Go packaging

[Leo Famulari]

Based on my work creating a go-build-system and packaging a non-trivial
Go application [0], I want to start a discussion on how we can
efficiently package Go software in Guix.

Go software is developed rather differently from most of what we
package, and I think our package abstraction does not fit Go libraries
well.

The primary differences are 0) Go libraries are typically bundled as
source code, 1) Go software is typically unversioned, 2) static archives
(.a) are the norm, and 3) Go libraries are expected to be built as part
of the build process of the calling application. That is, there is no
standard way to build an entire Go library on its own.

Elaboration:

0,1) The problem with every application bundling unversioned libraries
is that we need to package a different Git commit of the library for
every application we package, or risk our packages not working. Adding
package variants per-version is a bit messy currently.

3) As an example of how Go libaries are built piecemeal, take the core
networking library, 'golang.org/x/net' [1]. It includes dozens of
submodules such as bpf, icmp, ipv4, ipv6, etc. There is no way to build
all these submodules with a single command. Instead, each one is built
when it is needed during the build process of the calling application.
There are no build scripts. The library compilation process is
standardized as, for example, `go install golang.org/x/net/ipv4`.

This means that the entire networking library would consist of several
dozen Guix packages, multiplied by the number of different Git commits
required by the calling applications. It's unreasonable, in my opinion.

My suggestion is that we have two layers of Go library packages: a) a
template layer that includes the source URI, unpack-path, and other
package metadata, and b) a layer to create instances of the package
within the inputs field of the calling package.

Perhaps the instantiation layer could look like this in practice:

(define-public my-go-program
  [...]
  (inputs
   `(("golang.org/x/net"
      ,(go-package golang-org-x-net
        (version "ffcf1bedda")
        (import-paths '("golang.org/x/net/ipv4"
                        "golang.org/x/net/context"
                        "golang.org/x/net/ipv6"
                        "golang.org/x/net/internal/iana")))))))

Or something like that. What do you think?

[0]
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28586#26
[1]
https://godoc.org/golang.org/x/net

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