that tests and tools will not accidentally add extraneous files to them.
+# Windows does not respect FILE_ATTRIBUTE_READONLY on directories, according
+# to MSDN, so there we disable testing whether the directory itself is
+# unwritable.
 go get -d rsc.io/quote@latest
-[!windows] [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
 [!windows] [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 ! exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
 

src/io/ioutil/ioutil_test.go

@@ -5,7 +5,9 @@
 package ioutil
 
 import (
+	"bytes"
 	"os"
+	"path/filepath"
 	"testing"
 )
 
@@ -63,6 +65,35 @@ func TestWriteFile(t *testing.T) {
 	os.Remove(filename) // ignore error
 }
 
+func TestReadOnlyWriteFile(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skipf("Root can write to read-only files anyway, so skip the read-only test.")
+	}
+
+	// We don't want to use TempFile directly, since that opens a file for us as 0600.
+	tempDir, err := TempDir("", t.Name())
+	defer os.RemoveAll(tempDir)
+	filename := filepath.Join(tempDir, "blurp.txt")
+
+	shmorp := []byte("shmorp")
+	florp := []byte("florp")
+	err = WriteFile(filename, shmorp, 0444)
+	if err != nil {
+		t.Fatalf("WriteFile %s: %v", filename, err)
+	}
+	err = WriteFile(filename, florp, 0444)
+	if err == nil {
+		t.Fatalf("Expected an error when writing to read-only file %s", filename)
+	}
+	got, err := ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+	if !bytes.Equal(got, shmorp) {
+		t.Fatalf("want %s, got %s", shmorp, got)
+	}
+}
+
 func TestReadDir(t *testing.T) {
 	dirname := "rumpelstilzchen"
 	_, err := ReadDir(dirname)

src/syscall/syscall_windows.go

@@ -312,7 +312,11 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 	default:
 		createmode = OPEN_EXISTING
 	}
-	h, e := CreateFile(pathp, access, sharemode, sa, createmode, FILE_ATTRIBUTE_NORMAL, 0)
+	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
+	if perm&S_IWRITE == 0 {
+		attrs = FILE_ATTRIBUTE_READONLY
+	}
+	h, e := CreateFile(pathp, access, sharemode, sa, createmode, attrs, 0)
 	return h, e
 }