Reject structurally invalid request IDs.

The JSON-RPC 2.0 spec stipulates that request IDs should be strings or integral
number values. Prior to this commit, the library would accept basically any
JSON value as an ID. Now we reject objects (other than null), Booleans, and
arrays. The spec also says that fractional numbers should not be used, but we
still accept them.

Explicitly setting "id" to null is treated as equivalent to omitting the "id".

base.go

@@ -286,6 +286,20 @@ type jmessage struct {
 	err   error // if not nil, this message is invalid and err is why
 }
 
+// isValidID reports whether v is a valid JSON encoding of a request ID.
+// Precondition: v is a valid JSON value, or empty.
+func isValidID(v json.RawMessage) bool {
+	if len(v) == 0 || isNull(v) {
+		return true // nil or empty is OK, as is "null"
+	} else if v[0] == '"' || v[0] == '-' || (v[0] >= '0' && v[0] <= '9') {
+		return true // strings and numbers are OK
+
+		// N.B. This definition does not reject fractional numbers, although the
+		// spec says numeric IDs should not have fractional parts.
+	}
+	return false // anything else is garbage
+}
+
 func (j *jmessage) fail(code code.Code, msg string) error {
 	j.err = Errorf(code, msg)
 	return j.err
@@ -312,7 +326,11 @@ func (j *jmessage) parseJSON(data []byte) error {
 				j.fail(code.ParseError, "invalid version key")
 			}
 		case "id":
-			j.ID = val
+			if isValidID(val) {
+				j.ID = val
+			} else {
+				j.fail(code.InvalidRequest, "invalid request ID")
+			}
 		case "method":
 			if json.Unmarshal(val, &j.M) != nil {
 				j.fail(code.ParseError, "invalid method name")

jrpc2_test.go

@@ -588,6 +588,7 @@ func TestOtherClient(t *testing.T) {
 		}
 	}()
 
+	const invalidIDMessage = `{"jsonrpc":"2.0","id":null,"error":{"code":-32600,"message":"invalid request ID"}}`
 	tests := []struct {
 		input, want string
 	}{
@@ -663,6 +664,13 @@ func TestOtherClient(t *testing.T) {
 		// A broken single request should report a top-level error.
 		{`{"bogus"][++`,
 			`{"jsonrpc":"2.0","id":null,"error":{"code":-32700,"message":"invalid request message"}}`},
+
+		// Various invalid ID checks.
+		{`{"jsonrpc":"2.0", "id":[], "method":"X"}`, invalidIDMessage},    // invalid ID: array
+		{`{"jsonrpc":"2.0", "id":["q"], "method":"X"}`, invalidIDMessage}, // "
+		{`{"jsonrpc":"2.0", "id":{}, "method":"X"}`, invalidIDMessage},    // invalid ID: object
+		{`{"jsonrpc":"2.0", "id":true, "method":"X"}`, invalidIDMessage},  // invalid ID: Boolean
+		{`{"jsonrpc":"2.0", "id":false, "method":"X"}`, invalidIDMessage}, // "
 	}
 	for _, test := range tests {
 		if err := cli.Send([]byte(test.input)); err != nil {