haskellPackages: support Template Haskell when cross compiling

This makes TH work when cross-compiling between aarch64 and x86_64 for
both glibc and musl.

pkgsStatic does not work, yet and neither does darwin.

pkgs/development/haskell-modules/configuration-nix.nix

@@ -2141,6 +2141,21 @@ builtins.intersectAttrs super {
   cpython = doJailbreak super.cpython;
 
   botan-bindings = super.botan-bindings.override { botan = pkgs.botan3; };
+
+  # Avoids a cycle by disabling use of the external interpreter for the packages that are dependencies of iserv-proxy.
+  # These in particular can't rely on template haskell for cross-compilation anyway as they can't rely on iserv-proxy.
+  inherit
+    (
+      let
+        noExternalInterpreter = overrideCabal {
+          enableExternalInterpreter = false;
+        };
+      in
+      lib.mapAttrs (_: noExternalInterpreter) { inherit (super) iserv-proxy network; }
+    )
+    iserv-proxy
+    network
+    ;
 }
 
 // lib.optionalAttrs pkgs.config.allowAliases (

pkgs/development/haskell-modules/configuration-windows.nix

@@ -14,6 +14,23 @@ with haskellLib;
     else
       super.network;
 
+  # Avoids a cycle by disabling use of the external interpreter for the packages that are dependencies of iserv-proxy.
+  # See configuration-nix.nix, where iserv-proxy and network are handled.
+  # On Windows, network depends on temporary (see above), which depends on random, which depends on splitmix.
+  inherit
+    (
+      let
+        noExternalInterpreter = overrideCabal {
+          enableExternalInterpreter = false;
+        };
+      in
+      lib.mapAttrs (_: noExternalInterpreter) { inherit (super) random splitmix temporary; }
+    )
+    random
+    splitmix
+    temporary
+    ;
+
   # https://github.com/fpco/streaming-commons/pull/84
   streaming-commons = appendPatch (fetchpatch {
     name = "fix-headers-case.patch";

pkgs/development/haskell-modules/generic-builder.nix

@@ -10,12 +10,60 @@
   runCommandCC,
   ghcWithHoogle,
   ghcWithPackages,
+  haskellLib,
+  iserv-proxy,
   nodejs,
+  writeShellScriptBin,
 }:
 
 let
   isCross = stdenv.buildPlatform != stdenv.hostPlatform;
 
+  crossSupport = rec {
+    emulator = stdenv.hostPlatform.emulator buildPackages;
+
+    canProxyTH =
+      # iserv-proxy currently does not build on GHC 9.6
+      lib.versionAtLeast ghc.version "9.8" && stdenv.hostPlatform.emulatorAvailable buildPackages;
+
+    iservWrapper =
+      let
+        wrapperScript =
+          enableProfiling:
+          let
+            overrides = haskellLib.overrideCabal {
+              enableLibraryProfiling = enableProfiling;
+              enableExecutableProfiling = enableProfiling;
+            };
+            buildProxy = lib.getExe' iserv-proxy.build "iserv-proxy";
+            hostProxy = lib.getExe' (overrides iserv-proxy.host) "iserv-proxy-interpreter";
+          in
+          buildPackages.writeShellScriptBin ("iserv-wrapper" + lib.optionalString enableProfiling "-prof") ''
+            set -euo pipefail
+            PORT=$((5000 + $RANDOM % 5000))
+            (>&2 echo "---> Starting interpreter on port $PORT")
+            ${emulator} ${hostProxy} tmp $PORT &
+            RISERV_PID="$!"
+            trap "kill $RISERV_PID" EXIT # Needs cleanup when building without sandbox
+            ${buildProxy} $@ 127.0.0.1 "$PORT"
+            (>&2 echo "---> killing interpreter...")
+          '';
+
+        # GHC will add `-prof` to the external interpreter when doing a profiled build.
+        # Since a single derivation can build with both profiling and non-profiling versions
+        # we need both versions made available
+        both = buildPackages.symlinkJoin {
+          name = "iserv-wrapper-both";
+          paths = map wrapperScript [
+            false
+            true
+          ];
+        };
+
+      in
+      "${both}/bin/iserv-wrapper";
+  };
+
   # Pass the "wrong" C compiler rather than none at all so packages that just
   # use the C preproccessor still work, see
   # https://github.com/haskell/cabal/issues/6466 for details.
@@ -205,10 +253,15 @@ in
   # of `meta.pkgConfigModules`. This option defaults to false for now, since
   # this metadata is far from complete in nixpkgs.
   __onlyPropagateKnownPkgConfigModules ? false,
+
+  enableExternalInterpreter ? isCross && crossSupport.canProxyTH,
 }@args:
 
 assert editedCabalFile != null -> revision != null;
 
+# We only use iserv-proxy for the external interpreter
+assert enableExternalInterpreter -> crossSupport.canProxyTH;
+
 # --enable-static does not work on windows. This is a bug in GHC.
 # --enable-static will pass -staticlib to ghc, which only works for mach-o and elf.
 assert stdenv.hostPlatform.isWindows -> enableStaticLibraries == false;
@@ -298,7 +351,15 @@ let
     "--hsc2hs-option=--cross-compile"
     (optionalString enableHsc2hsViaAsm "--hsc2hs-option=--via-asm")
   ]
-  ++ optional (allPkgconfigDepends != [ ]) "--with-pkg-config=${pkg-config.targetPrefix}pkg-config";
+  ++ optional (allPkgconfigDepends != [ ]) "--with-pkg-config=${pkg-config.targetPrefix}pkg-config"
+
+  ++ optionals enableExternalInterpreter (
+    map (opt: "--ghc-option=${opt}") [
+      "-fexternal-interpreter"
+      "-pgmi"
+      crossSupport.iservWrapper
+    ]
+  );
 
   makeGhcOptions = opts: lib.concatStringsSep " " (map (opt: "--ghc-option=${opt}") opts);
 

pkgs/development/haskell-modules/make-package-set.nix

@@ -49,7 +49,7 @@ let
   inherit (haskellLib) overrideCabal;
 
   mkDerivationImpl = pkgs.callPackage ./generic-builder.nix {
-    inherit stdenv;
+    inherit stdenv haskellLib;
     nodejs = buildPackages.nodejs-slim;
     inherit (self)
       buildHaskellPackages
@@ -57,6 +57,10 @@ let
       ghcWithHoogle
       ghcWithPackages
       ;
+    iserv-proxy = {
+      build = buildHaskellPackages.iserv-proxy;
+      host = self.iserv-proxy;
+    };
     inherit (self.buildHaskellPackages) jailbreak-cabal;
     hscolour = overrideCabal (drv: {
       isLibrary = false;

pkgs/top-level/release-haskell.nix

@@ -520,7 +520,24 @@ let
               ghc948
               ;
           };
-        };
+        }
+        //
+          removePlatforms
+            [
+              # Testing cross from x86_64-linux
+              "aarch64-darwin"
+              "aarch64-linux"
+              "x86_64-darwin"
+            ]
+            {
+              haskellPackages = {
+                inherit (packagePlatforms pkgs.pkgsCross.aarch64-multiplatform.haskellPackages)
+                  ghc
+                  hello
+                  th-orphans
+                  ;
+              };
+            };
       };
     })
     (versionedCompilerJobs {