feat: execute binary specified by $0

thread argv0 through the arx call stack so we can select the binary in
the target's bin path based on it.

this also fixes an issue with fakedir where running `Dir.pwd` within
ruby would cause a segfault due to a null ptr dereference.

Author: Cassandra Comar

arx-multi-bin.patch

@@ -0,0 +1,44 @@
+diff --git a/model-scripts/tmpx.sh b/model-scripts/tmpx.sh
+index 9c2ec6f..aa6fea9 100755
+--- a/model-scripts/tmpx.sh
++++ b/model-scripts/tmpx.sh
+@@ -4,6 +4,7 @@ unset rm_ dir
+ tmp=true ; run=true
+ tmpdir= ; rm0=true ; rm1=true ; shared=false ; hash="" # To be set by tool.
+ token=`date -u +%FT%TZ | tr -d :-`-`hexdump -n4 -e '"%08x"' </dev/urandom`
++prog_name="$0"
+ opts() {
+   cmd="$1" ; shift
+   n=$#
+@@ -39,9 +40,9 @@ opts() {
+     if $shared
+     then
+       rm_=false
+-      dir="$tmpdir"/tmpx-"$hash"
++      dir="$HOME"/.cache/tmpx-"$hash"
+     else
+-      dir="$tmpdir"/tmpx-"$token"
++      dir="$HOME"/.cache/tmpx-"$token"
+     fi
+     : ${rm_:=true}
+     if $rm_
+@@ -53,6 +54,7 @@ opts() {
+       trap 'exit 2' HUP INT QUIT BUS SEGV PIPE TERM
+     fi
+     mkdir -p "$dir"
++    export TMPX_RESTORE_PWD=$(pwd)
+     cd "$dir"
+   fi
+   # Call the command with the reassembled ARGV, options removed.
+@@ -72,7 +74,10 @@ go () {
+   fi
+   if $run
+   then
+-    ( . ../env && exec ../run "$@" )
++    (
++      . ../env
++      exec -a "${prog_name}" sh -c ". ../run" "${prog_name}" "$@"
++    )
+   fi
+ }
+ unpack_env () { : # NOOP

default.nix

@@ -7,10 +7,7 @@ with nixpkgs;
 
 let
   arx' = haskellPackages.arx.overrideAttrs (o: {
-    patchPhase = (o.patchPhase or "") + ''
-      substituteInPlace model-scripts/tmpx.sh \
-        --replace /tmp/ \$HOME/.cache/
-    '';
+    patches = (o.patches or []) ++ [./arx-multi-bin.patch];
   });
 in rec {
   toStorePath = target:
@@ -27,7 +24,7 @@ in rec {
     stdenv.mkDerivation {
       name = "arx";
       buildCommand = ''
-        ${arx'}/bin/arx tmpx --shared -rm! ${archive} -o $out // ${startup}
+        ${arx'}/bin/arx tmpx --shared -rm! ${archive} -o $out -e ${startup}
         chmod +x $out
       '';
     };
@@ -94,15 +91,17 @@ in rec {
     # Avoid re-adding a store path into the store
     path = toStorePath target;
     script-linux = ''
-      .${nix-user-chroot'}/bin/nix-user-chroot -n ./nix ${nixUserChrootFlags} -- ${path}${run} "$@"
+      exec .${nix-user-chroot'}/bin/nix-user-chroot -n ./nix ${nixUserChrootFlags} -- ".$(basename "$0")/$(dirname ${path}${run})" "$@"
     '';
     script-macos = ''
       # use absolute paths so the environment variables don't get reinterpreted after a cd
-      cur_dir=$(pwd)
-      export DYLD_INSERT_LIBRARIES="''${cur_dir}/lib/libfakedir.dylib"
+      __TMPX_DAT_PATH=$(pwd)
+      cd "''${TMPX_RESTORE_PWD}"
+      export DYLD_INSERT_LIBRARIES="''${__TMPX_DAT_PATH}/lib/libfakedir.dylib"
       export FAKEDIR_PATTERN=/nix
-      export FAKEDIR_TARGET="''${cur_dir}/nix"
-      exec .${path}${run} "$@"
+      export FAKEDIR_TARGET="''${__TMPX_DAT_PATH}/nix"
+
+      exec "''${__TMPX_DAT_PATH}$(basename "$0")/$(dirname ${path}${run})/$(basename "$0")" "$@"
     '';
     script = if stdenv.isDarwin then script-macos else script-linux;
   in
@@ -116,7 +115,7 @@ in rec {
     let
       script = makeStartup { inherit target nixUserChrootFlags nix-user-chroot' run initScript; };
     in makebootstrap {
-      startup = ".${script} '\"$@\"'";
+      startup = script;
       targets = [ "${script}" ] ++ extraTargets;
     };
 

fakedir-null-buffer.patch

@@ -0,0 +1,23 @@
+diff --git a/trivial_replacements.c b/trivial_replacements.c
+index 8de580f..c33ef25 100644
+--- a/trivial_replacements.c
++++ b/trivial_replacements.c
+@@ -1,4 +1,5 @@
+ #include "common.h"
++#include <string.h>
+ 
+ /**
+  * @file        trivial_replacements.c
+@@ -309,8 +310,11 @@ ENDSUBST
+ 
+ SUBST(char const *, getcwd, (char *buf, size_t size))
+     pthread_mutex_unlock(&_lock);
+-    getcwd(buf, size);
++    char const *cwd = getcwd(buf, size);
+     pthread_mutex_lock(&_lock);
++    if (buf == NULL) {
++        buf = strdup(cwd);
++    }
+     strlcpy(buf, rewrite_path_rev(buf), size);
+     buf;
+ ENDSUBST

flake.nix

@@ -4,32 +4,31 @@
   inputs.nixpkgs.url = "nixpkgs/nixos-24.05-small";
   inputs.fakedir-pkgs.url = "github:nixie-dev/fakedir";
 
-  outputs = { self, nixpkgs, fakedir-pkgs }: let
-    systems = [ "x86_64-linux" "i686-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
-    forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f system);
-  in {
+  outputs = { self, nixpkgs, fakedir-pkgs }: {
     bundlers = {
       nix-bundle = { program, system }: let
         nixpkgs' = nixpkgs.legacyPackages.${system};
-        fakedir = fakedir-pkgs.packages.${system}.fakedir-universal;
+        fakedir = fakedir-pkgs.packages.${system}.fakedir.overrideAttrs (old: { patches = [./fakedir-null-buffer.patch]; });
         nix-bundle = import self { nixpkgs = nixpkgs'; inherit fakedir; };
         script-linux = nixpkgs'.writeScript "startup" ''
           #!/bin/sh
-          .${nix-bundle.nix-user-chroot}/bin/nix-user-chroot -n ./nix -- ${program} "$@"
+          exec .${nix-bundle.nix-user-chroot}/bin/nix-user-chroot -n ./nix -- "$(dirname ${program})/$(basename "$0")" "$@"
         '';
         script-darwin = nixpkgs'.writeScript "startup" ''
           #!/bin/sh
           # use absolute paths so the environment variables don't get reinterpreted after a cd
-          cur_dir=$(pwd)
-          export DYLD_INSERT_LIBRARIES="''${cur_dir}/lib/libfakedir.dylib"
+          __TMPX_DAT_PATH=$(pwd)
+          cd "''${TMPX_RESTORE_PWD}"
+          export DYLD_INSERT_LIBRARIES="''${__TMPX_DAT_PATH}/lib/libfakedir.dylib"
           export FAKEDIR_PATTERN=/nix
-          export FAKEDIR_TARGET="''${cur_dir}/nix"
-          exec .${program} "$@"
+          export FAKEDIR_TARGET="''${__TMPX_DAT_PATH}/nix"
+
+          exec "''${__TMPX_DAT_PATH}$(dirname ${program})/$(basename "$0")" "$@"
         '';
         script = if nixpkgs'.stdenv.isDarwin then script-darwin else script-linux;
       in nix-bundle.makebootstrap {
         targets = [ script ];
-        startup = ".${builtins.unsafeDiscardStringContext script} '\"$@\"'";
+        startup = script;
       };
     };