beamPackages.buildMix: compile non-deterministic deps in $out/src

pkgs/development/beam-modules/build-mix.nix

@@ -3,6 +3,7 @@
   erlang,
   hex,
   beamCopySourceHook,
+  beamCopySourceToSourceRootHook,
   beamModuleInstallHook,
   mixBuildDirHook,
   mixCompileHook,
@@ -33,7 +34,12 @@ lib.extendMkDerivation {
       # Deterministic Erlang builds remove full system paths from debug information
       # among other things to keep builds more reproducible. See their docs for more:
       # https://www.erlang.org/doc/man/compile
-      erlangDeterministicBuilds ? true,
+      #
+      # Default to `true` to reduce the closure size
+      # and prevent `mix` to leave artifacts in `$out/src`
+      # but be aware that it breaks some apps (eg. those using Surface)
+      # see https://github.com/surface-ui/surface/issues/762#issuecomment-3577960256
+      erlangDeterministicBuilds ? finalAttrs.erlangDeterministicBuilds or true,
       ...
     }@args:
     {
@@ -74,7 +80,7 @@ lib.extendMkDerivation {
         elixir
         hex
 
-        beamCopySourceHook
+        (if erlangDeterministicBuilds then beamCopySourceHook else beamCopySourceToSourceRootHook)
         beamModuleInstallHook
         mixBuildDirHook
         mixCompileHook

pkgs/development/beam-modules/default.nix

@@ -92,6 +92,7 @@ let
 
       inherit (callPackages ./hooks { })
         beamCopySourceHook
+        beamCopySourceToSourceRootHook
         beamModuleInstallHook
         mixBuildDirHook
         mixCompileHook

pkgs/development/beam-modules/hooks/beam-copy-source-to-sourceRoot-hook.sh

@@ -0,0 +1,19 @@
+# When using some libraries (eg. surface or fine)
+# building needs to access files in the build path of dependencies
+# as returned by `module_info/1` (which requires `erlangDeterministicBuilds == false`).
+# Hence this hook makes the build happen inside `$out/src` instead of /build/source`.
+#
+# Remark: any `patchPhase` will (correctly) be applied to `$out/src`.
+#
+beamCopySourceToSourceRootHook() {
+  echo "Executing beamCopySourceToSourceRootHook"
+
+  mkdir -p "$out"
+  src="$out/src"
+  cp -r "$sourceRoot" "$src"
+  sourceRoot="$src"
+
+  echo "Finished beamCopySourceToSourceRootHook"
+}
+
+postUnpackHooks+=(beamCopySourceToSourceRootHook)

pkgs/development/beam-modules/hooks/default.nix

@@ -8,6 +8,10 @@
     meta.license = lib.licenses.mit;
   } ./beam-copy-source-hook.sh;
 
+  beamCopySourceToSourceRootHook = makeSetupHook {
+    name = "beam-copy-source-to-sourceRoot-hook.sh";
+  } ./beam-copy-source-to-sourceRoot-hook.sh;
+
   beamModuleInstallHook = makeSetupHook {
     name = "beam-module-install-hook.sh";
     meta.license = lib.licenses.mit;