Safely set parent for shared objects

Author: edi33416

std/experimental/allocator/building_blocks/affix_allocator.d

@@ -71,6 +71,12 @@ struct AffixAllocator(Allocator, Prefix, Suffix = void)
 
             static if (is(Allocator == RCIAllocator) || is(Allocator == RCISharedAllocator))
             {
+                static if (is(Allocator == RCISharedAllocator))
+                {
+                    shared bool nullParent = true;
+                    shared bool nullParentWait = true;
+                }
+
                 @nogc nothrow pure @safe
                 Allocator parent()
                 {
@@ -104,9 +110,22 @@ struct AffixAllocator(Allocator, Prefix, Suffix = void)
                         }
                         else
                         {
-                            auto fn = (() @trusted =>
-                                    cast(RCIAllocator function() @nogc nothrow pure @safe)(&wrapProcAllocatorObject))();
-                            _parent = fn();
+                            import core.atomic : cas, atomicLoad, atomicStore;
+
+                            if ((() @trusted => cas(&nullParent, true, false))())
+                            {
+                                auto fn = (() @trusted =>
+                                        cast(RCISharedAllocator function() @nogc nothrow pure @safe)
+                                        (&wrapProcAllocatorObject))();
+                                _parent = fn();
+                                // Notify other threads that the parent has been set
+                                atomicStore(nullParentWait, false);
+                            }
+                            else
+                            {
+                                // Busy-wait for the parent to be set
+                                while(atomicLoad(nullParentWait)) {}
+                            }
                         }
                     }
 
@@ -600,3 +619,10 @@ struct AffixAllocator(Allocator, Prefix, Suffix = void)
     assert(a.parent.bytesUsed == 100 + uint.sizeof);
     assert((() nothrow @nogc => a.deallocate(b))());
 }
+
+@system unittest
+{
+    import std.experimental.allocator : RCISharedAllocator;
+
+    AffixAllocator!(RCISharedAllocator, size_t) a;
+}