feat: Add Linux implementation of ReflinkCopy FileCopyMethod

.github/workflows/go.yml

@@ -23,7 +23,7 @@ jobs:
            copymethod: ReflinkCopy
          - runner: ubuntu-latest
            filesystem: btrfs
-           copymethod: GetBytes
+           copymethod: ReflinkCopy
         go: ['1.18', '1.19', '1.20']
     steps:
     - name: Set up Go

copy_methods_linux.go

@@ -0,0 +1,79 @@
+//go:build linux
+
+package copy
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// ReflinkCopy tries to copy the file by creating a reflink from the source
+// file to the destination file. This asks the filesystem to share the
+// contents between the files using a copy-on-write method.
+//
+// Reflinks are the fastest way to copy large files, but have a few limitations:
+//
+//   - Requires using a supported filesystem (btrfs, xfs, apfs)
+//   - Source and destination must be on the same filesystem.
+//
+// See: https://btrfs.readthedocs.io/en/latest/Reflink.html
+//
+// -------------------- PLATFORM SPECIFIC INFORMATION --------------------
+//
+// Linux implementation uses the `ficlone` ioctl:
+// https://manpages.debian.org/testing/manpages-dev/ioctl_ficlone.2.en.html
+//
+// Support:
+//   - BTRFS or XFS filesystem
+//
+// Considerations:
+//   - Ownership is not preserved.
+//   - Setuid and Setgid are not preserved.
+//   - Times are not preserved.
+var ReflinkCopy = FileCopyMethod{
+	fcopy: func(src, dest string, info os.FileInfo, opt Options) (err error, skipFile bool) {
+		if opt.FS != nil {
+			return fmt.Errorf("%w: cannot create reflink from Go's fs.FS interface", ErrUnsupportedCopyMethod), false
+		}
+
+		if opt.WrapReader != nil {
+			return fmt.Errorf("%w: cannot create reflink when WrapReader option is used", ErrUnsupportedCopyMethod), false
+		}
+
+		// Open source file.
+		readcloser, err := os.OpenFile(src, os.O_RDONLY, 0)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil, true
+			}
+			return
+		}
+		defer fclose(readcloser, &err)
+
+		// Open dest file.
+		f, err := os.Create(dest)
+		if err != nil {
+			return
+		}
+		defer fclose(f, &err)
+
+		// Do copy.
+		srcFd := readcloser.Fd()
+		destFd := f.Fd()
+		err = unix.IoctlFileClone(int(destFd), int(srcFd))
+
+		// Return an error if cloning is not possible.
+		if err != nil {
+			_ = os.Remove(dest) // remove the empty file on error
+			return &os.PathError{
+				Op:   "create reflink",
+				Path: src,
+				Err:  err,
+			}, false
+		}
+
+		return nil, false
+	},
+}

copy_methods_x.go

@@ -1,4 +1,4 @@
-//go:build !darwin
+//go:build !darwin && !linux
 
 package copy