Allow for empty extension values

While not spec compliant, some implementations allow for empty extension
values. This aligns with our behavior for empty foundation values.
And makes the parser more forgiving for bad implementations.

candidate_base.go

@@ -1032,19 +1032,17 @@ func unmarshalCandidateExtensions(raw string) (extensions []CandidateExtension,
 		}
 		i = next
 
-		if i >= len(raw) {
-			return extensions, "", fmt.Errorf(
-				"%w: missing value for %s in %s", errParseExtension, key, raw, //nolint: errorlint // we are wrapping the error
-			)
-		}
-
-		value, next, err := readCandidateByteString(raw, i)
-		if err != nil {
-			return extensions, "", fmt.Errorf(
-				"%w: failed to read value %v", errParseExtension, err, //nolint: errorlint // we are wrapping the error
-			)
+		// while not spec-compliant, we allow for empty values, as seen in the wild
+		var value string
+		if i < len(raw) {
+			value, next, err = readCandidateByteString(raw, i)
+			if err != nil {
+				return extensions, "", fmt.Errorf(
+					"%w: failed to read value %v", errParseExtension, err, //nolint: errorlint // we are wrapping the error
+				)
+			}
+			i = next
 		}
-		i = next
 
 		if key == "tcptype" {
 			rawTCPTypeRaw = value

candidate_test.go

@@ -698,8 +698,20 @@ func TestCandidateExtensionsMarshal(t *testing.T) {
 			"1052353102 1 tcp 2128609279 192.168.0.196 0 typ host",
 		},
 		{
-			[]CandidateExtension{},
-			"1052353102 1 tcp 2128609279 192.168.0.196 0 typ host",
+			[]CandidateExtension{
+				{"tcptype", "active"},
+				{"empty-value-1", ""},
+				{"empty-value-2", ""},
+			},
+			"1052353102 1 tcp 2128609279 192.168.0.196 0 typ host tcptype active empty-value-1  empty-value-2",
+		},
+		{
+			[]CandidateExtension{
+				{"tcptype", "active"},
+				{"empty-value-1", ""},
+				{"empty-value-2", ""},
+			},
+			"1052353102 1 tcp 2128609279 192.168.0.196 0 typ host tcptype active empty-value-1  empty-value-2 ",
 		},
 	}
 
@@ -967,16 +979,10 @@ func TestUnmarshalCandidateExtensions(t *testing.T) {
 			},
 			fail: false,
 		},
-		{
-			name:     "invalid extension string",
-			value:    "invalid",
-			expected: []CandidateExtension{},
-			fail:     true,
-		},
 		{
 			name:     "invalid extension",
-			value:    " a b",
-			expected: []CandidateExtension{{"a", "b"}, {"c", "d"}},
+			value:    " a b d",
+			expected: []CandidateExtension{{"", "a"}, {"b", "d"}},
 			fail:     true,
 		},
 	}
@@ -1357,6 +1363,27 @@ func TestCandidateAddExtension(t *testing.T) {
 
 		require.Error(t, candidate.AddExtension(CandidateExtension{"tcptype", "INVALID"}))
 	})
+
+	t.Run("Add empty extension", func(t *testing.T) {
+		candidate, err := NewCandidateHost(&CandidateHostConfig{
+			Network:    NetworkTypeUDP4.String(),
+			Address:    "fcd9:e3b8:12ce:9fc5:74a5:c6bb:d8b:e08a",
+			Port:       53987,
+			Priority:   500,
+			Foundation: "750",
+		})
+		if err != nil {
+			t.Error(err)
+		}
+
+		require.Error(t, candidate.AddExtension(CandidateExtension{"", ""}))
+
+		require.NoError(t, candidate.AddExtension(CandidateExtension{"a", ""}))
+
+		extensions := candidate.Extensions()
+
+		require.Equal(t, []CandidateExtension{{"a", ""}}, extensions)
+	})
 }
 
 func TestCandidateRemoveExtension(t *testing.T) {