From 4b9acf4e21a834276b7d061942e7b5d3692662b6 Mon Sep 17 00:00:00 2001
From: Matthew Bauer <mjbauer95@gmail.com>
Date: Thu, 3 Dec 2020 15:41:59 -0600
Subject: [PATCH] Use posix_spawn_setbinpref_np to advise which architecture to
 run

When running universal binaries like /bin/bash, Darwin XNU will choose
which architecture of the binary to use based on "binary preferences".
This change sets that to the current platform for aarch64 and x86_64
builds. In addition it now uses posix_spawn instead of the usual
execve. Note, that this does not prevent the other architecture from
being run, just advises which to use.

Unfortunately, posix_spawnattr_setbinpref_np does not appear to be
inherited by child processes in x86_64 Rosetta 2 translations, meaning
that this will not always work as expected.

For example:

  {
    arm = derivation {
      name = "test";
      system = "aarch64-darwin";
      builder = "/bin/bash";
      args = [ "-e" (builtins.toFile "test" ''
        set -x
        /usr/sbin/sysctl sysctl.proc_translated
        /usr/sbin/sysctl sysctl.proc_native
        [ "$(/usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };
    rosetta = derivation {
      name = "test";
      system = "x86_64-darwin";
      builder = "/bin/bash";
      args = [ "-e" (builtins.toFile "test" ''
        set -x
        /usr/sbin/sysctl sysctl.proc_translated
        /usr/sbin/sysctl sysctl.proc_native
        [ "$(/usr/bin/arch)" = i386 ]
        echo It works!
        /usr/bin/touch $out
      '') ];
    };
  }

`arm' fails on x86_64-compiled Nix, but `arm' and `rosetta' succeed on
aarch64-compiled Nix. I suspect there is a way to fix this since:

  $ /usr/bin/arch -arch x86_64 /bin/bash \
    -c '/usr/bin/arch -arch arm64e /bin/bash -c /usr/bin/arch'
  arm64

seems to work correctly. We may need to wait for Apple to update
system_cmds in opensource.apple.com to find out how though.
---
 src/libstore/build/derivation-goal.cc | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 1db85bd37..f370fd82d 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -50,6 +50,10 @@
 #define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root, put_old))
 #endif
 
+#if __APPLE__
+#include <spawn.h>
+#endif
+
 #include <pwd.h>
 #include <grp.h>
 
@@ -2844,7 +2848,27 @@ void DerivationGoal::runChild()
             }
         }
 
+#if __APPLE__
+        posix_spawnattr_t attrp;
+
+        if (posix_spawnattr_init(&attrp))
+            throw SysError("failed to initialize builder");
+
+        if (posix_spawnattr_setflags(&attrp, POSIX_SPAWN_SETEXEC))
+            throw SysError("failed to initialize builder");
+
+        if (drv->platform == "aarch64-darwin") {
+            cpu_type_t cpu = CPU_TYPE_ARM64;
+            posix_spawnattr_setbinpref_np(&attrp, 1, &cpu, NULL);
+        } else if (drv->platform == "x86_64-darwin") {
+            cpu_type_t cpu = CPU_TYPE_X86_64;
+            posix_spawnattr_setbinpref_np(&attrp, 1, &cpu, NULL);
+        }
+
+        posix_spawn(NULL, builder, NULL, &attrp, stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
+#else
         execve(builder, stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
+#endif
 
         throw SysError("executing '%1%'", drv->builder);