[Libguestfs] [PATCH nbdkit UNFINISHED] Add the ability to write plugins in golang.

Sorry Dan, but I really do dislike golang with a passion :-) Here is a patch that allows you to write nbdkit plugins in golang. As with C, OCaml and Rust, you can write a plugin in Go which compiles directly to a .so file that can be loaded into golang, so in that sense it works completely differently from scripting language plugins like Perl and Python where there's an nbdkit-<lang>-plugin that intermediates between nbdkit and the user’s code. With that said, there are many problems. The root cause of most of them is that you cannot pass Go pointers to C, perhaps because the golang developers never heard of registering extra GC roots[1], even though that is common in non-Blub languages like OCaml and Haskell. This leads to awkward complications nicely summarised in this page: https://eli.thegreenplace.net/2019/passing-callbacks-and-pointers-to-cgo/ This requires that every callback has a wrapper across 3 files. Oh and these wrappers cannot be in a library module, they must be copied into the plugin source code, AND they must be commented out by the end-user by hand if the callback is not called. (There may be a way around the latter issues, but I could not work out how.) This means plugins have tons of duplicated source code. Other issues that are not related to that one: - We have to link the nbdkit module with -Wl,--unresolved-symbols=ignore-in-object-files because of the way nbdkit plugins have deliberately unresolved symbols. See the libnbdkit.so proposal on the mailing list for another way to solve this. - Be nice if ./configure could check that golang >= 1.5 since that was the first version that introduced shared libraries. - Since initialization is not synchronous in golang, you cannot rely on anything being initialized in the plugin before nbdkit starts calling in. For this reason I worked around it by having plugin_init() call a start up function (func init_plugin()) where all golang initialization must be done. Otherwise: nbdkit: golang plugin: dlopen ("plugin.so"); let's start initializing init = dlsym ("plugin_init"); return &plugin (uninitialized!) plugin = init (); plugin.load () hey, I'm still initializing! - Related to the previous point: Be nice to move plugin_init() into the nbdkit module. However I don't believe this is possible because this function has to call into the plugin (in main module). - Tests are a joke at the moment. We would really need a test which properly exercises threads / parallel client connections, so we can be sure that the nbdkit thread & golang goroutine models do not conflict in some way. (I don't think they do, but need to check). - Current test func pluginPRead() needs to be completed. - Documentation needs fixing. I didn't want to write too many docs until I know finally how plugins would work. Rich. [1] Reading the proposal here confirms my suspicions, since there is a much simpler, better and more obvious solution than what is proposed: https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md