nix run: Fix chroot execution

Running "nix run" with a diverted store, e.g.

  $ nix run --store local?root=/tmp/nix nixpkgs.hello

stopped working when Nix became multithreaded, because
unshare(CLONE_NEWUSER) doesn't work in multithreaded processes. The
obvious solution is to terminate all other threads first, but 1) there
is no way to terminate Boehm GC marker threads; and 2) it appears that
the kernel has a race where unshare(CLONE_NEWUSER) will still fail for
some indeterminate amount of time after joining other threads.

So instead, "nix run" will now exec() a single-threaded helper ("nix
__run_in_chroot") that performs the actual unshare()/chroot()/exec().

src/nix/main.cc

@@ -9,6 +9,10 @@
 #include "store-api.hh"
 #include "progress-bar.hh"
 
+extern std::string chrootHelperName;
+
+void chrootHelper(int argc, char * * argv);
+
 namespace nix {
 
 struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
@@ -57,6 +61,13 @@ void mainWrapped(int argc, char * * argv)
     verbosity = lvlError;
     settings.verboseBuild = false;
 
+    /* The chroot helper needs to be run before any threads have been
+       started. */
+    if (argc > 0 && argv[0] == chrootHelperName) {
+        chrootHelper(argc, argv);
+        return;
+    }
+
     initNix();
     initGC();