Rethink the usage of global variables

[adutra]

We have a few global variables, e.g.:

• message.DefaultMessageCodecs
• client.HandshakeHandler
• and all the datatypes and data codecs in datatype package.

Global variables are not guaranteed to be immutable, and even if they are (because the type is an interface with only accessor methods), one could still set the variable to nil.

For example you cannot forbid a user from doing:

message.DefaultMessageCodecs = nil
datatype.Int = datatype.Varchar

... which would screw up the library.

The Go community itself seems to hesitate about using global variables. Several packages have global variables, e.g. time.UTC.

We need to asses if it's safe to keep the global variables around or if we should remove them.

Relevant articles on this topic:

• https://stackoverflow.com/questions/38552095/when-to-use-global-variables
• https://www.reddit.com/r/golang/comments/el49xz/global_variable_vs_function_for_constant_structs/

[adutra]

I'm torn on this one. I think trying to defend ourselves against malicious usage of the library is going to be inefficient. On the other hand, using functions everywhere could hurt the performance.

I'd be in favor of changing just DefaultMessageCodecs as it is a slice and indeed slices could be accidentally modified by client code.