dotcloud/docker

// Package pidfile provides structure and helper functions to create and remove
// PID file. A PID file is usually a file used to store the process ID of a
// running process.
package pidfile // import "github.com/docker/docker/pkg/pidfile"

import (
    "bytes"
    "fmt"
    "os"
    "strconv"

    "github.com/docker/docker/pkg/process"
)

// Read reads the "PID file" at path, and returns the PID if it contains a
// valid PID of a running process, or 0 otherwise. It returns an error when
// failing to read the file, or if the file doesn't exist, but malformed content
// is ignored. Consumers should therefore check if the returned PID is a non-zero
// value before use.
func Read(path string) (pid int, err error) {
    pidByte, err := os.ReadFile(path)
    if err != nil {
        return 0, err
    }
    pid, err = strconv.Atoi(string(bytes.TrimSpace(pidByte)))
    if err != nil {
        return 0, nil
    }
    if pid != 0 && process.Alive(pid) {
        return pid, nil
    }
    return 0, nil
}

// Write writes a "PID file" at the specified path. It returns an error if the
// file exists and contains a valid PID of a running process, or when failing
// to write the file.
func Write(path string, pid int) error {
    if pid < 1 {
        // We might be running as PID 1 when running docker-in-docker,
        // but 0 or negative PIDs are not acceptable.
        return fmt.Errorf("invalid PID (%d): only positive PIDs are allowed", pid)
    }
    oldPID, err := Read(path)
    if err != nil && !os.IsNotExist(err) {
        return err
    }
    if oldPID != 0 {
        return fmt.Errorf("process with PID %d is still running", oldPID)
    }
    return os.WriteFile(path, []byte(strconv.Itoa(pid)), 0o644)
}