fileutil: add ZeroToEnd for zeroing files

pkg/fileutil/fileutil.go

@@ -96,3 +96,26 @@ func Exist(name string) bool {
 	_, err := os.Stat(name)
 	return err == nil
 }
+
+// ZeroToEnd zeros a file starting from SEEK_CUR to its SEEK_END. May temporarily
+// shorten the length of the file.
+func ZeroToEnd(f *os.File) error {
+	// TODO: support FALLOC_FL_ZERO_RANGE
+	off, err := f.Seek(0, os.SEEK_CUR)
+	if err != nil {
+		return err
+	}
+	lenf, lerr := f.Seek(0, os.SEEK_END)
+	if lerr != nil {
+		return lerr
+	}
+	if err = f.Truncate(off); err != nil {
+		return err
+	}
+	// make sure blocks remain allocated
+	if err = Preallocate(f, lenf, true); err != nil {
+		return err
+	}
+	_, err = f.Seek(off, os.SEEK_SET)
+	return err
+}

pkg/fileutil/fileutil_test.go

@@ -118,3 +118,42 @@ func TestExist(t *testing.T) {
 		t.Errorf("exist = %v, want false", g)
 	}
 }
+
+func TestZeroToEnd(t *testing.T) {
+	f, err := ioutil.TempFile(os.TempDir(), "fileutil")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	b := make([]byte, 1024)
+	for i := range b {
+		b[i] = 12
+	}
+	if _, err = f.Write(b); err != nil {
+		t.Fatal(err)
+	}
+	if _, err = f.Seek(512, os.SEEK_SET); err != nil {
+		t.Fatal(err)
+	}
+	if err = ZeroToEnd(f); err != nil {
+		t.Fatal(err)
+	}
+	off, serr := f.Seek(0, os.SEEK_CUR)
+	if serr != nil {
+		t.Fatal(serr)
+	}
+	if off != 512 {
+		t.Fatalf("expected offset 512, got %d", off)
+	}
+
+	b = make([]byte, 512)
+	if _, err = f.Read(b); err != nil {
+		t.Fatal(err)
+	}
+	for i := range b {
+		if b[i] != 0 {
+			t.Errorf("expected b[%d] = 0, got %d", i, b[i])
+		}
+	}
+}