// Copyright 2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package sysapi

import (
	"bytes"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"syscall"
)

// AtomicWriteFile atomically writes a file.
func AtomicWriteFile(name string, content []byte, perm os.FileMode) error {

	// Write to a temporary file in the same directory first. This is to
	// guarantee the the temporary file and the intended file are on the
	// same filesystem.
	dir := filepath.Dir(name)
	tmpFile, err := ioutil.TempFile(dir, "atom_write")
	if err != nil {
		return err
	}

	// Write the file. os.Write() is guaranteed to return a non-nil error
	// when the number of bytes written != len(content).
	_, err = tmpFile.Write(content)
	if err1 := tmpFile.Close(); err == nil {
		err = err1
	}
	if err != nil {
		os.Remove(tmpFile.Name())
		return err
	}

	// Rename the temporary file to the desired name. This is an atomic
	// operation on POSIX compliant file systems.
	return syscall.Rename(tmpFile.Name(), name)
}

// RunCommand returns the stdout and stderr produced by running the named
// program with the given arguments.
func RunCommand(name string, args ...string) ([]byte, []byte, error) {
	cmd := exec.Command(name, args...)

	var stdout, stderr bytes.Buffer
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr

	if err := cmd.Run(); err != nil {
		return nil, nil, err
	}

	return stdout.Bytes(), stderr.Bytes(), nil
}

// FileExists checks if the named file exists.
func FileExists(path string) bool {
	if _, err := os.Stat(path); os.IsNotExist(err) {
		return false
	}
	return true
}