Preflight report output filenames

internal/fileutil/fileutil.go

@@ -8,11 +8,28 @@ import (
 	"path/filepath"
 )
 
+const (
+	maxFileNameBytes     = 255
+	temporaryFilePattern = ".weatherreporter-*.tmp"
+)
+
+// ValidateAtomicPath verifies that the final path can be safely used with this
+// package's same-directory atomic-write implementation.
+func ValidateAtomicPath(path string) error {
+	if len(filepath.Base(path)) > maxFileNameBytes {
+		return fmt.Errorf("final file name exceeds the %d-byte limit", maxFileNameBytes)
+	}
+	return nil
+}
+
 func WriteFileAtomic(path string, data []byte) error {
+	if err := ValidateAtomicPath(path); err != nil {
+		return err
+	}
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return fmt.Errorf("create directory %q: %w", filepath.Dir(path), err)
 	}
-	tmp, err := os.CreateTemp(filepath.Dir(path), "."+filepath.Base(path)+".*.tmp")
+	tmp, err := os.CreateTemp(filepath.Dir(path), temporaryFilePattern)
 	if err != nil {
 		return fmt.Errorf("create temporary file for %q: %w", path, err)
 	}

internal/fileutil/fileutil_test.go

@@ -42,6 +42,31 @@ func TestWriteFileAtomicOverwritesTarget(t *testing.T) {
 	}
 }
 
+func TestWriteFileAtomicSupportsLongestFileName(t *testing.T) {
+	directory := t.TempDir()
+	name := strings.Repeat("a", maxFileNameBytes-len(".md")) + ".md"
+	path := filepath.Join(directory, name)
+	if err := os.WriteFile(path, []byte("old"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := WriteFileAtomic(path, []byte("new")); err != nil {
+		t.Fatalf("WriteFileAtomic() error = %v", err)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil || string(data) != "new" {
+		t.Fatalf("output/error = %q/%v", data, err)
+	}
+	info, err := os.Stat(path)
+	if err != nil || info.Mode().Perm() != 0o600 {
+		t.Fatalf("output mode/error = %o/%v", info.Mode().Perm(), err)
+	}
+	matches, err := filepath.Glob(filepath.Join(directory, ".weatherreporter-*.tmp"))
+	if err != nil || len(matches) != 0 {
+		t.Fatalf("temporary files/error = %v/%v", matches, err)
+	}
+}
+
 func TestWriteFileAtomicCleansTemporaryFileAfterRenameError(t *testing.T) {
 	dir := t.TempDir()
 	target := filepath.Join(dir, "target")
@@ -56,7 +81,7 @@ func TestWriteFileAtomicCleansTemporaryFileAfterRenameError(t *testing.T) {
 	if !strings.Contains(err.Error(), "save") {
 		t.Fatalf("error = %q, want save context", err.Error())
 	}
-	matches, err := filepath.Glob(filepath.Join(dir, ".target.*.tmp"))
+	matches, err := filepath.Glob(filepath.Join(dir, ".weatherreporter-*.tmp"))
 	if err != nil {
 		t.Fatalf("Glob() error = %v", err)
 	}