Move PreallocateFile to fs package

internal/fs/preallocate_darwin.go

@@ -0,0 +1,28 @@
+package fs
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func PreallocateFile(wr *os.File, size int64) error {
+	// try contiguous first
+	fst := unix.Fstore_t{
+		Flags:   unix.F_ALLOCATECONTIG | unix.F_ALLOCATEALL,
+		Posmode: unix.F_PEOFPOSMODE,
+		Offset:  0,
+		Length:  size,
+	}
+	err := unix.FcntlFstore(wr.Fd(), unix.F_PREALLOCATE, &fst)
+
+	if err == nil {
+		return nil
+	}
+
+	// just take preallocation in any form, but still ask for everything
+	fst.Flags = unix.F_ALLOCATEALL
+	err = unix.FcntlFstore(wr.Fd(), unix.F_PREALLOCATE, &fst)
+
+	return err
+}

internal/fs/preallocate_linux.go

@@ -0,0 +1,16 @@
+package fs
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func PreallocateFile(wr *os.File, size int64) error {
+	if size <= 0 {
+		return nil
+	}
+	// int fallocate(int fd, int mode, off_t offset, off_t len)
+	// use mode = 0 to also change the file size
+	return unix.Fallocate(int(wr.Fd()), 0, 0, size)
+}

internal/fs/preallocate_other.go

@@ -0,0 +1,12 @@
+//go:build !linux && !darwin
+// +build !linux,!darwin
+
+package fs
+
+import "os"
+
+func PreallocateFile(wr *os.File, size int64) error {
+	// Maybe truncate can help?
+	// Windows: This calls SetEndOfFile which preallocates space on disk
+	return wr.Truncate(size)
+}

internal/fs/preallocate_test.go

@@ -0,0 +1,38 @@
+package fs
+
+import (
+	"os"
+	"path"
+	"strconv"
+	"syscall"
+	"testing"
+
+	"github.com/restic/restic/internal/test"
+)
+
+func TestPreallocate(t *testing.T) {
+	for _, i := range []int64{0, 1, 4096, 1024 * 1024} {
+		t.Run(strconv.FormatInt(i, 10), func(t *testing.T) {
+			dirpath := test.TempDir(t)
+
+			flags := os.O_CREATE | os.O_TRUNC | os.O_WRONLY
+			wr, err := os.OpenFile(path.Join(dirpath, "test"), flags, 0600)
+			test.OK(t, err)
+			defer func() {
+				test.OK(t, wr.Close())
+			}()
+
+			err = PreallocateFile(wr, i)
+			if err == syscall.ENOTSUP {
+				t.SkipNow()
+			}
+			test.OK(t, err)
+
+			fi, err := wr.Stat()
+			test.OK(t, err)
+
+			efi := ExtendedStat(fi)
+			test.Assert(t, efi.Size == i || efi.Blocks > 0, "Preallocated size of %v, got size %v block %v", i, efi.Size, efi.Blocks)
+		})
+	}
+}