Reapply "work around objc crash on macOS"

This reverts commit b6169e0.

default.nix

@@ -17,6 +17,7 @@ stdenv.mkDerivation {
     nlohmann_json
     nix
     boost
+    curl
   ];
   nativeBuildInputs = with pkgs; [
     meson

meson.build

@@ -3,6 +3,7 @@ project('nix-eval-jobs', 'cpp',
   license : 'GPL-3.0',
 )
 
+libcurl_dep = dependency('libcurl', required: true)
 nix_main_dep = dependency('nix-main', required: true)
 nix_store_dep = dependency('nix-store', required: true)
 nix_expr_dep = dependency('nix-expr', required: true)

src/meson.build

@@ -22,6 +22,7 @@ configure_file(
 
 executable('nix-eval-jobs', src,
            dependencies : [
+             libcurl_dep,
              nix_main_dep,
              nix_store_dep,
              nix_expr_dep,

src/nix-eval-jobs.cc

@@ -1,5 +1,6 @@
 #include <nix/config.h> // IWYU pragma: keep
 
+#include <curl/curl.h>
 #include <nix/eval-settings.hh>
 #include <nix/shared.hh>
 #include <nix/sync.hh>
@@ -347,6 +348,19 @@ int main(int argc, char **argv) {
     /* We are doing the garbage collection by killing forks */
     setenv("GC_DONT_GC", "1", 1);
 
+    /* Because of an objc quirk[1], calling curl_global_init for the first time
+       after fork() will always result in a crash.
+       Up until now the solution has been to set
+       OBJC_DISABLE_INITIALIZE_FORK_SAFETY for every nix process to ignore that
+       error. Instead of working around that error we address it at the core -
+       by calling curl_global_init here, which should mean curl will already
+       have been initialized by the time we try to do so in a forked process.
+
+       [1]
+       https://github.com/apple-oss-distributions/objc4/blob/01edf1705fbc3ff78a423cd21e03dfc21eb4d780/runtime/objc-initialize.mm#L614-L636
+    */
+    curl_global_init(CURL_GLOBAL_ALL);
+
     return handleExceptions(argv[0], [&]() {
         initNix();
         initGC();