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

[codedninja]

Reference issue: #120 & #118

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[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: 9f90eaa

[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 497c678

[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.