Added a mutex for Listeners struct for thread safety.

internal/radio/listeners.go

@@ -4,10 +4,12 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"sync"
 )
 
 // Listeners stores the current and peak numbers of listeners.
 type Listeners struct {
+	mut           sync.Mutex
 	current, peak int
 }
 
@@ -18,16 +20,22 @@ func NewListeners() *Listeners {
 
 // Current returns a number of current listeners.
 func (l *Listeners) Current() int {
+	l.mut.Lock()
+	defer l.mut.Unlock()
 	return l.current
 }
 
 // Current returns a number of peak listeners.
 func (l *Listeners) Peak() int {
+	l.mut.Lock()
+	defer l.mut.Unlock()
 	return l.peak
 }
 
 // Inc increments by 1 a current number of listeners and updates a peak number.
 func (l *Listeners) Inc() int {
+	l.mut.Lock()
+	defer l.mut.Unlock()
 	if l.current == math.MaxInt {
 		// We panic here because if this will ever happen, then something's going certainly wrong.
 		panic(fmt.Sprint("a current number of listeners exceeded MaxInt which is", math.MaxInt))
@@ -42,6 +50,8 @@ func (l *Listeners) Inc() int {
 // Dec decrements by 1 a current number of listeners. An error will occur if
 // a resulting number is less than 0.
 func (l *Listeners) Dec() (int, error) {
+	l.mut.Lock()
+	defer l.mut.Unlock()
 	if l.current == 0 {
 		return l.current, errors.New("an attempt to decrement a number of current listeners down to less than 0")
 	}