Fix memory corruption caused by GC-invisible coroutine stacks

Crucially this introduces BoehmGCStackAllocator, but it also
adds a bunch of wiring to avoid making libutil depend on bdw-gc.

Part of the solutions for #4178, #4200

src/libutil/serialise.hh

@@ -5,6 +5,7 @@
 #include "types.hh"
 #include "util.hh"
 
+namespace boost::context { struct stack_context; }
 
 namespace nix {
 
@@ -497,5 +498,18 @@ struct FramedSink : nix::BufferedSink
     };
 };
 
+/* Stack allocation strategy for sinkToSource.
+   Mutable to avoid a boehm gc dependency in libutil.
+
+   boost::context doesn't provide a virtual class, so we define our own.
+ */
+struct StackAllocator {
+    virtual boost::context::stack_context allocate() = 0;
+    virtual void deallocate(boost::context::stack_context sctx) = 0;
+
+    /* The stack allocator to use in sinkToSource and potentially elsewhere.
+       It is reassigned by the initGC() method in libexpr. */
+    static StackAllocator *defaultAllocator;
+};
 
 }