feat(go): expose rule's metadata in Go API

[codedninja]

Reference issue: #120 & #118

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[plusvic]

Now that I see an implementation I'm reconsidering the use of JSON as a format for exchanging metadata information. There are some issues with JSON that make it far from ideal:

• We lose information about whether a number is actually a float or an integer.
• For non-UTF8 strings we are going to get an array of numbers, which will be translated to []float instead of []byte.

[codedninja]

When I wrote the tests I noticed that as well, maybe a similar implementation as the original Yara did? Which is what I was originally aiming for when attempting to implement it in the C-API.

struct YR_META
{
  DECLARE_REFERENCE(const char*, identifier);
  DECLARE_REFERENCE(const char*, string);

  int64_t integer;
  int32_t type;
  int32_t flags;
};

[plusvic]

I've refactored the C API for obtaining the rule's metadata. See: 9f90eaab37a764cc90c40881fc60e448483bb7bf

[codedninja]

There is a slight error with the float variant inside of the YRX_METADATA_VALUE union. When the header file gets generated it makes it into "two or more data types in declared specifier" due to float being a reserved keyword.

While you decide on the new name for the float variant, I shall temporary rename to get the go package working.

[plusvic]

There is a slight error with the float variant inside of the YRX_METADATA_VALUE union. When the header file gets generated it makes it into "two or more data types in declared specifier" due to float being a reserved keyword.

While you decide on the new name for the float variant, I shall temporary rename to get the go package working.

Fixed in 497c678cb288dbfdfc12415741e301d724735c97

[codedninja]

Union types are represented as byte arrays in golang but I was able to get it working with that limitation.

[plusvic]

I've added some helper functions for accessing the metadata values in a safer, more legible way.