Allow musl cross interpreters to load C dependencies

Keep static Haskell libraries and fully static application outputs while planning ghc914-sh iserv projects with a dynamic musl C loader. Scope the policy to Linux-built musl Cabal v2 interpreters and check that scope in static Linux CI.

Normal and profiled target plans preserve all unit IDs, source records and Haskell ways; only the iserv executable-static flag changes. The shared V2 project skeleton changes auxiliary derivation hashes. A small i686 musl probe reproduces static dlopen failure and verifies the dynamic loader under QEMU; full interpreter TH validation remains for canonical CI.

scripts/nix/README.md

@@ -33,6 +33,15 @@ Musl projects declare OpenSSL in Cabal's `extra-libraries-static` field for
 `direct-sqlcipher` and `simplexmq`. GHC's fully static link path reads that field,
 so an ordinary `extra-libraries` entry alone does not supply `libcrypto`. These
 source metadata patches run before planning and require Cabal specification 3.8.
+
+Linux-built musl `ghc914-sh` iserv projects use a package-specific
+`executable-static: False` policy before Cabal v2 planning. The build-time
+interpreter needs musl's dynamic C loader for Template Haskell dependencies;
+a fully static musl executable has a `dlopen` stub. Its Haskell library ways
+and profiling selection stay unchanged, and the final CLI remains fully static.
+The pure scope check runs in the Linux static CI group:
+`nix eval --json --file scripts/nix/check-musl-iserv-scope.nix`.
+
 Windows patches Basement's `INVALID_HANDLE_VALUE` expression to `INT_PTR`, so
 hsc2hs receives the signed pointer-sized integer expected by the Haskell API.
 

scripts/nix/check-musl-iserv-scope.nix

@@ -0,0 +1,38 @@
+# Pure policy regression: nix eval --json --file scripts/nix/check-musl-iserv-scope.nix
+let
+  overlay = import ./musl-iserv-overlay.nix;
+  lib = {
+    optionalAttrs = condition: attrs: if condition then attrs else {};
+    mkIf = condition: attrs: if condition then attrs else {};
+    mkAfter = value: value;
+  };
+  apply = { musl ? true, linux ? true, compiler ? "ghc914-sh", name ? "iserv-proxy", builderVersion ? 2 }:
+    let
+      prev = {
+        inherit lib;
+        stdenv = { hostPlatform.isMusl = musl; buildPlatform.isLinux = linux; };
+        haskell-nix.cabalProject' = modules: modules;
+      };
+      result = overlay prev prev;
+    in if result ? haskell-nix then
+      (builtins.head (result.haskell-nix.cabalProject' [])) {
+        inherit lib;
+        pkgs = prev;
+        config = { inherit name builderVersion; compiler-nix-name = compiler; };
+      }
+    else {};
+  selected = apply {};
+in
+assert builtins.attrNames selected == [ "cabalProjectLocal" ];
+assert builtins.match ".*executable-static: False.*" (builtins.replaceStrings [ "\n" ] [ " " ] selected.cabalProjectLocal) != null;
+assert apply { name = "simplex-chat"; } == {};
+assert apply { musl = false; } == {};
+assert apply { linux = false; } == {};
+assert apply { compiler = "ghc963"; } == {};
+assert apply { builderVersion = 1; } == {};
+{
+  muslInterpreterCLoadingEnabled = true;
+  applicationPolicyUnchanged = true;
+  nonMuslAndDarwinBuildersUnchanged = true;
+  otherCompilersAndLegacyBuilderUnchanged = true;
+}

scripts/nix/musl-iserv-overlay.nix

@@ -0,0 +1,27 @@
+final: prev:
+prev.lib.optionalAttrs (
+  prev.stdenv.hostPlatform.isMusl && prev.stdenv.buildPlatform.isLinux
+) {
+  haskell-nix = prev.haskell-nix // {
+    cabalProject' = projectModule: prev.haskell-nix.cabalProject' (
+      [ ({ config, lib, pkgs, ... }:
+        lib.mkIf (
+          pkgs.stdenv.hostPlatform.isMusl
+          && pkgs.stdenv.buildPlatform.isLinux
+          && config.compiler-nix-name == "ghc914-sh"
+          && config.name == "iserv-proxy"
+          && config.builderVersion == 2
+        ) {
+          # The build-time interpreter loads foreign libraries for TH.
+          # Fully static musl uses a dlopen stub; retain static Haskell
+          # libraries while allowing its C loader. Final CLI projects keep
+          # haskell.nix's default executable-static setting.
+          cabalProjectLocal = lib.mkAfter ''
+            package iserv-proxy
+              executable-static: False
+          '';
+        }) ]
+      ++ (if builtins.isList projectModule then projectModule else [ projectModule ])
+    );
+  };
+}