preallocate: use custom preallocate directives where possible

Updates #4

sync/preallocate.go

@@ -1,9 +1,13 @@
-// +build !linux
-
 package sync
 
 import "os"
 
-func fallocate(f *os.File, size int64) error {
+func Preallocate(f *os.File, size int64) error {
+	err := preallocate(f, size)
+	if err == nil {
+		return nil
+	}
+
+	// use default truncation if platform specific allocation fails
 	return f.Truncate(size)
 }

sync/preallocate_linux.go

@@ -5,10 +5,6 @@ import (
 	"syscall"
 )
 
-func fallocate(f *os.File, size int64) error {
-	err := syscall.Fallocate(int(f.Fd()), 0, 0, size)
-	if err == syscall.ENOTSUP {
-		return f.Truncate(size)
-	}
-	return err
+func preallocate(f *os.File, size int64) error {
+	return syscall.Fallocate(int(f.Fd()), 0, 0, size)
 }

sync/preallocate_other.go

@@ -0,0 +1,12 @@
+// +build !linux,!darwin
+
+package sync
+
+import (
+	"fmt"
+	"os"
+)
+
+func preallocate(f *os.File, size int64) error {
+	return fmt.Errorf("Operation not supported on this platform")
+}

sync/sync.go

@@ -405,7 +405,7 @@ func (c *Client) download(ctx context.Context, t *task) error {
 	defer f.Close()
 
 	// pre-allocate file space
-	_ = fallocate(f, t.state.FileLength)
+	_ = Preallocate(f, t.state.FileLength)
 
 	t.state.DownloadStartedAt = time.Now()
 	t.state.DownloadStatus = DownloadInProgress