add AtomicWriteFile for replacing a file in one step

AtomicWriteFile writes to a temporary file in the same directory, flushes it,
sets the mode and renames it over the target, so a reader never sees a
half-written file and a failure leaves the old one untouched. Comes with a doc
comment and table-driven tests. (closes #17)

Model: opus-5

atomicwrite_test.go

@@ -0,0 +1,110 @@
+package util
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAtomicWriteFile(t *testing.T) {
+	tests := []struct {
+		name     string
+		existing string
+		data     []byte
+		perm     os.FileMode
+	}{
+		{"a new file", "", []byte("hello"), 0644},
+		{"replacing a shorter file", "old", []byte("a much longer set of contents"), 0644},
+		{"replacing a longer file", "a much longer set of contents", []byte("new"), 0644},
+		{"an empty payload", "something", []byte{}, 0644},
+		{"a nil payload", "something", nil, 0644},
+		{"a private mode", "", []byte("secret"), 0600},
+		{"an executable mode", "", []byte("#!/bin/sh\n"), 0755},
+		{"binary contents", "", []byte{0x00, 0xff, 0x10, 0x00}, 0644},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			path := filepath.Join(directory, "target")
+
+			if test.existing != "" {
+				if err := os.WriteFile(path, []byte(test.existing), 0666); err != nil {
+					t.Fatalf("could not write the file to be replaced: %v", err)
+				}
+			}
+
+			if err := AtomicWriteFile(path, test.data, test.perm); err != nil {
+				t.Fatalf("did not expect an error, got %v", err)
+			}
+
+			written, err := os.ReadFile(path)
+			if err != nil {
+				t.Fatalf("could not read the file back: %v", err)
+			}
+			if string(written) != string(test.data) {
+				t.Errorf("expected contents %q got %q", string(test.data), string(written))
+			}
+
+			info, err := os.Stat(path)
+			if err != nil {
+				t.Fatalf("could not look at the file: %v", err)
+			}
+			if info.Mode().Perm() != test.perm {
+				t.Errorf("expected mode %v got %v", test.perm, info.Mode().Perm())
+			}
+
+			entries, err := os.ReadDir(directory)
+			if err != nil {
+				t.Fatalf("could not list the directory: %v", err)
+			}
+			if len(entries) != 1 {
+				names := make([]string, 0, len(entries))
+				for _, entry := range entries {
+					names = append(names, entry.Name())
+				}
+				t.Errorf("expected only the target file to be left, got %v", names)
+			}
+		})
+	}
+}
+
+func TestAtomicWriteFileReportsAMissingDirectory(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "not-there", "target")
+
+	if err := AtomicWriteFile(path, []byte("hello"), 0644); err == nil {
+		t.Errorf("expected an error for a directory that does not exist")
+	}
+}
+
+func TestAtomicWriteFileLeavesTheOldFileAloneOnFailure(t *testing.T) {
+	directory := t.TempDir()
+	path := filepath.Join(directory, "target")
+
+	if err := os.WriteFile(path, []byte("original"), 0644); err != nil {
+		t.Fatalf("could not write the file to be replaced: %v", err)
+	}
+
+	// A directory that cannot be written to means the temporary file cannot
+	// be created, so the write has to fail before anything is replaced.
+	if err := os.Chmod(directory, 0500); err != nil {
+		t.Fatalf("could not change the directory mode: %v", err)
+	}
+	defer os.Chmod(directory, 0700)
+
+	if os.Geteuid() == 0 {
+		t.Skip("running as root, which ignores the directory mode")
+	}
+
+	if err := AtomicWriteFile(path, []byte("replacement"), 0644); err == nil {
+		t.Fatalf("expected an error for a directory that cannot be written to")
+	}
+
+	existing, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("could not read the file back: %v", err)
+	}
+	if string(existing) != "original" {
+		t.Errorf("expected the original contents to survive, got %q", string(existing))
+	}
+}