Centralize atomic artifact writes

internal/fileutil/fileutil.go

@@ -0,0 +1,48 @@
+// Package fileutil provides narrow filesystem helpers for durable artifacts.
+package fileutil
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+func WriteFileAtomic(path string, data []byte) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("create directory %q: %w", filepath.Dir(path), err)
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(path), "."+filepath.Base(path)+".*.tmp")
+	if err != nil {
+		return fmt.Errorf("create temporary file for %q: %w", path, err)
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName)
+	if _, err := tmp.Write(data); err != nil {
+		tmp.Close()
+		return fmt.Errorf("write temporary file for %q: %w", path, err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("close temporary file for %q: %w", path, err)
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		return fmt.Errorf("save %q: %w", path, err)
+	}
+	return nil
+}
+
+func WriteJSONAtomic(path string, value any) error {
+	data, err := json.MarshalIndent(value, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal %q: %w", path, err)
+	}
+	return WriteFileAtomic(path, data)
+}
+
+func CopyFileAtomic(source string, target string) error {
+	data, err := os.ReadFile(source)
+	if err != nil {
+		return fmt.Errorf("read %q: %w", source, err)
+	}
+	return WriteFileAtomic(target, data)
+}

internal/fileutil/fileutil_test.go

@@ -0,0 +1,103 @@
+package fileutil
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestWriteFileAtomicCreatesParentDirectory(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "nested", "artifact.txt")
+
+	if err := WriteFileAtomic(path, []byte("artifact")); err != nil {
+		t.Fatalf("WriteFileAtomic() error = %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	if string(data) != "artifact" {
+		t.Fatalf("data = %q, want artifact", data)
+	}
+}
+
+func TestWriteFileAtomicOverwritesTarget(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "artifact.txt")
+	if err := WriteFileAtomic(path, []byte("old")); err != nil {
+		t.Fatalf("WriteFileAtomic() initial error = %v", err)
+	}
+
+	if err := WriteFileAtomic(path, []byte("new")); err != nil {
+		t.Fatalf("WriteFileAtomic() overwrite error = %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	if string(data) != "new" {
+		t.Fatalf("data = %q, want new", data)
+	}
+}
+
+func TestWriteFileAtomicCleansTemporaryFileAfterRenameError(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "target")
+	if err := os.Mkdir(target, 0o755); err != nil {
+		t.Fatalf("Mkdir() error = %v", err)
+	}
+
+	err := WriteFileAtomic(target, []byte("data"))
+	if err == nil {
+		t.Fatal("WriteFileAtomic() error = nil, want rename error")
+	}
+	if !strings.Contains(err.Error(), "save") {
+		t.Fatalf("error = %q, want save context", err.Error())
+	}
+	matches, err := filepath.Glob(filepath.Join(dir, ".target.*.tmp"))
+	if err != nil {
+		t.Fatalf("Glob() error = %v", err)
+	}
+	if len(matches) != 0 {
+		t.Fatalf("temporary files = %v, want none", matches)
+	}
+}
+
+func TestWriteJSONAtomic(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "artifact.json")
+
+	if err := WriteJSONAtomic(path, map[string]string{"status": "ok"}); err != nil {
+		t.Fatalf("WriteJSONAtomic() error = %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	if string(data) != "{\n  \"status\": \"ok\"\n}" {
+		t.Fatalf("json = %q, want indented object", data)
+	}
+}
+
+func TestCopyFileAtomic(t *testing.T) {
+	dir := t.TempDir()
+	source := filepath.Join(dir, "source.txt")
+	target := filepath.Join(dir, "nested", "target.txt")
+	if err := os.WriteFile(source, []byte("copied"), 0o600); err != nil {
+		t.Fatalf("WriteFile() error = %v", err)
+	}
+
+	if err := CopyFileAtomic(source, target); err != nil {
+		t.Fatalf("CopyFileAtomic() error = %v", err)
+	}
+
+	data, err := os.ReadFile(target)
+	if err != nil {
+		t.Fatalf("ReadFile() error = %v", err)
+	}
+	if string(data) != "copied" {
+		t.Fatalf("data = %q, want copied", data)
+	}
+}