hydra-eval-jobs: Reinitialize Boehm GC in the child

This is necessary to prevent hangs in the garbage collector.

src/hydra-eval-jobs/hydra-eval-jobs.cc

@@ -1,6 +1,7 @@
 #include <map>
 #include <iostream>
 
+#define GC_LINUX_THREADS 1
 #include <gc/gc_allocator.h>
 
 #include "shared.hh"
@@ -265,9 +266,14 @@ int main(int argc, char * * argv)
             ProcessOptions options;
             options.allowVfork = false;
 
+            GC_atfork_prepare();
+
             auto pid = startProcess([&]() {
                 pipe.readSide = -1;
 
+                GC_atfork_child();
+                GC_start_mark_threads();
+
                 if (lastAttrPath != "") debug("resuming from '%s'", lastAttrPath);
 
                 /* FIXME: The build hook in conjunction with import-from-derivation is causing "unexpected EOF" during eval */
@@ -300,6 +306,8 @@ int main(int argc, char * * argv)
                 exit(0);
             }, options);
 
+            GC_atfork_parent();
+
             pipe.writeSide = -1;
 
             int status;