Fix musl TH code using C++ & external interpreter (#1832)

* Fix musl TH code using C++ & external interpreter

This makes the gcc_s.so library available to iserv.

* Pass LD_LIBRARY_PATH as nix attr

builder/comp-builder.nix

@@ -315,6 +315,10 @@ let
     }
     // lib.optionalAttrs (stdenv.buildPlatform.libc == "glibc") {
       LOCALE_ARCHIVE = "${buildPackages.glibcLocales}/lib/locale/locale-archive";
+    }
+    // lib.optionalAttrs stdenv.hostPlatform.isMusl {
+      # This fixes musl compilation of TH code that depends on C++ (for instance TH code that uses the double-conversion package)
+      LD_LIBRARY_PATH="${pkgs.buildPackages.gcc-unwrapped.lib}/x86_64-unknown-linux-musl/lib";
     };
 
   haddock = haddockBuilder {

test/th-dlls/default.nix

@@ -4,12 +4,18 @@
 with lib;
 
 let
-  project = project' {
+  project = externalInterpreter: project' {
     inherit compiler-nix-name evalPackages;
     src = testSrc "th-dlls";
+    modules = [({pkgs, ...}: lib.optionalAttrs externalInterpreter {
+      packages.th-dlls.components.library.ghcOptions = [ "-fexternal-interpreter" ];
+      # Static openssl seems to fail to load in iserv for musl
+      packages.HsOpenSSL.components.library.libs = lib.optional pkgs.stdenv.hostPlatform.isMusl (pkgs.openssl.override { static = false; });
+    })];
   };
 
-  packages = project.hsPkgs;
+  packages = (project false).hsPkgs;
+  packages-ei = (project true).hsPkgs;
 
 in recurseIntoAttrs {
   meta.disabled = stdenv.hostPlatform.isGhcjs ||
@@ -20,10 +26,13 @@ in recurseIntoAttrs {
     (stdenv.hostPlatform.isDarwin && __elem compiler-nix-name ["ghc941" "ghc942" "ghc943" "ghc944"]);
 
   ifdInputs = {
-    inherit (project) plan-nix;
+    inherit (project true) plan-nix;
   };
 
   build = packages.th-dlls.components.library;
   build-profiled = packages.th-dlls.components.library.profiled;
   just-template-haskell = packages.th-dlls.components.exes.just-template-haskell;
+  build-ei = packages-ei.th-dlls.components.library;
+  build-profiled-ei = packages-ei.th-dlls.components.library.profiled;
+  just-template-haskell-ei = packages-ei.th-dlls.components.exes.just-template-haskell;
 }