From c295ebde639a7ecc77fde921cac1c61190ee0c28 Mon Sep 17 00:00:00 2001
From: "Ethan P." <eth-p+git@hidden.email>
Date: Sat, 24 Aug 2024 16:06:28 -0700
Subject: [PATCH] feat: Add Darwin implementation of ReflinkCopy FileCopyMethod

---
 .github/workflows/go.yml |  2 +-
 all_test.go              |  3 ++
 copy_methods_darwin.go   | 87 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 91 insertions(+), 1 deletion(-)
 create mode 100644 copy_methods_darwin.go

diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index b050a7b..916b8e9 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -20,7 +20,7 @@ jobs:
          - runner: windows-latest
          - runner: macos-latest
            filesystem: APFS
-           copymethod: GetBytes
+           copymethod: ReflinkCopy
          - runner: ubuntu-latest
            filesystem: btrfs
            copymethod: GetBytes
diff --git a/all_test.go b/all_test.go
index 6604bf7..2ce4d23 100644
--- a/all_test.go
+++ b/all_test.go
@@ -24,6 +24,9 @@ func setupFileCopyMethod(m *testing.M) {
 	case "CopyBytes":
 		defaultCopyMethodName = "CopyBytes"
 		defaultCopyMethod = CopyBytes
+	case "ReflinkCopy":
+		defaultCopyMethodName = "ReflinkCopy"
+		defaultCopyMethod = ReflinkCopy
 	}
 }
 
diff --git a/copy_methods_darwin.go b/copy_methods_darwin.go
new file mode 100644
index 0000000..578a39d
--- /dev/null
+++ b/copy_methods_darwin.go
@@ -0,0 +1,87 @@
+//go:build darwin
+
+package copy
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"time"
+
+	"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 --------------------
+//
+// Darwin implementation uses the `clonefile` syscall:
+// https://www.manpagez.com/man/2/clonefile/
+//
+// Support:
+//   - MacOS 10.14 or newer
+//   - APFS filesystem
+//
+// Considerations:
+//   - Ownership is not preserved.
+//   - Setuid and Setgid are not preserved.
+//   - Times are copied by default.
+//   - Flag CLONE_NOFOLLOW is not used, we use lcopy instead of fcopy for
+//     symbolic links.
+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
+		}
+
+		// Do copy.
+		const clonefileFlags = 0
+		err = unix.Clonefile(src, dest, clonefileFlags)
+
+		// If the error is the file already exists, delete it and try again.
+		if errors.Is(err, os.ErrExist) {
+			if err = os.Remove(dest); err != nil {
+				return err, false
+			}
+
+			err = unix.Clonefile(src, dest, clonefileFlags) // retry
+		}
+
+		// Return error if clone is not possible.
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil, true // but not if source file doesn't exist
+			}
+
+			return &os.PathError{
+				Op:   "create reflink",
+				Path: src,
+				Err:  err,
+			}, false
+		}
+
+		// Copy-on-write preserves the modtime by default.
+		// If PreserveTimes is not true, update the time to now.
+		if !opt.PreserveTimes {
+			now := time.Now()
+			if err := os.Chtimes(dest, now, now); err != nil {
+				return err, false
+			}
+		}
+
+		return nil, false
+	},
+}