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.go

@@ -0,0 +1,56 @@
+package util
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// AtomicWriteFile writes data to path without ever leaving a half-written file
+// behind. It writes to a temporary file in the same directory, flushes it to
+// disk, gives it the mode in perm and renames it over path, so anything reading
+// path sees either the previous contents or the complete new contents and
+// never something in between. If any step fails, the temporary file is removed
+// and path is left as it was.
+//
+// The temporary file has to live in the same directory as path, because a
+// rename across two filesystems is not possible and would not be instant if it
+// were.
+//
+// Two details differ from os.WriteFile. The mode is applied after the file is
+// created, so perm is what the finished file ends up with rather than perm with
+// the process umask taken out of it. And the directory holding the file is not
+// flushed, so a machine that loses power immediately after this returns may
+// come back with the rename undone, even though the data itself was written.
+func AtomicWriteFile(path string, data []byte, perm os.FileMode) error {
+	directory := filepath.Dir(path)
+
+	temporary, err := os.CreateTemp(directory, "."+filepath.Base(path)+".tmp")
+	if err != nil {
+		return err
+	}
+	temporaryPath := temporary.Name()
+
+	// Does nothing once the rename below has succeeded, because by then the
+	// temporary name no longer refers to anything.
+	defer os.Remove(temporaryPath)
+
+	if _, err := temporary.Write(data); err != nil {
+		temporary.Close()
+		return err
+	}
+
+	if err := temporary.Sync(); err != nil {
+		temporary.Close()
+		return err
+	}
+
+	if err := temporary.Close(); err != nil {
+		return err
+	}
+
+	if err := os.Chmod(temporaryPath, perm); err != nil {
+		return err
+	}
+
+	return os.Rename(temporaryPath, path)
+}

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))
+	}
+}

chunk.go

@@ -1,26 +0,0 @@
-package util
-
-// ChunkStrings splits input into consecutive slices of at most size elements.
-// The last slice holds whatever is left over and may be shorter than the
-// others. Each returned slice is a copy, so appending to one of them cannot
-// disturb another. A size below one, or an empty input, returns nil.
-func ChunkStrings(input []string, size int) [][]string {
-	if size < 1 || len(input) == 0 {
-		return nil
-	}
-
-	out := make([][]string, 0, (len(input)+size-1)/size)
-
-	for start := 0; start < len(input); start += size {
-		end := start + size
-		if end > len(input) {
-			end = len(input)
-		}
-
-		chunk := make([]string, end-start)
-		copy(chunk, input[start:end])
-		out = append(out, chunk)
-	}
-
-	return out
-}

chunk_test.go

@@ -1,43 +0,0 @@
-package util
-
-import (
-	"reflect"
-	"testing"
-)
-
-func TestChunkStrings(t *testing.T) {
-	tests := []struct {
-		name     string
-		input    []string
-		size     int
-		expected [][]string
-	}{
-		{"exact multiple", []string{"a", "b", "c", "d"}, 2, [][]string{{"a", "b"}, {"c", "d"}}},
-		{"with a remainder", []string{"a", "b", "c"}, 2, [][]string{{"a", "b"}, {"c"}}},
-		{"size larger than input", []string{"a", "b"}, 5, [][]string{{"a", "b"}}},
-		{"size of one", []string{"a", "b"}, 1, [][]string{{"a"}, {"b"}}},
-		{"size of zero", []string{"a", "b"}, 0, nil},
-		{"negative size", []string{"a", "b"}, -1, nil},
-		{"empty input", []string{}, 2, nil},
-		{"nil input", nil, 2, nil},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			got := ChunkStrings(test.input, test.size)
-			if !reflect.DeepEqual(got, test.expected) {
-				t.Errorf("expected %#v got %#v", test.expected, got)
-			}
-		})
-	}
-}
-
-func TestChunkStringsReturnsCopies(t *testing.T) {
-	input := []string{"a", "b", "c", "d"}
-	chunks := ChunkStrings(input, 2)
-	chunks[0][0] = "changed"
-
-	if input[0] != "a" {
-		t.Errorf("expected the input to be unchanged, got %#v", input)
-	}
-}