1 files changed, 59 insertions, 0 deletions
diff --git a/internal/file/hardlink.go b/internal/file/hardlink.go
new file mode 100644
index 0000000..8913674
--- /dev/null
+++ b/internal/file/hardlink.go
@@ -0,0 +1,59 @@
+package file
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"syscall"
+)
+
+// haveHardlink ensures f.path is a hard link to f.hardlinkTarget.
+//
+// Idempotency and clobber policy:
+//   - already the same inode as the target: nothing to do.
+//   - a different file/link is in the way: it is renamed to "<path>.old"
+//     before the link is created (matching the symlink resource's behavior).
+func (f *File) haveHardlink() error {
+	log.Printf("processing hardlink: %s -> %s", f.path, f.hardlinkTarget)
+
+	if f.hardlinkTarget == "" {
+		return fmt.Errorf("hardlink %s has no target", f.path)
+	}
+
+	targetInfo, err := os.Stat(f.hardlinkTarget)
+	if err != nil {
+		return fmt.Errorf("failed to stat hardlink target %s: %w", f.hardlinkTarget, err)
+	}
+
+	if info, err := os.Lstat(f.path); err == nil {
+		if sameInode(info, targetInfo) {
+			log.Printf("hardlink %s already links to %s", f.path, f.hardlinkTarget)
+			return nil
+		}
+		old := f.path + ".old"
+		log.Printf("%s already exists, renaming to %s", f.path, old)
+		if err := os.Rename(f.path, old); err != nil {
+			return fmt.Errorf("failed to move existing %s aside: %w", f.path, err)
+		}
+	} else if !os.IsNotExist(err) {
+		return fmt.Errorf("failed to stat %s: %w", f.path, err)
+	}
+
+	if err := os.Link(f.hardlinkTarget, f.path); err != nil {
+		return fmt.Errorf("failed to create hardlink %s -> %s: %w", f.path, f.hardlinkTarget, err)
+	}
+
+	log.Printf("created hardlink %s -> %s", f.path, f.hardlinkTarget)
+	return nil
+}
+
+// sameInode reports whether two FileInfos refer to the same underlying inode
+// (same device and inode number), i.e. they are already hard-linked.
+func sameInode(a, b os.FileInfo) bool {
+	as, aok := a.Sys().(*syscall.Stat_t)
+	bs, bok := b.Sys().(*syscall.Stat_t)
+	if !aok || !bok {
+		return false
+	}
+	return as.Dev == bs.Dev && as.Ino == bs.Ino
+}