Add locks to safelog

The safelog Write function can be called from multiple go routines, and
it was not thread safe. These locks in particular allow us to pass the
logscrubber's output io.Writer to other libraries, such as pion.

common/safelog/log.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"io"
 	"regexp"
+	"sync"
 )
 
 const ipv4Address = `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}`
@@ -30,8 +31,13 @@ var addressRegexp = regexp.MustCompile(addressPattern)
 type LogScrubber struct {
 	Output io.Writer
 	buffer []byte
+
+	lock sync.Mutex
 }
 
+func (ls *LogScrubber) Lock()   { (*ls).lock.Lock() }
+func (ls *LogScrubber) Unlock() { (*ls).lock.Unlock() }
+
 func scrub(b []byte) []byte {
 	scrubbedBytes := b
 	for _, pattern := range scrubberPatterns {
@@ -45,6 +51,9 @@ func scrub(b []byte) []byte {
 }
 
 func (ls *LogScrubber) Write(b []byte) (n int, err error) {
+	ls.Lock()
+	defer ls.Unlock()
+
 	n = len(b)
 	ls.buffer = append(ls.buffer, b...)
 	for {