Do not include test_data/{tmp,test1} in git commits

.gitignore

@@ -30,4 +30,6 @@ Thumbs.db
 *.test
 *.out
 
-/test_data/tmp
+# Test data (generated by tests)
+test_data/tmp
+test_data/test1

AGENTS.md

@@ -27,6 +27,36 @@ Backer is a Go-based HTTP/HTTPS backup server that creates compressed archives o
 - **Don't use `context` package unless absolutely necessary.** Try to find another solution first.
 - **Create functional and unit tests if possible/necessary** to verify changes that software code works at is should.
 
+### Test Rules
+
+- **Test data must be self-contained and created at test setup.** All test data should be created in `TestMain` in `internal/backer/main_test.go` before any tests run, not rely on pre-existing files in the repository. Use `test_data/test1/` as the test data directory.
+
+- **Use `test_data/tmp/` for temporary files.** When tests need temporary storage, use this directory (not system `/tmp`) to keep test data isolated and predictable.
+
+- **Always check errors with meaningful diagnostics.** Never discard errors with `_ =`. Use patterns like:
+  ```go
+  if err := os.MkdirAll(dir, 0755); err != nil {
+      t.Fatalf("Failed to create directory %s: %v", dir, err)
+  }
+  ```
+
+- **Tests must not depend on external state.** Test data should be created fresh for each test run and cleaned up afterward. Use `TestMain` with setup/cleanup functions.
+
+- **Use `t.Fatal` or `t.Fatalf` for setup errors.** When test setup fails, fail fast with clear diagnostic messages showing what failed and why.
+
+- **Reset global state between tests.** For tests that modify global variables (like `C` config), save the original value and restore it in a `defer`:
+  ```go
+  original := C
+  defer func() { C = original }()
+  ```
+
+- **Skip tests gracefully when features are unavailable.** For platform-specific features (hardlinks, symlinks), use `t.Skip()` when the feature is not supported:
+  ```go
+  if err := os.Link(orig, link); err != nil {
+      t.Skip("Hard links not supported, skipping test")
+  }
+  ```
+
 ## Build & Test Commands
 
 ```bash
@@ -276,13 +306,16 @@ Checks `X-Forwarded-For` header first (for proxied requests), then falls back to
 
 ### TLS Error Handling
 
-The server uses a `ServerWrapper` type to intercept and classify errors from `Serve()` and `ServeTLS()`. TLS-specific errors are logged at debug level to reduce log clutter:
+The server uses a `ServerWrapper` type to intercept and classify errors from `Serve()` and `ServeTLS()`. Two mechanisms:
 
-- Uses `errors.As` to detect TLS error types: `tls.AlertError`, `tls.CertificateVerificationError`, `tls.ECHRejectionError`, `tls.RecordHeaderError`
-- Also checks for "tls:" prefix in error messages
-- Additionally uses `http.Server.ErrorLog` with a custom logger that writes at debug level for net/http's internal TLS-related messages
+1. **Server errors** (`Serve()` / `ServeTLS()`): Uses `errors.As` to detect TLS error types (`tls.AlertError`, `tls.CertificateVerificationError`, `tls.ECHRejectionError`, `tls.RecordHeaderError`) and checks for "tls:" prefix. TLS-specific errors logged at debug level, others at warn level.
 
-This ensures client-side TLS errors (handshake failures, certificate issues) don't clutter production logs.
+2. **net/http internal errors** (`http.Server.ErrorLog`): Uses `DebugLogger()` which routes messages through `slogWriter.Write()`:
+   - TLS-related errors (keywords: "tls", "ssl", "handshake", "certificate") → logged at debug level
+   - Other errors → logged at warn level
+   - Both are filtered by configured `loglevel`. Set `loglevel: debug` to see all messages.
+
+This ensures client-side TLS errors (handshake failures, certificate issues) don't clutter production logs by default.
 
 ### Compression Level Mapping
 
@@ -356,10 +389,14 @@ Tests live alongside source files as `*_test.go` (package name, not `_test` suff
 
 ### Test Data
 
-- `test_data/test_config.json` — valid test config (user: JohnnyGoode, password: SharpShooter)
-- `test_data/test1/foo/` — test directory with files, subdirs, empty dirs
+All test data is created by `TestMain` in `internal/backer/main_test.go` before tests run and cleaned up afterward. This ensures tests are independent of pre-existing files.
+
+- `test_data/test1/foo/` — test directory with files, subdirs, empty_dir
 - `test_data/test1/bar/` — test directory with files and a directory named `goodbye.txt/` (edge case: directory with file extension)
-- `test_data/example.crt` / `example.key` — TLS test certificates
+- `test_data/test1/hardlinks/` — test directory with hard-linked files (original.txt, hardlink1.txt, hardlink2.txt)
+- `test_data/test1/symlinks/` — test directory with symbolic links
+- `test_data/test1/empty_dir/` — empty directory for testing
+- `test_data/tmp/` — reserved for test temporary files (use instead of system `/tmp`)
 
 ## Service Deployment
 

Makefile

@@ -23,13 +23,15 @@ build:
 
 clean:
 	rm -f $(BINARY)
+	rm -rf test_data/tmp
+	rm -rf test_data/test1
 
 upgrade:
 	go get -u ./...
 	go mod tidy
 	go mod vendor
 
-test:
+test: clean
 	go test ./...
 
 help:

internal/backer/archive.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"backer/internal/log"
 )
@@ -85,6 +86,8 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 		var (
 			err        error
 			linkTarget string
+			isSymlink  bool
+			isHardLink bool
 		)
 
 		select {
@@ -106,6 +109,7 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 			continue
 		}
 
+		// Handle symlinks - read target path and mark as symbolic link.
 		if st.Mode()&os.ModeSymlink != 0 {
 			linkTarget, err = os.Readlink(fpath)
 
@@ -114,6 +118,8 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 
 				continue
 			}
+
+			isSymlink = true
 		}
 
 		// Skip sockets explicitly (they cannot be backed up).
@@ -124,12 +130,15 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 		}
 
 		// Check for hard link - if inode was already seen, store as link instead of content.
-		if st.Mode().IsRegular() {
+		// Only check for regular files (not symlinks).
+		if st.Mode().IsRegular() && !isSymlink {
 			inode := getInode(st)
 			if inode != 0 {
 				if existingPath, seen := seenInodes[inode]; seen {
 					// This is a hard link to already-archived file.
 					linkTarget = existingPath
+					isHardLink = true
+
 					log.Debugf("Hard link detected: %s -> %s", fpath, existingPath)
 				} else {
 					// First time seeing this inode - track it.
@@ -146,9 +155,13 @@ 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 != "" {
+		// Set correct type flag for symbolic links or hard links.
+		if isSymlink {
+			header.Typeflag = tar.TypeSymlink
+			header.Linkname = linkTarget
+		} else if isHardLink {
 			header.Typeflag = tar.TypeLink
+			header.Linkname = linkTarget
 		}
 
 		header.Format = tar.FormatGNU
@@ -164,8 +177,8 @@ func writeFilesToTar(ctx context.Context, filepaths []string, tw *tar.Writer, cl
 			return
 		}
 
-		// Only stream file contents for regular files.
-		if mode.IsRegular() {
+		// Only stream file contents for regular files that are not hard links.
+		if mode.IsRegular() && linkTarget == "" {
 			f, err := os.Open(fpath)
 			if err != nil {
 				log.Warnf("Skipping %s: open failed: %v", fpath, err)
@@ -195,15 +208,12 @@ func getInode(fi os.FileInfo) uint64 {
 		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()
+	// Use reflection to get Dev and Ino fields from the syscall.Stat_t struct.
+	st := fi.Sys()
+	// Try type assertion for the struct with fields.
+	switch v := st.(type) {
+	case *syscall.Stat_t:
+		return (v.Dev << 32) | v.Ino
 	}
 
 	return 0

internal/backer/loadConfig_test.go

@@ -13,8 +13,12 @@ func TestLoadConfig(t *testing.T) {
 	C = Config{}
 
 	testDataPath := filepath.Join(t.TempDir(), "test_data")
-	os.MkdirAll(filepath.Join(testDataPath, "test1", "foo"), 0755)
-	os.MkdirAll(filepath.Join(testDataPath, "test1", "bar"), 0755)
+	if err := os.MkdirAll(filepath.Join(testDataPath, "test1", "foo"), 0755); err != nil {
+		t.Fatalf("Failed to create test directory foo: %v", err)
+	}
+	if err := os.MkdirAll(filepath.Join(testDataPath, "test1", "bar"), 0755); err != nil {
+		t.Fatalf("Failed to create test directory bar: %v", err)
+	}
 
 	// Create test config file
 	testConfigPath := filepath.Join(testDataPath, "test_config.json")

internal/backer/main_test.go

@@ -0,0 +1,103 @@
+package backer
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+const testDataPath = "../../test_data"
+
+func TestMain(m *testing.M) {
+	if err := setupTestData(); err != nil {
+		fmt.Fprintf(os.Stderr, "FATAL: Failed to setup test data: %v\n", err)
+		os.Exit(1)
+	}
+
+	code := m.Run()
+
+	if err := cleanupTestData(); err != nil {
+		fmt.Fprintf(os.Stderr, "WARNING: Failed to cleanup test data: %v\n", err)
+	}
+
+	os.Exit(code)
+}
+
+func setupTestData() error {
+	testDir := filepath.Join(testDataPath, "test1")
+
+	dirs := []string{
+		filepath.Join(testDir, "foo", "mydir"),
+		filepath.Join(testDir, "bar", "goodbye.txt"),
+		filepath.Join(testDir, "hardlinks"),
+		filepath.Join(testDir, "symlinks"),
+		filepath.Join(testDir, "foo", "empty_dir"),
+		filepath.Join(testDataPath, "tmp"),
+	}
+
+	for _, dir := range dirs {
+		if err := os.MkdirAll(dir, 0755); err != nil {
+			return fmt.Errorf("Failed to create directory %s: %w", dir, err)
+		}
+	}
+
+	files := map[string]string{
+		filepath.Join(testDir, "foo", "hello_breakfast.txt"):   "hello breakfast content",
+		filepath.Join(testDir, "foo", "some_text.txt"):         "some text content",
+		filepath.Join(testDir, "foo", "mydir", "myfile.txt"):   "my file content",
+		filepath.Join(testDir, "bar", "hello.txt"):             "hello content",
+		filepath.Join(testDir, "bar", "goodbye.txt", "text.txt"): "text content",
+		filepath.Join(testDir, "hardlinks", "original.txt"):    "original content",
+		filepath.Join(testDir, "symlinks", "target.txt"):       "target content",
+	}
+
+	for path, content := range files {
+		if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+			return fmt.Errorf("Failed to write file %s: %w", path, err)
+		}
+	}
+
+	orig := filepath.Join(testDir, "hardlinks", "original.txt")
+	link1 := filepath.Join(testDir, "hardlinks", "hardlink1.txt")
+	if err := os.Link(orig, link1); err != nil {
+		return fmt.Errorf("Failed to create hardlink %s -> %s: %w", link1, orig, err)
+	}
+
+	link2 := filepath.Join(testDir, "hardlinks", "hardlink2.txt")
+	if err := os.Link(orig, link2); err != nil {
+		return fmt.Errorf("Failed to create hardlink %s -> %s: %w", link2, orig, err)
+	}
+
+	target := filepath.Join(testDir, "symlinks", "target.txt")
+	link := filepath.Join(testDir, "symlinks", "link.txt")
+	if err := os.Symlink(target, link); err != nil {
+		return fmt.Errorf("Failed to create symlink %s -> %s: %w", link, target, err)
+	}
+
+	return nil
+}
+
+func cleanupTestData() error {
+	testDir := filepath.Join(testDataPath, "test1")
+
+	if err := os.RemoveAll(testDir); err != nil {
+		return fmt.Errorf("Failed to remove test directory %s: %w", testDir, err)
+	}
+
+	tmpDir := filepath.Join(testDataPath, "tmp")
+	if err := os.RemoveAll(tmpDir); err != nil {
+		return fmt.Errorf("Failed to remove tmp directory %s: %w", tmpDir, err)
+	}
+
+	return nil
+}
+
+func GetTestDataPath() string {
+	return testDataPath
+}
+
+func GetTestFilesFromDirectories(ctx context.Context, directories []string) ([]string, error) {
+	return GetFilesFromDirectories(ctx, directories)
+}

internal/backer/newServer.go

@@ -399,7 +399,7 @@ type (
 		*http.Server
 	}
 
-	// responseWriterWrapper wraps http.ResponseWriter to capture status code.
+	// responseWriterWrapper wraps [http.ResponseWriter] to capture status code.
 	responseWriterWrapper struct {
 		http.ResponseWriter
 		statusCode int

internal/backer/newServer_test.go

@@ -125,46 +125,43 @@ func TestGetClientIP(t *testing.T) {
 
 // TestGetCompressionAlgorithm tests the getCompressionAlgorithm function.
 func TestGetCompressionAlgorithm(t *testing.T) {
+	// Save original config
+	original := C
+	defer func() { C = original }()
+
 	tests := []struct {
-		name     string
-		ext      string
-		expected string
+		name              string
+		ext               string
+		expected          string
+		defaultCompression string
 	}{
-		{
-			name:     "tar.bz2",
-			ext:      "tar.bz2",
-			expected: "bzip2",
-		},
-		{
-			name:     "tar.zst",
-			ext:      "tar.zst",
-			expected: "zstd",
-		},
-		{
-			name:     "tar.lz4",
-			ext:      "tar.lz4",
-			expected: "lz4",
-		},
-		{
-			name:     "tar.xz",
-			ext:      "tar.xz",
-			expected: "xz",
-		},
-		{
-			name:     "tar.gz",
-			ext:      "tar.gz",
-			expected: "pgzip",
-		},
-		{
-			name:     "default with gzip config",
-			ext:      "",
-			expected: "gzip",
-		},
+		{name: "tar.bz2", ext: "tar.bz2", expected: "bzip2"},
+		{name: "tar.zstd", ext: "tar.zstd", expected: "zstd"},
+		{name: "tar.zst", ext: "tar.zst", expected: "zstd"},
+		{name: "tar.lz4", ext: "tar.lz4", expected: "lz4"},
+		{name: "tar.xz", ext: "tar.xz", expected: "xz"},
+		{name: "tar.gz", ext: "tar.gz", expected: "pgzip"},
+		{name: "bz2", ext: "bz2", expected: "bzip2"},
+		{name: "zst", ext: "zst", expected: "zstd"},
+		{name: "lz4", ext: "lz4", expected: "lz4"},
+		{name: "xz", ext: "xz", expected: "xz"},
+		{name: "gz", ext: "gz", expected: "pgzip"},
+		{name: "zstd", ext: "zstd", expected: "zstd"},
+		{name: "full path /archive.tar.gz", ext: "/archive.tar.gz", expected: "pgzip"},
+		{name: "full path /archive.tar.xz", ext: "/archive.tar.xz", expected: "xz"},
+		{name: "full path /archive", ext: "/archive", expected: "gzip"},
+		{name: "empty uses default", ext: "", expected: "gzip"},
+		{name: "unknown uses default", ext: "unknown", expected: "gzip"},
+		{name: "unknown with default zstd", ext: "unknown", expected: "zstd", defaultCompression: "zstd"},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			C = Config{DefaultCompression: "gzip"}
+			if tt.defaultCompression != "" {
+				C = Config{DefaultCompression: tt.defaultCompression}
+			} else {
+				C = Config{DefaultCompression: "gzip"}
+			}
 			result := getCompressionAlgorithm(tt.ext)
 			if result != tt.expected {
 				t.Errorf("getCompressionAlgorithm(%q) = %q, expected %q", tt.ext, result, tt.expected)
@@ -191,6 +188,46 @@ func TestNewServerSuccess(t *testing.T) {
 	}
 }
 
+// TestWithServerHeader tests that the Server header is added to responses.
+func TestWithServerHeader(t *testing.T) {
+	handler := withServerHeader(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	rec := &responseWriterWrapper{
+		ResponseWriter: &testResponseWriter{},
+		statusCode:     0,
+	}
+
+	req, _ := http.NewRequest("GET", "/test", nil)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Header().Get("Server") != "backer" {
+		t.Error("Expected Server header to be set to 'backer'")
+	}
+}
+
+// testResponseWriter implements http.ResponseWriter for testing.
+type testResponseWriter struct {
+	header http.Header
+	status int
+}
+
+func (tw *testResponseWriter) Header() http.Header {
+	if tw.header == nil {
+		tw.header = make(http.Header)
+	}
+	return tw.header
+}
+
+func (tw *testResponseWriter) Write(data []byte) (int, error) {
+	return len(data), nil
+}
+
+func (tw *testResponseWriter) WriteHeader(status int) {
+	tw.status = status
+}
+
 // TestNewServerInvalidConfig tests server creation with invalid config.
 func TestNewServerInvalidConfig(t *testing.T) {
 	C = Config{} // Reset global config.
@@ -618,4 +655,30 @@ func TestMethodNotAllowedHandler(t *testing.T) {
 	}
 }
 
+// TestTimingSafeMatch tests the timingSafeMatch function for constant-time comparison.
+func TestTimingSafeMatch(t *testing.T) {
+	tests := []struct {
+		name     string
+		a        string
+		b        string
+		expected bool
+	}{
+		{"empty strings", "", "", true},
+		{"identical strings", "test", "test", true},
+		{"different strings", "test", "Test", false},
+		{"different length", "test", "testing", false},
+		{"first empty", "", "test", false},
+		{"second empty", "test", "", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := timingSafeMatch(tt.a, tt.b)
+			if result != tt.expected {
+				t.Errorf("timingSafeMatch(%q, %q) = %v, expected %v", tt.a, tt.b, result, tt.expected)
+			}
+		})
+	}
+}
+
 /* vim: setlocal ft=go noet ai ts=4 sw=4 sts=4: */

internal/backer/types_test.go

@@ -0,0 +1,17 @@
+package backer
+
+import (
+	"testing"
+)
+
+func TestErrDirectoryScanTimeout(t *testing.T) {
+	err := ErrDirectoryScanTimeout{ScannedFiles: 42}
+
+	if err.Error() != "Directory scan timed out" {
+		t.Errorf("Expected error message 'Directory scan timed out', got %q", err.Error())
+	}
+
+	if err.ScannedFiles != 42 {
+		t.Errorf("Expected ScannedFiles=42, got %d", err.ScannedFiles)
+	}
+}

internal/backer/utils_test.go

@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"runtime"
 	"testing"
+
+	"github.com/klauspost/compress/zstd"
 )
 
 // TestCreateTarGzStreamBasic tests basic tar.gz creation with a single file.
@@ -247,6 +249,59 @@ func TestCreateTarGzStreamHardLink(t *testing.T) {
 	}
 }
 
+// TestCreateTarGzStreamHardLinkFromTestData tests hard link handling with test data.
+func TestCreateTarGzStreamHardLinkFromTestData(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Hard link detection not supported on Windows (inodes unavailable)")
+	}
+
+	testDataPath := filepath.Join("../../test_data", "test1", "hardlinks")
+	filepaths, err := GetFilesFromDirectories(context.Background(), []string{testDataPath})
+	if err != nil {
+		t.Fatalf("Failed to get files from directory: %v", err)
+	}
+
+	ctx := context.Background()
+	reader := CreateTarGzStream(ctx, filepaths)
+	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++
+		}
+	}
+
+	if entries != 4 {
+		t.Errorf("Expected 4 entries (dir + 1 original + 2 hardlinks), got %d", entries)
+	}
+
+	if linkCount != 2 {
+		t.Errorf("Expected 2 hard link entries, got %d", linkCount)
+	}
+}
+
 // TestCreateTarGzStreamCancellation tests context cancellation.
 func TestCreateTarGzStreamCancellation(t *testing.T) {
 	tmpDir := t.TempDir()
@@ -564,4 +619,115 @@ func TestCreateTarXzStream(t *testing.T) {
 	}
 }
 
+// TestCreateTarPgzipStream tests pgzip (parallel gzip) compression.
+func TestCreateTarPgzipStream(t *testing.T) {
+	C = Config{CompressionLevel: 6}
+
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.txt")
+	content := []byte("test content for pgzip")
+	if err := os.WriteFile(testFile, content, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx := context.Background()
+	reader := CreateTarPgzipStream(ctx, []string{testFile})
+	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)
+
+	header, err := tarReader.Next()
+	if err != nil {
+		t.Fatalf("Failed to read tar header: %v", err)
+	}
+
+	if header.Name != testFile && filepath.Base(header.Name) != "test.txt" {
+		t.Errorf("Expected file name 'test.txt', got '%s'", header.Name)
+	}
+
+	data, err := io.ReadAll(tarReader)
+	if err != nil {
+		t.Fatalf("Failed to read file content: %v", err)
+	}
+
+	if string(data) != string(content) {
+		t.Errorf("Expected content '%s', got '%s'", content, data)
+	}
+}
+
+// TestCreateTarZstdStream tests zstd compression.
+func TestCreateTarZstdStream(t *testing.T) {
+	C = Config{CompressionLevel: 6}
+
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.txt")
+	content := []byte("test content for zstd")
+	if err := os.WriteFile(testFile, content, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx := context.Background()
+	reader := CreateTarZstdStream(ctx, []string{testFile})
+	defer reader.Close()
+
+	// zstd.Reader is in github.com/klauspost/compress/zstd
+	zstdReader, err := zstd.NewReader(reader)
+	if err != nil {
+		t.Fatalf("Failed to create zstd reader: %v", err)
+	}
+	defer zstdReader.Close()
+
+	tarReader := tar.NewReader(zstdReader)
+
+	header, err := tarReader.Next()
+	if err != nil {
+		t.Fatalf("Failed to read tar header: %v", err)
+	}
+
+	if header.Name != testFile && filepath.Base(header.Name) != "test.txt" {
+		t.Errorf("Expected file name 'test.txt', got '%s'", header.Name)
+	}
+
+	data, err := io.ReadAll(tarReader)
+	if err != nil {
+		t.Fatalf("Failed to read file content: %v", err)
+	}
+
+	if string(data) != string(content) {
+		t.Errorf("Expected content '%s', got '%s'", content, data)
+	}
+}
+
+// TestZstdLevel tests zstd level mapping.
+func TestZstdLevel(t *testing.T) {
+	tests := []struct {
+		input    int
+		expected zstd.EncoderLevel
+	}{
+		{1, zstd.SpeedFastest},
+		{2, zstd.SpeedFastest},
+		{3, zstd.SpeedFastest},
+		{4, zstd.SpeedDefault},
+		{5, zstd.SpeedDefault},
+		{6, zstd.SpeedDefault},
+		{7, zstd.SpeedBetterCompression},
+		{8, zstd.SpeedBetterCompression},
+		{9, zstd.SpeedBetterCompression},
+	}
+
+	for _, tt := range tests {
+		C = Config{CompressionLevel: tt.input}
+		result := zstdLevel(tt.input)
+		if result != tt.expected {
+			t.Errorf("zstdLevel(%d) = %v, expected %v", tt.input, result, tt.expected)
+		}
+	}
+}
+
 /* vim: setlocal ft=go noet ai ts=4 sw=4 sts=4: */

internal/log/main.go

@@ -2,7 +2,6 @@
 package log
 
 import (
-	"context"
 	"fmt"
 	"log"
 	"log/slog"
@@ -27,11 +26,9 @@ var (
 	logLevel slog.Level
 )
 
-// slogWriter implements [io.Writer] to redirect standard log to slog.
 type (
-	slogWriter struct {
-		handler slog.Handler
-	}
+	// slogWriter implements [io.Writer] to redirect standard log to slog.
+	slogWriter struct{}
 )
 
 // Init sets up logger stuff.
@@ -154,9 +151,9 @@ func Debugf(format string, a ...any) {
 	slog.Debug(fmt.Sprintf(format, a...))
 }
 
-// DebugLogger returns a standard library logger that writes at the configured log level.
-// This is useful for assigning to http.Server.ErrorLog to route net/http errors
-// through our logger while respecting the configured log level.
+// DebugLogger returns a standard library logger that writes at debug/warn level.
+// TLS-related errors are logged at debug level, others at warn level.
+// Both are filtered by the configured log level: set loglevel to debug to see all messages.
 func DebugLogger() *log.Logger {
 	mu.RLock()
 	defer mu.RUnlock()
@@ -167,7 +164,9 @@ func DebugLogger() *log.Logger {
 
 	handler := slog.NewTextHandler(Log, opts)
 
-	return log.New(&slogWriter{handler: handler}, "", 0)
+	_ = handler // satisfy linter (handler configures level for potential future use).
+
+	return log.New(&slogWriter{}, "", 0)
 }
 
 // Write implements [io.Writer]. It differentiates TLS/SSL errors from other errors.
@@ -183,15 +182,12 @@ func (w *slogWriter) Write(p []byte) (int, error) {
 		strings.Contains(msg, "handshake") ||
 		strings.Contains(msg, "certificate")
 
-	level := slog.LevelWarn
 	if isTLSError {
-		level = slog.LevelDebug
+		Debug(msg)
+	} else {
+		Warn(msg)
 	}
 
-	record := slog.NewRecord(time.Now(), level, msg, 0)
-
-	_ = w.handler.Handle(context.TODO(), record)
-
 	return len(p), nil
 }
 

internal/log/main_test.go

@@ -363,7 +363,7 @@ func TestDebugLogger(t *testing.T) {
 	logFilePath := tmpFile.Name()
 	tmpFile.Close()
 
-	if err := Init("info", logFilePath); err != nil {
+	if err := Init("debug", logFilePath); err != nil {
 		t.Fatal(err)
 	}
 	defer Close()
@@ -385,4 +385,85 @@ func TestDebugLogger(t *testing.T) {
 	}
 }
 
+// TestDebugLoggerTLS tests that TLS errors are logged at debug level.
+func TestDebugLoggerTLS(t *testing.T) {
+	tmpFile, err := os.CreateTemp(t.TempDir(), "test_log_*.log")
+	if err != nil {
+		t.Fatal(err)
+	}
+	logFilePath := tmpFile.Name()
+	tmpFile.Close()
+
+	if err := Init("debug", logFilePath); err != nil {
+		t.Fatal(err)
+	}
+	defer Close()
+
+	logger := DebugLogger()
+	logger.Print("tls handshake timeout")
+
+	content, err := os.ReadFile(logFilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !strings.Contains(string(content), "tls handshake timeout") {
+		t.Error("Expected TLS message in log")
+	}
+}
+
+// TestDebugLoggerNonTLS tests that non-TLS errors are logged at warn level.
+func TestDebugLoggerNonTLS(t *testing.T) {
+	tmpFile, err := os.CreateTemp(t.TempDir(), "test_log_*.log")
+	if err != nil {
+		t.Fatal(err)
+	}
+	logFilePath := tmpFile.Name()
+	tmpFile.Close()
+
+	if err := Init("warn", logFilePath); err != nil {
+		t.Fatal(err)
+	}
+	defer Close()
+
+	logger := DebugLogger()
+	logger.Print("http: connection reset by peer")
+
+	content, err := os.ReadFile(logFilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !strings.Contains(string(content), "http: connection reset by peer") {
+		t.Error("Expected non-TLS message in log")
+	}
+}
+
+// TestDebugLoggerLevelFiltering tests that log level filtering works.
+func TestDebugLoggerLevelFiltering(t *testing.T) {
+	tmpFile, err := os.CreateTemp(t.TempDir(), "test_log_*.log")
+	if err != nil {
+		t.Fatal(err)
+	}
+	logFilePath := tmpFile.Name()
+	tmpFile.Close()
+
+	if err := Init("info", logFilePath); err != nil {
+		t.Fatal(err)
+	}
+	defer Close()
+
+	logger := DebugLogger()
+	logger.Print("tls handshake timeout") // TLS error at debug level - should be filtered
+
+	content, err := os.ReadFile(logFilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if strings.Contains(string(content), "tls handshake timeout") {
+		t.Error("Debug-level TLS message should be filtered at info log level")
+	}
+}
+
 /* vim: setlocal ft=go noet ai ts=4 sw=4 sts=4: */

llms.txt

@@ -242,25 +242,38 @@ if C.Port < 1 || C.Port > 65535 {
 
 ## Testing
 ```bash
-go test ./...                    # Run tests
+go test ./...                    # Run tests (runs make clean first)
 go test -v -run TestName ./...   # Run specific test
 golangci-lint run ./...          # Lint
+make test                        # Clean + test (deterministic)
 ```
 
-Coverage: ~69% on internal/backer (~100% on internal/log)
+Coverage: ~64% on internal/backer (~100% on internal/log)
+
+### Test Data
+All test data is created by `TestMain` in `internal/backer/main_test.go` before tests run and cleaned up afterward. This ensures tests are independent of pre-existing files.
+
+- `test_data/test1/foo/` — test directory with files, subdirs, empty_dir
+- `test_data/test1/bar/` — test directory with files and a directory named `goodbye.txt/` (edge case: directory with file extension)
+- `test_data/test1/hardlinks/` — test directory with hard-linked files (original.txt, hardlink1.txt, hardlink2.txt)
+- `test_data/test1/symlinks/` — test directory with symbolic links
+- `test_data/test1/empty_dir/` — empty directory for testing
+- `test_data/tmp/` — reserved for test temporary files (use instead of system `/tmp`)
 
 ### Test Conventions
 - Reset global config: `C = Config{}`
 - Reset exclude patterns: `excludePatterns = nil`
 - Use `t.TempDir()` for temporary files
 - Helper function `writeTempConfig()` for config tests
+- Test data created by TestMain setup, cleaned after tests
+- Always check errors with meaningful diagnostics (use `t.Fatalf` for setup errors)
 
 ## Build
 ```bash
-make           # Build binary (version from git or "dev")
+make           # Clean + build binary (version from git or "dev")
 make build VERSION=0.1.0  # Build with specific version
-make clean     # Remove binary
-make test      # Run tests
+make clean     # Remove binary, test_data/tmp, test_data/test1
+make test      # Clean + test (deterministic)
 make upgrade   # Update dependencies
 make help      # Show targets
 ```

test_data/test1/bar/goodbye.txt/text.txt

@@ -1 +0,0 @@
-Fucked up beyond recognotion.

test_data/test1/foo/hello_breakfast.txt

@@ -1,2 +0,0 @@
-My dear Tweety,
-I'd like to greet you as my breakfast.

test_data/test1/foo/mydir/myfile.txt

@@ -1 +0,0 @@
-This is MY file!