JSON -> Blob conversion makes number parsing hard, particularly for doubles

The root issue is that Blobs' have a lot of numeric types and JSON only has one. The JSON-C library attempts to guess the type and this guess is reflected in the blob format:

    case json_type_int: {
        int64_t i64 = json_object_get_int64(obj);
        if (i64 >= INT32_MIN && i64 <= INT32_MAX) {
            blobmsg_add_u32(b, name, (uint32_t)i64);
        } else {
            blobmsg_add_u64(b, name, (uint64_t)i64);
        }
        break;
    }
    case json_type_double:
        blobmsg_add_double(b, name, json_object_get_double(obj));
        break;

However, consider the case of passing a double in via JSON. The upstream code might pass something like 2.1, or it might pass something like 2, noting that JSON/Javascript thinks of all things as Numbers; the former will be turned into a BLOBMSG_TYPE_DOUBLE while the latter will be turned into a BLOBMSG_TYPE_INT32. As far as I understand it, the only way to handle this with a policy when parsing the resulting message is to use BLOBMSG_TYPE_UNSPEC and then write your own conversion. e.g.

static inline double blobmsg_cast_double(struct blob_attr *attr)
{
    double tmp = 0;

    if (blobmsg_type(attr) == BLOBMSG_TYPE_INT64)
        tmp = blobmsg_get_u64(attr);
    else if (blobmsg_type(attr) == BLOBMSG_TYPE_INT32)
        tmp = (int32_t)blobmsg_get_u32(attr);
    else if (blobmsg_type(attr) == BLOBMSG_TYPE_INT16)
        tmp = (int16_t)blobmsg_get_u16(attr);
    else if (blobmsg_type(attr) == BLOBMSG_TYPE_INT8)
        tmp = (int8_t)blobmsg_get_u8(attr);
    else if (blobmsg_type(attr) == BLOBMSG_TYPE_DOUBLE)
        tmp = blobmsg_get_double(attr);

    return tmp;
}

This whole situation makes me a little uncomfortable. My preference would be for all JSON numbers to be represented as doubles in the blobmsg, but this is hardly backwards compatible. A conservative approach would be to add a BLOBMSG_CAST_DOUBLE type (cf BLOBMSG_CAST_INT64) and the helper functions above. I'd also like there to be equivalent double->int casting functions, potentially changing the behaviour of BLOBMSG_CAST_INT64 to support doubles.

Thoughts?

Start by tracing the JSON-to-blob conversion and the existing BLOBMSG_CAST_INT64 policy and helper APIs. Review the compatibility implications described in the issue, then establish the agreed numeric casting behavior and add coverage for integer and double inputs once the design is settled.