SourceExprCommand: allocate the vSourceExpr via uncollectable memory

Previously the memory would occasionally be collected during eval since
the GC doesn't consider the member variable as alive / doesn't scan the
region of memory where the pointer lives.

By using the traceable_allocator<T> allocator provided by Boehm GC we
can ensure the memory isn't collected. It should be properly freed when
SourceExprCommand goes out of scope.

src/nix/command.hh

@@ -41,7 +41,7 @@ private:
 
     std::shared_ptr<EvalState> evalState;
 
-    Value * vSourceExpr = 0;
+    std::shared_ptr<Value> vSourceExpr;
 };
 
 enum RealiseMode { Build, NoBuild, DryRun };

src/nix/installables.cc

@@ -8,10 +8,13 @@
 #include "store-api.hh"
 #include "shared.hh"
 
+#include <gc/gc.h>
+
 #include <regex>
 
 namespace nix {
 
+
 SourceExprCommand::SourceExprCommand()
 {
     mkFlag()
@@ -24,11 +27,14 @@ SourceExprCommand::SourceExprCommand()
 
 Value * SourceExprCommand::getSourceExpr(EvalState & state)
 {
-    if (vSourceExpr) return vSourceExpr;
+    if (vSourceExpr) return vSourceExpr.get();
 
     auto sToplevel = state.symbols.create("_toplevel");
 
-    vSourceExpr = state.allocValue();
+    // Allocate the vSourceExpr Value as uncollectable. Boehm GC doesn't
+    // consider the member variable "alive" during execution causing it to be
+    // GC'ed in the middle of evaluation.
+    vSourceExpr = std::allocate_shared<Value>(traceable_allocator<Value>());
 
     if (file != "")
         state.evalFile(lookupFileArg(state, file), *vSourceExpr);
@@ -69,7 +75,7 @@ Value * SourceExprCommand::getSourceExpr(EvalState & state)
         vSourceExpr->attrs->sort();
     }
 
-    return vSourceExpr;
+    return vSourceExpr.get();
 }
 
 ref<EvalState> SourceExprCommand::getEvalState()