lib/osutil: Fix "atomic" rename on Windows (ref #6495, ref #6493) (#6510)

So, in a funny plot twist, it turns out that WriteFile in Go 1.13
doesn't actually set the read only bit on Windows when called with
permissions 0444 so my test was broken. With an improved test it turns
out that Rename does not, in fact, overwrite a read-only file on
Windows. This adds a fix for that.

(Rename might get improved in Go 1.15...)

lib/osutil/atomic_test.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"testing"
 )
 
@@ -52,27 +53,45 @@ func TestCreateAtomicCreate(t *testing.T) {
 }
 
 func TestCreateAtomicReplace(t *testing.T) {
-	testCreateAtomicReplace(t, 0644)
+	testCreateAtomicReplace(t, 0666)
 }
 func TestCreateAtomicReplaceReadOnly(t *testing.T) {
-	testCreateAtomicReplace(t, 0400)
+	testCreateAtomicReplace(t, 0444) // windows compatible read-only bits
 }
 
 func testCreateAtomicReplace(t *testing.T, oldPerms os.FileMode) {
 	t.Helper()
 
-	os.RemoveAll("testdata")
-	defer os.RemoveAll("testdata")
+	testdir, err := ioutil.TempDir("", "syncthing")
+	if err != nil {
+		t.Fatal(err)
+	}
+	testfile := filepath.Join(testdir, "testfile")
 
-	if err := os.Mkdir("testdata", 0755); err != nil {
+	os.RemoveAll(testdir)
+	defer os.RemoveAll(testdir)
+
+	if err := os.Mkdir(testdir, 0755); err != nil {
 		t.Fatal(err)
 	}
 
-	if err := ioutil.WriteFile("testdata/file", []byte("some old data"), oldPerms); err != nil {
+	if err := ioutil.WriteFile(testfile, []byte("some old data"), oldPerms); err != nil {
 		t.Fatal(err)
 	}
 
-	w, err := CreateAtomic("testdata/file")
+	// Go < 1.14 has a bug in WriteFile where it does not use the requested
+	// permissions on Windows. Chmod to make sure.
+	if err := os.Chmod(testfile, oldPerms); err != nil {
+		t.Fatal(err)
+	}
+	// Trust, but verify.
+	if info, err := os.Stat(testfile); err != nil {
+		t.Fatal(err)
+	} else if info.Mode() != oldPerms {
+		t.Fatalf("Wrong perms 0%o", info.Mode())
+	}
+
+	w, err := CreateAtomic(testfile)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -84,7 +103,7 @@ func testCreateAtomicReplace(t *testing.T, oldPerms os.FileMode) {
 		t.Fatal(err)
 	}
 
-	bs, err := ioutil.ReadFile("testdata/file")
+	bs, err := ioutil.ReadFile(testfile)
 	if err != nil {
 		t.Fatal(err)
 	}