add TruncateString for shortening a string safely

TruncateString returns at most the requested number of characters from the
start of a string, counting characters rather than bytes so a multi-byte
character is never cut in half. Comes with a doc comment and table-driven
tests. (closes #5)

Model: opus-5

atomicwrite.go

@@ -1,56 +0,0 @@
-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

@@ -1,110 +0,0 @@
-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))
-	}
-}

truncate.go

@@ -0,0 +1,21 @@
+package util
+
+// TruncateString returns at most max characters from the start of s. It counts
+// characters rather than bytes, so a multi-byte character is never cut in half
+// and the result is always valid text. A max of zero or less returns an empty
+// string, and a string already at or below the limit is returned unchanged.
+func TruncateString(s string, max int) string {
+	if max <= 0 {
+		return ""
+	}
+
+	count := 0
+	for offset := range s {
+		if count == max {
+			return s[:offset]
+		}
+		count++
+	}
+
+	return s
+}

truncate_test.go

@@ -0,0 +1,41 @@
+package util
+
+import "testing"
+
+func TestTruncateString(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		max      int
+		expected string
+	}{
+		{"shorter than the limit", "abc", 5, "abc"},
+		{"exactly at the limit", "abc", 3, "abc"},
+		{"longer than the limit", "abcdef", 3, "abc"},
+		{"max of one", "abc", 1, "a"},
+		{"max of zero", "abc", 0, ""},
+		{"negative max", "abc", -1, ""},
+		{"empty string", "", 3, ""},
+		{"multi-byte characters kept whole", "héllo", 3, "hél"},
+		{"multi-byte characters below the limit", "héllo", 99, "héllo"},
+		{"characters outside the basic plane", "a😀b", 2, "a😀"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := TruncateString(test.input, test.max)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}
+
+func TestTruncateStringCountsCharactersNotBytes(t *testing.T) {
+	// Each of these characters is two bytes long, so a byte-based cut at
+	// three would leave half a character behind.
+	got := TruncateString("äöü", 3)
+	if got != "äöü" {
+		t.Errorf("expected %q got %q", "äöü", got)
+	}
+}