WIP: try to cross-compile using proot instead of binfmt

but it doesn't seem to be emulating child processes...

overlays/cross.nix

@@ -84,7 +84,7 @@ let
     # so hack its components into something which *does* handle cross compilation
     lib.overrideDerivation pkg ({ builder, args, ... }: {
       builder = "${final.buildPackages.bash}/bin/sh";
-      args = ["-e" (final.buildPackages.vmTools.vmRunCommand qemuCommandLinux)];
+      args = [ "-e" (final.buildPackages.vmTools.vmRunCommand qemuCommandLinux) ];
       # orig{Builder,Args} gets used by the vmRunCommand script:
       origBuilder = builder;
       origArgs = args;
@@ -98,12 +98,44 @@ let
       override = attrs: emulateBuilder (pkg.override attrs);
       overrideAttrs = mergeFn: emulateBuilder (pkg.overrideAttrs mergeFn);
     }
-    # alternatively, `proot` could let us get per-package binfmt:
-    # - <https://proot-me.github.io/>
-    # - i.e., execute host programs *and* build programs, mixed
   ;
 
-  emulateBuildMachine =
+  # given a package that's defined for build == host,
+  # build it from a "proot": a chroot-like environment where `exec` is hooked to invoke qemu instead.
+  # this is like binfmt, but configured to run *only* the emulated host and not the build machine
+  # see: <https://proot-me.github.io/>
+  # hinted at by: <https://www.tweag.io/blog/2022-03-31-running-wasm-native-hybrid-code/>
+  emulateBuilderProot = pkg:
+    lib.overrideDerivation pkg ({ builder, args, ... }: {
+      builder = "${final.buildPackages.bash}/bin/sh";
+      args = [ "-e" prootBuilder ];
+      origBuilder = builder;
+      origArgs = args;
+
+      enableParallelBuilding = true;  # TODO: inherit from `pkg`?
+      NIX_DEBUG = "6";
+
+      # finally, let nix know that this package should be built by the build system
+      system = final.stdenv.buildPlatform.system;
+    }) // {
+      override = attrs: emulateBuilderProot (pkg.override attrs);
+      overrideAttrs = mergeFn: emulateBuilderProot (pkg.overrideAttrs mergeFn);
+    };
+
+    prootBuilder = let
+      proot = "${final.buildPackages.proot}/bin/proot";
+      # prootFlags = "-r / -b /:/";
+      prootFlags = "-b /nix:/nix -b /tmp:/tmp";
+      # prootFlags = "-b /:/ -b ${final.bash}/bin/sh:/bin/sh";  # --mixed-mode false
+      qemu = "${final.buildPackages.qemu}/bin/qemu-aarch64";
+    in
+      final.pkgs.writeText "proot-run" ''
+        echo "proot: ${proot} -q ${qemu} ${prootFlags} $origBuilder $origArgs"
+        ${proot} -q ${qemu} ${prootFlags} $origBuilder $origArgs
+        echo "exited proot"
+      '';
+
+  buildOnHost =
     let
       # patch packages which can't ordinarily exist in buildPackages
       preFixPkg = p:
@@ -122,7 +154,7 @@ let
       unsplicePkg = p: p.__spliced.hostTarget or p;
       unsplicePkgs = ps: map (p: unsplicePkg (preFixPkg p)) ps;
     in
-      pkg: emulateBuilder ((pkg.override {
+      pkg: (pkg.override {
         inherit (emulated) stdenv;
       }).overrideAttrs (upstream: {
         # for this purpose, the naming in `depsAB` is "inputs build for A, used to create packages in B" (i think).
@@ -141,7 +173,10 @@ let
 
         nativeCheckInputs = unsplicePkgs (upstream.nativeCheckInputs or []);
         nativeInstallCheckInputs = unsplicePkgs (upstream.nativeInstallCheckInputs or []);
-      }));
+      });
+
+  emulateBuildMachine = pkg: emulateBuilder (buildOnHost pkg);
+  buildInProot = pkg: emulateBuilderProot (buildOnHost pkg);
 in {
   inherit emulated;
 
@@ -352,7 +387,7 @@ in {
     unwrapped = super.unwrapped // {
       browserpass-extension = super.unwrapped.browserpass-extension.override {
         # this overlay is optional for binfmt machines, but non-binfmt can't cross-compile the modules (for use at runtime)
-        mkYarnModules = args: emulateBuildMachine {
+        mkYarnModules = args: buildInProot {
           override = { stdenv }: (
             (final.yarn2nix-moretea.override {
               pkgs = final.pkgs.__splicedPackages // { inherit stdenv; };
@@ -791,7 +826,7 @@ in {
   koreader = (prev.koreader.override {
     # fixes runtime error: luajit: ./ffi/util.lua:757: attempt to call field 'pack' (a nil value)
     # inherit (emulated) luajit;
-    luajit = emulateBuildMachine (final.luajit.override {
+    luajit = buildInProot (final.luajit.override {
       buildPackages.stdenv = emulated.stdenv;  # it uses buildPackages.stdenv for HOST_CC
     });
   }).overrideAttrs (upstream: {
@@ -802,7 +837,7 @@ in {
   koreader-from-src = prev.koreader-from-src.override {
     # fixes runtime error: luajit: ./ffi/util.lua:757: attempt to call field 'pack' (a nil value)
     # inherit (emulated) luajit;
-    luajit = emulateBuildMachine (final.luajit.override {
+    luajit = buildInProot (final.luajit.override {
       buildPackages.stdenv = emulated.stdenv;  # it uses buildPackages.stdenv for HOST_CC
     });
   };
@@ -1651,7 +1686,7 @@ in {
   tangram = (prev.tangram.override {
     # N.B. blueprint-compiler is in nativeBuildInputs.
     # the trick here is to force the aarch64 versions to be used during build (via emulation),
-    blueprint-compiler = emulateBuildMachine (final.blueprint-compiler.overrideAttrs (upstream: {
+    blueprint-compiler = buildInProot (final.blueprint-compiler.overrideAttrs (upstream: {
       # default is to propagate gobject-introspection *as a buildInput*, when it's supposed to be native.
       propagatedBuildInputs = [];
       # "Namespace Gtk not available"