feat(platform): add WriteUserFile atomic user-space writer

internal/platform/writefile_test.go

@@ -0,0 +1,54 @@
+package platform
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteUserFileCreatesParent(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "a", "b", "c.txt")
+	if err := WriteUserFile(dest, []byte("hi"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := os.ReadFile(dest)
+	if string(got) != "hi" {
+		t.Fatalf("content %q", got)
+	}
+	info, _ := os.Stat(dest)
+	if info.Mode().Perm() != 0o644 {
+		t.Fatalf("mode %v", info.Mode().Perm())
+	}
+}
+
+func TestWriteUserFileOverwritesAtomically(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "x")
+	if err := WriteUserFile(dest, []byte("one"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := WriteUserFile(dest, []byte("two"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := os.ReadFile(dest)
+	if string(got) != "two" {
+		t.Fatalf("got %q want two", got)
+	}
+	// No .tmp sibling should be left behind.
+	if _, err := os.Stat(dest + ".tmp"); !os.IsNotExist(err) {
+		t.Fatalf(".tmp left behind: %v", err)
+	}
+}
+
+func TestWriteUserFileExecMode(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "bin")
+	if err := WriteUserFile(dest, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	info, _ := os.Stat(dest)
+	if info.Mode().Perm() != 0o755 {
+		t.Fatalf("mode %v want 0755", info.Mode().Perm())
+	}
+}