feat(fsx): add atomic and durable file writes

internal/fsx/fsx_test.go

@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: MIT
+
+package fsx
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteFileAtomicCreatesAndReplaces(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "nested", "file.json")
+
+	if err := WriteFileAtomic(path, []byte("first"), 0o600); err != nil {
+		t.Fatalf("WriteFileAtomic: %v", err)
+	}
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	if string(got) != "first" {
+		t.Fatalf("content = %q, want %q", got, "first")
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("Stat: %v", err)
+	}
+	if info.Mode().Perm() != 0o600 {
+		t.Fatalf("perm = %o, want 600", info.Mode().Perm())
+	}
+
+	if err := WriteFileAtomic(path, []byte("second"), 0o600); err != nil {
+		t.Fatalf("second WriteFileAtomic: %v", err)
+	}
+	got, _ = os.ReadFile(path)
+	if string(got) != "second" {
+		t.Fatalf("content = %q, want %q", got, "second")
+	}
+}
+
+func TestWriteFileAtomicLeavesNoTempFiles(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "state.json")
+	if err := WriteFileAtomic(path, []byte("x"), 0o644); err != nil {
+		t.Fatalf("WriteFileAtomic: %v", err)
+	}
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("ReadDir: %v", err)
+	}
+	if len(entries) != 1 || entries[0].Name() != "state.json" {
+		t.Fatalf("unexpected directory contents: %v", entries)
+	}
+}