weatherreporter/internal/fileutil/fileutil.go

// Package fileutil provides narrow filesystem helpers for operator-owned outputs.
package fileutil

import (
	"context"
	"fmt"
	"os"
	"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. It accepts only an
// absent path or an existing regular file.
func ValidateAtomicPath(path string) error {
	if len(filepath.Base(path)) > maxFileNameBytes {
		return fmt.Errorf("final file name exceeds the %d-byte limit", maxFileNameBytes)
	}
	info, err := os.Lstat(path)
	if os.IsNotExist(err) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("inspect final destination %q: %w", path, err)
	}
	if !info.Mode().IsRegular() {
		return fmt.Errorf("final destination %q must be absent or a regular file", path)
	}
	return nil
}

func WriteFileAtomic(path string, data []byte) error {
	return WriteFileAtomicContext(context.Background(), path, data)
}

// WriteFileAtomicContext writes data through a same-directory temporary file.
// It checks cancellation immediately before the rename that publishes the file.
func WriteFileAtomicContext(ctx context.Context, path string, data []byte) error {
	if ctx == nil {
		ctx = context.Background()
	}
	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), temporaryFilePattern)
	if err != nil {
		return fmt.Errorf("create temporary file for %q: %w", path, err)
	}
	tmpName := tmp.Name()
	defer os.Remove(tmpName)
	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		return fmt.Errorf("write temporary file for %q: %w", path, err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("close temporary file for %q: %w", path, err)
	}
	if err := ValidateAtomicPath(path); err != nil {
		return err
	}
	if err := ctx.Err(); err != nil {
		return err
	}
	if err := os.Rename(tmpName, path); err != nil {
		return fmt.Errorf("save %q: %w", path, err)
	}
	return nil
}