local: fix preallocate warning on Linux with ZFS

Under Linux, rclone attempts to preallocate files for efficiency.

Before this change, pre-allocation would fail on ZFS with the error

    Failed to pre-allocate: operation not supported

After this change rclone tries a different flag combination for ZFS
then disables pre-allocate if that doesn't work.

Fixes #3066

backend/local/preallocate_unix.go

@@ -4,16 +4,40 @@ package local
 
 import (
 	"os"
+	"sync/atomic"
 
+	"github.com/ncw/rclone/fs"
 	"golang.org/x/sys/unix"
 )
 
+var (
+	fallocFlags = [...]uint32{
+		unix.FALLOC_FL_KEEP_SIZE,                             // Default
+		unix.FALLOC_FL_KEEP_SIZE | unix.FALLOC_FL_PUNCH_HOLE, // for ZFS #3066
+	}
+	fallocFlagsIndex int32
+)
+
 // preAllocate the file for performance reasons
 func preAllocate(size int64, out *os.File) error {
 	if size <= 0 {
 		return nil
 	}
-	err := unix.Fallocate(int(out.Fd()), unix.FALLOC_FL_KEEP_SIZE, 0, size)
+	index := atomic.LoadInt32(&fallocFlagsIndex)
+again:
+	if index >= int32(len(fallocFlags)) {
+		return nil // Fallocate is disabled
+	}
+	flags := fallocFlags[index]
+	err := unix.Fallocate(int(out.Fd()), flags, 0, size)
+	if err == unix.ENOTSUP {
+		// Try the next flags combination
+		index++
+		atomic.StoreInt32(&fallocFlagsIndex, index)
+		fs.Debugf(nil, "preAllocate: got error on fallocate, trying combination %d/%d: %v", index, len(fallocFlags), err)
+		goto again
+
+	}
 	// FIXME could be doing something here
 	// if err == unix.ENOSPC {
 	// 	log.Printf("No space")