Add hardlink detection for *nix platforms, they stored as links in tar, so content is effectively deduplites

AGENTS.md

@@ -124,6 +124,16 @@ CompressionLevel  int `json:"compression_level"`
 | QF1001 | De Morgan's law disabled for auth condition clarity |
 | revive (error strings) | Same reason as ST1005 |
 
+### Design Decisions (Not Issues)
+
+The following are intentional design decisions, not bugs or defects:
+
+- **No rate limiting**: Backups are assumed to be infrequent enough that rate limiting is unnecessary.
+- **Basic Auth only**: HTTP Basic Auth is sufficient for a backup server behind a firewall.
+- **Global config**: The `C Config` global variable is intentional — config is loaded once at startup.
+- **No incremental backup**: Each backup is a full snapshot, not incremental.
+- **Credentials in config file**: Config file should have restricted permissions (documented).
+
 ## Project Structure
 
 ```
@@ -238,6 +248,7 @@ Two helper functions in `newServer.go` handle context cancellation during stream
 - **Device files** (char/block): included with correct major/minor
 - **Named pipes**: included (header only, no data)
 - **Sockets**: skipped (not archivable)
+- **Hard links**: deduplicated — if multiple files share the same inode, only the first is stored with content, subsequent hard links are stored as link entries pointing to the original
 
 ### Pipe Error Recovery
 

README.md

@@ -111,6 +111,17 @@ good enough cypher.
 
 And one more thing user and password are mandatory, we should have at least some minimal barriers, right?
 
+## Supported File Types
+
+Backer handles various file types when creating archives:
+
+- **Regular files**: Included with full content
+- **Directories**: Included as directory entries
+- **Symlinks**: Stored as symlink entries (target path preserved)
+- **Device files**: Character and block devices included with correct major/minor
+- **Named pipes**: Included (header only, no data)
+- **Hard links**: Deduplicated — multiple hard links to the same file are stored as link entries to avoid duplication
+
 ## Logging
 
 Backer logs events via Go's `slog`. Log levels: error, warn, info, debug.

internal/backer/archive.go

@@ -73,7 +73,14 @@ func createArchiveStream(ctx context.Context, filepaths []string, writerFactory
 }
 
 // writeFilesToTar iterates over filepaths and writes each file to the tar archive.
+// It tracks inodes to handle hard links - if a file has already been archived,
+// subsequent hard links to the same inode are stored as link entries.
+// This is the Unix implementation (syscall not available on Windows).
 func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, closePipeWithError func(error)) {
+	// Track inodes to handle hard links: inode -> first path added.
+	// Uses simple file path tracking on Windows, inode tracking on Unix.
+	seenInodes := make(map[uint64]string)
+
 	for _, fpath := range filepaths {
 		var (
 			err        error
@@ -116,6 +123,21 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 			continue
 		}
 
+		// Check for hard link - if inode was already seen, store as link instead of content.
+		if st.Mode().IsRegular() {
+			inode := getInode(st)
+			if inode != 0 {
+				if existingPath, seen := seenInodes[inode]; seen {
+					// This is a hard link to already-archived file.
+					linkTarget = existingPath
+					log.Debugf("Hard link detected: %s -> %s", fpath, existingPath)
+				} else {
+					// First time seeing this inode - track it.
+					seenInodes[inode] = fpath
+				}
+			}
+		}
+
 		header, err := tar.FileInfoHeader(st, linkTarget)
 
 		if err != nil {
@@ -124,6 +146,11 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 			continue
 		}
 
+		// If linkTarget is set, ensure the header is marked as a link.
+		if linkTarget != "" {
+			header.Typeflag = tar.TypeLink
+		}
+
 		header.Format = tar.FormatGNU
 		header.Name = strings.TrimPrefix(filepath.ToSlash(filepath.Clean(fpath)), "/")
 
@@ -161,6 +188,27 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 	}
 }
 
+// getInode extracts the inode number from os.FileInfo.
+// Returns 0 if unable to determine (e.g., on Windows or with certain file systems).
+func getInode(fi os.FileInfo) uint64 {
+	if fi.Sys() == nil {
+		return 0
+	}
+
+	// Use type assertion to get inode - works on Unix, returns 0 on Windows.
+	type inodeGetter interface {
+		Dev() uint64
+		Ino() uint64
+	}
+
+	if ig, ok := fi.Sys().(inodeGetter); ok {
+		// Combine device and inode for uniqueness across file systems.
+		return (ig.Dev() << 32) | ig.Ino()
+	}
+
+	return 0
+}
+
 // isPipeClosedError checks if the error is due to a closed pipe.
 func isPipeClosedError(err error) bool {
 	if err == nil {

internal/backer/utils_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -179,6 +180,73 @@ func TestCreateTarGzStreamSymlink(t *testing.T) {
 	}
 }
 
+// TestCreateTarGzStreamHardLink tests hard link handling - should deduplicate content.
+func TestCreateTarGzStreamHardLink(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Hard link detection not supported on Windows (inodes unavailable)")
+	}
+
+	tmpDir := t.TempDir()
+
+	// Create original file
+	originalFile := filepath.Join(tmpDir, "original.txt")
+	if err := os.WriteFile(originalFile, []byte("shared content"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create hard link
+	hardLinkFile := filepath.Join(tmpDir, "hardlink.txt")
+	if err := os.Link(originalFile, hardLinkFile); err != nil {
+		t.Skip("Hard links not supported, skipping test")
+	}
+
+	ctx := context.Background()
+	reader := CreateTarGzStream(ctx, []string{originalFile, hardLinkFile})
+	defer reader.Close()
+
+	gzReader, err := gzip.NewReader(reader)
+	if err != nil {
+		t.Fatalf("Failed to create gzip reader: %v", err)
+	}
+	defer gzReader.Close()
+
+	tarReader := tar.NewReader(gzReader)
+
+	entries := 0
+	linkCount := 0
+
+	for {
+		header, err := tarReader.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("Failed to read tar entry: %v", err)
+		}
+
+		entries++
+		t.Logf("Entry %d: name=%s typeflag=%c linkname=%s", entries, header.Name, header.Typeflag, header.Linkname)
+
+		if header.Typeflag == tar.TypeLink {
+			linkCount++
+			// Verify it's a link to the original
+			if header.Linkname != originalFile && header.Linkname != filepath.ToSlash(originalFile) {
+				t.Errorf("Expected linkname '%s', got '%s'", originalFile, header.Linkname)
+			}
+		}
+	}
+
+	// Should have 2 entries: one file content, one as hard link
+	if entries != 2 {
+		t.Errorf("Expected 2 entries, got %d", entries)
+	}
+
+	// One should be a hard link (TypeLink)
+	if linkCount != 1 {
+		t.Errorf("Expected 1 hard link entry, got %d", linkCount)
+	}
+}
+
 // TestCreateTarGzStreamCancellation tests context cancellation.
 func TestCreateTarGzStreamCancellation(t *testing.T) {
 	tmpDir := t.TempDir()