Anonymous functions and NIF resources when working with multiple processes & nodes

Hi.

I’ve started experimenting with adding distributed support to a project of mine:

https://github.com/almightycouch/gitgud

You can see a demo running on fly.io here:

For a little bit of context:

• I have a NIF wrapper around libgit2 written in C.
• Most functions take and return wrapped C pointers (see erl_nif - Resource objects).
• I have implemented a GenServer in order to safely (thread-safety) manipulate a repository from multiple processes simultaneously.

Here’s a brief example of the API:

alias GitRekt.GitAgent

# load repository
{:ok, agent} = GitAgent.start_link("tmp/my-repo.git")

# fetch master branch
{:ok, branch} = GitAgent.branch(agent, "master")

# fetch commit pointed by master
{:ok, commit} = GitAgent.peel(agent, branch)

# fetch commit author & message
{:ok, author} = GitAgent.commit_author(agent, commit)
{:ok, message} = GitAgent.commit_message(agent, commit)

IO.puts "Last commit by #{author.name} <#{author.email}>:"
IO.puts message

In this example,

• agent is a PID,
• branch is a struct in the form of %GitRef{oid: <<...>>, name: "master", type: :branch},
• commit is a struct in the form of %GitCommit{oid: <<...>>, __ref__: c_ptr},
• author is a map in the form of %{name: "...", email: "..."},
• message is a binary.

I’m not very familiar with the internals of the BEAM and I’m not sure what really happens when passing NIF resources around processes. In the example above, commit contains the referenced pointer to an actual C pointer (:__ref__ field).

I assume that because the current process is only holding the pointer, thread-safety is not compromised (manipulating the C pointer is always done in the dedicated GitAgent process).

So here are my first questions:

1. Is a NIF resource object a simple safe-pointer that can be passed around processes?
2. Does any copy operations happens?
3. Is it garbage collected like everything else?

Now GitAgent provides a subset of libgit2 functions. In order to implement more complex things on top, it provides GitAgent.transaction/2:

def resolve_commit_all(agent, commit) do
  GitAgent.transaction(agent, fn repo ->
    with {:ok, author} <- GitAgent.commit_author(repo, commit),
        {:ok, committer} <- GitAgent.commit_committer(repo, commit),
        {:ok, message} <- GitAgent.commit_message(repo, commit),
        {:ok, parents} <- GitAgent.commit_parents(repo, commit),
        {:ok, timestamp} <- GitAgent.commit_timestamp(repo, commit),
        {:ok, gpg_sig} <- GitAgent.commit_gpg_signature(repo, commit) do
      {:ok, %{
        oid: commit.oid,
        author: author,
        committer: committer,
        message: message,
        parents: Enum.to_list(parents),
        timestamp: timestamp,
        gpg_sig: gpg_sig
      }}
    end
  end)
end

• The anonymous functions is executed by the agent process,
• repo is a NIF resource object handle (wraps git_repository),
• commit is not explicitly passed anywhere to it but is captured from the scope,

Again, I assume that things just work™ because my NIF resources (repo and commit) are not violating anything because they are always manipulated from the same process.

So in my adventure of running my project in a distributed environment. I have a few things I’d like to understand.

1. Is it OK to share NIF resources across different nodes with my current approach? Basically, the NIF safe-pointers are used like simple references and are only manipulated from a unique dedicated process.

2. How does the garbage collector handle NIF resources that are shared between multiple processes? What about running process on a different node?

3. How are captured variables handled when running anonymous functions on different nodes? What about NIF resources?

Also I would be very interested in any good papers, docs on the internals of the BEAM for this kind of things. I’ve came across a few interesting topics in the forum

Any tips on how to debug and profile code (memory leaks, garbage collector, etc.) when working with NIFs are also very welcome.