Simplify JSON number decoding

Simplifiy implementation and, as a bonus, allow overflowing floating numbers to convert to +/-Infinity like they do in JSON.parse.

Also update outdated comment about NaN and Infinity being encoded as `null`.

c++/src/capnp/compat/json-test.c++

@@ -258,6 +258,8 @@ KJ_TEST("decode all types") {
   CASE_NO_ROUNDTRIP(R"({"float32Field":"-infinity"})", root.getFloat32Field() == -kj::inf());
   CASE_NO_ROUNDTRIP(R"({"float32Field":"INF"})", root.getFloat32Field() == kj::inf());
   CASE_NO_ROUNDTRIP(R"({"float32Field":"-INF"})", root.getFloat32Field() == -kj::inf());
+  CASE_NO_ROUNDTRIP(R"({"float32Field":1e39})", root.getFloat32Field() == kj::inf());
+  CASE_NO_ROUNDTRIP(R"({"float32Field":-1e39})", root.getFloat32Field() == -kj::inf());
   CASE_NO_ROUNDTRIP(R"({"float64Field":0})", root.getFloat64Field() == 0);
   CASE(R"({"float64Field":4.5})", root.getFloat64Field() == 4.5);
   CASE_NO_ROUNDTRIP(R"({"float64Field":null})", kj::isNaN(root.getFloat64Field()));
@@ -268,6 +270,8 @@ KJ_TEST("decode all types") {
   CASE_NO_ROUNDTRIP(R"({"float64Field":"-infinity"})", root.getFloat64Field() == -kj::inf());
   CASE_NO_ROUNDTRIP(R"({"float64Field":"INF"})", root.getFloat64Field() == kj::inf());
   CASE_NO_ROUNDTRIP(R"({"float64Field":"-INF"})", root.getFloat64Field() == -kj::inf());
+  CASE_NO_ROUNDTRIP(R"({"float64Field":1e309})", root.getFloat64Field() == kj::inf());
+  CASE_NO_ROUNDTRIP(R"({"float64Field":-1e309})", root.getFloat64Field() == -kj::inf());
   CASE(R"({"textField":"hello"})", kj::str("hello") == root.getTextField());
   CASE(R"({"dataField":[7,0,122]})",
       kj::heapArray<byte>({7,0,122}).asPtr() == root.getDataField());
@@ -531,20 +535,6 @@ KJ_TEST("basic json decoding") {
     KJ_EXPECT_THROW_MESSAGE("Unexpected", json.decodeRaw("+123", root));
   }
 
-  {
-    MallocMessageBuilder message;
-    auto root = message.initRoot<JsonValue>();
-
-    KJ_EXPECT_THROW_MESSAGE("Overflow", json.decodeRaw("1e1024", root));
-  }
-
-  {
-    MallocMessageBuilder message;
-    auto root = message.initRoot<JsonValue>();
-
-    KJ_EXPECT_THROW_MESSAGE("Underflow", json.decodeRaw("1e-1023", root));
-  }
-
   {
     MallocMessageBuilder message;
     auto root = message.initRoot<JsonValue>();

c++/src/capnp/compat/json.c++

@@ -658,19 +658,7 @@ public:
   }
 
   void parseNumber(JsonValue::Builder& output) {
-    auto numberStr = consumeNumber();
-    char *endPtr;
-
-    errno = 0;
-    double value = strtod(numberStr.begin(), &endPtr);
-
-    KJ_ASSERT(endPtr != numberStr.begin(), "strtod should not fail! Is consumeNumber wrong?");
-    KJ_REQUIRE((value != HUGE_VAL && value != -HUGE_VAL) || errno != ERANGE,
-        "Overflow in JSON number.");
-    KJ_REQUIRE(value != 0.0 || errno != ERANGE,
-        "Underflow in JSON number.");
-
-    output.setNumber(value);
+    output.setNumber(consumeNumber().parseAs<double>());
   }
 
   void parseString(JsonValue::Builder& output) {

c++/src/capnp/compat/json.h

@@ -49,7 +49,7 @@ class JsonCodec {
   // - 64-bit integers are encoded as strings, since JSON "numbers" are double-precision floating
   //   points which cannot store a 64-bit integer without losing data.
   // - NaNs and infinite floating point numbers are not allowed by the JSON spec, and so are encoded
-  //   as null. This matches the behavior of `JSON.stringify` in at least Firefox and Chrome.
+  //   as strings.
   // - Data is encoded as an array of numbers in the range [0,255]. You probably want to register
   //   a handler that does something better, like maybe base64 encoding, but there are a zillion
   //   different ways people do this.