Make KnownHostsFile safe for concurrent use

tofu.go

@@ -25,18 +25,25 @@ type KnownHosts map[string]Fingerprint
 
 // KnownHostsFile represents a list of known hosts optionally loaded from a file.
 // The zero value for KnownHostsFile represents an empty list ready to use.
+//
+// KnownHostsFile is safe for concurrent use by multiple goroutines.
 type KnownHostsFile struct {
 	KnownHosts
 	out io.Writer
+	mu  sync.Mutex
 }
 
 // SetOutput sets the output to which new known hosts will be written to.
 func (k *KnownHostsFile) SetOutput(w io.Writer) {
+	k.mu.Lock()
+	defer k.mu.Unlock()
 	k.out = w
 }
 
 // Add adds a known host to the list of known hosts.
 func (k *KnownHostsFile) Add(hostname string, fingerprint Fingerprint) {
+	k.mu.Lock()
+	defer k.mu.Unlock()
 	if k.KnownHosts == nil {
 		k.KnownHosts = KnownHosts{}
 	}