detect builder from prefetched github repos

lib/deps_nix.ex

@@ -68,7 +68,7 @@ defmodule DepsNix do
          :ok <- File.write("#{dir}/deps-nix-tarball", body),
          {_, 0} <- System.cmd("tar", ["-xf", "#{dir}/deps-nix-tarball"], cd: dir),
          {output, 0} <- System.cmd("nix", ["hash", "path", path]) do
-      String.trim_trailing(output)
+      {String.trim_trailing(output), find_builder_from_path(path)}
     end
   end
 
@@ -124,6 +124,14 @@ defmodule DepsNix do
     end
   end
 
+  defp find_builder_from_path(path) do
+    cond do
+      File.exists?(Path.join(path, "mix.exs")) -> "buildMix"
+      File.exists?(Path.join(path, "rebar.config")) -> "buildRebar3"
+      true -> :unknown
+    end
+  end
+
   @spec to_opts({list(), any(), any()}) :: Options.t()
   defp to_opts({[], _, _}) do
     %Options{

lib/deps_nix/derivation.ex

@@ -48,21 +48,28 @@ defmodule DepsNix.Derivation do
   def from(%Mix.Dep{scm: Mix.SCM.Git} = dep, options) do
     {:git, url, rev, _} = dep.opts[:lock]
 
-    prefetcher = options.github_prefetcher
-
     case parse_git_url(url) do
       [owner: owner, repo: repo] ->
+        prefetcher = options.github_prefetcher
+
+        {hash, builder} =
+          if prefetcher do
+            prefetcher.(owner, repo, rev)
+          else
+            {"", "buildMix"}
+          end
+
         fetcher = %FetchFromGitHub{
           owner: owner,
           repo: repo,
           rev: rev,
-          hash: if(prefetcher, do: prefetcher.(owner, repo, rev), else: "")
+          hash: hash
         }
 
         new(dep,
           version: rev,
           src: fetcher,
-          builder: "buildMix",
+          builder: builder,
           app_config_path: app_config_path(options)
         )
 

test/deps_nix/derivation_test.exs

@@ -74,7 +74,7 @@ defmodule DepsNix.DerivationTest do
                Derivation.from(dep, %DepsNix.Options{
                  github_prefetcher: fn
                    "code-supply", "mudbrick", ^rev ->
-                     generated_hash
+                     {generated_hash, "buildMix"}
                  end
                })
     end

test/deps_nix_test.exs

@@ -10,15 +10,31 @@ defmodule DepsNixTest do
                "code-supply",
                "deps_nix",
                "8a6c3537c958fe3fd1810d56bdee6c13fb35d089"
-             ) == "sha256-zJOkGOSBBA0Y9HPRmwPmBpqaqsoRa0oR7VjMMyukvX4="
+             ) == {"sha256-zJOkGOSBBA0Y9HPRmwPmBpqaqsoRa0oR7VjMMyukvX4=", "buildMix"}
     end
 
     test "works for repos with ../ relative symlinks" do
       assert DepsNix.github_prefetcher(
                "Strech",
                "avrora",
                "a2df4d8f177dacc7be24aa3e6bc76b52c3f114a9"
-             ) == "sha256-msktKtQGBhe2UrZr9uiKiRFCiXCkFa0+zbOy8KQIhc4="
+             ) == {"sha256-msktKtQGBhe2UrZr9uiKiRFCiXCkFa0+zbOy8KQIhc4=", "buildMix"}
+    end
+
+    test "detects rebar3-only repositories" do
+      assert DepsNix.github_prefetcher(
+               "klarna",
+               "mnesia_eleveldb",
+               "af6d0556a78aec2918b3471f0c85121402a1f5b1"
+             ) == {"sha256-+ZZ5Uyoe/HK0wL0ev1vn9Tuiaps4X88izETtuRszKYE=", "buildRebar3"}
+    end
+
+    test "prefers buildMix over buildRebar3 for repos that have both" do
+      assert DepsNix.github_prefetcher(
+               "erlef",
+               "oidcc",
+               "fdf45b06d79813c7110171c5c6d334394c2f1190"
+             ) == {"sha256-QUwRH9GWMTrG2HsFqiPNNKm+K5+cwPigdtlx9OUNK58=", "buildMix"}
     end
 
     test "fails for nonsense repos" do