saferio

saferio is a small Go library for safely handling temporary files and logs in monitoring-forge Mackerel plugins.

It provides helpers to read and write JSON files while reducing the risk of symlink-based attacks and partial writes.

Features

• Atomic JSON writes: WriteJSON writes to a temporary file and renames it into place, minimizing the chance of leaving a partially written file.
• Safe file opening: OpenRD / OpenAD open files with O_NOFOLLOW on Unix-like systems to help prevent symlink attacks.
• Cross-platform: Separate implementations for Unix and Windows.
• Small surface: Only three public functions plus platform-specific open helpers.

Usage

package main

import (
	"log"

	"github.com/monitoring-forge/saferio"
)

func main() {
	dir := "/var/lib/mackerel-plugin"

	// Write metrics state atomically.
	state := map[string]any{"counter": 42}
	if err := saferio.WriteJSON(dir, "mackerel-plugin-foo.json", state); err != nil {
		log.Fatal(err)
	}

	// Read it back safely.
	var loaded map[string]any
	if err := saferio.ReadJSON(dir, "mackerel-plugin-foo.json", &loaded); err != nil {
		log.Fatal(err)
	}

	// Check for a file presence.
	if saferio.FileExists(dir, "mackerel-plugin-foo.json") {
		log.Println("state file exists")
	}
}

API

FileExists(dir, filename string) bool

Returns true if the file exists in dir.

WriteJSON(dir, filename string, v any) error

Encodes v as JSON to a temporary file in dir and renames it to filename. If filename already exists, it attempts to replace it atomically.

ReadJSON(dir, filename string, v any) error

Opens filename safely (with O_NOFOLLOW on Unix) and decodes its JSON content into v.

If the file is empty, ReadJSON returns io.EOF. Callers that want to treat an empty file as "no state" can check for this error explicitly.

OpenRD(filename string) (*os.File, error)

Opens a file for reading with O_RDONLY. On Unix-like systems, O_NOFOLLOW is added.

OpenAD(filename string) (*os.File, error)

Opens a file for appending with O_WRONLY | O_CREATE | O_APPEND. On Unix-like systems, O_NOFOLLOW is added.

Platform-specific behavior

Function	Unix-like	Windows
OpenRD	O_RDONLY | O_NOFOLLOW	O_RDONLY
OpenAD	O_WRONLY | O_CREATE | O_APPEND | O_NOFOLLOW	O_WRONLY | O_CREATE | O_APPEND

Windows does not support O_NOFOLLOW, so the flag is omitted on that platform.

License

MIT License - see LICENSE for details.