types/key: add missing ChallengePublic.UnmarshalText

Forgot it when adding the Challenge types earlier.

Change-Id: Ie0872c4e6dc25e5d832aa58c7b3f66d450bf6b71
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/key/chal.go

@@ -7,6 +7,7 @@ package key
 import (
 	"errors"
 
+	"go4.org/mem"
 	"tailscale.com/types/structs"
 )
 
@@ -76,3 +77,8 @@ func (k ChallengePublic) String() string {
 func (k ChallengePublic) MarshalText() ([]byte, error) {
 	return toHex(k.k[:], chalPublicHexPrefix), nil
 }
+
+// UnmarshalText implements encoding.TextUnmarshaler.
+func (k *ChallengePublic) UnmarshalText(b []byte) error {
+	return parseHex(k.k[:], mem.B(b), mem.S(chalPublicHexPrefix))
+}

types/key/node_test.go

@@ -142,3 +142,19 @@ func TestNodeWriteRawWithoutAllocating(t *testing.T) {
 		t.Fatalf("WriteRawWithoutAllocating got %f allocs, want %f", got, want)
 	}
 }
+
+func TestChallenge(t *testing.T) {
+	priv := NewChallenge()
+	pub := priv.Public()
+	txt, err := pub.MarshalText()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var back ChallengePublic
+	if err := back.UnmarshalText(txt); err != nil {
+		t.Fatal(err)
+	}
+	if back != pub {
+		t.Errorf("didn't round trip: %v != %v", back, pub)
+	}
+}