SingleWriterFixedHashMap: Clean up tombstones when copying from a full map

Summary:
Clean up tombstones when copying from a full map.
Equal capacity maps are copied with tombstones for speed but that optimization should not be used if it results in a full map.

Add guarantee that a copy from a map with tombstones will have at least one empty element.

Reviewed By: A5he

Differential Revision: D25230678

fbshipit-source-id: 900b734cc526be8da8be4e70137d19645f994f19

folly/experimental/SingleWriterFixedHashMap.h

@@ -48,6 +48,9 @@ namespace folly {
 /// - used()
 /// - available()
 ///
+/// This implementation guarantees that a copy from a map with
+/// tombstones will have at least one available empty element.
+///
 template <typename Key, typename Value>
 class SingleWriterFixedHashMap {
 #if __cpp_lib_atomic_is_always_lock_free
@@ -86,7 +89,8 @@ class SingleWriterFixedHashMap {
       return;
     }
     elem_ = std::make_unique<Elem[]>(capacity_);
-    if (capacity_ == o.capacity_) {
+    if (capacity_ == o.capacity_ &&
+        (o.used_ < o.capacity_ || o.size() == o.capacity_)) {
       memcpy(elem_.get(), o.elem_.get(), capacity_ * sizeof(Elem));
       used_ = o.used_;
       setSize(o.size());

folly/experimental/test/SingleWriterHashMapTest.cpp

@@ -151,6 +151,23 @@ TEST(SingleWriterFixedHashMap, drf) {
   drf_test();
 }
 
+void copy_tombstones_test() {
+  SWFHM* m = new SWFHM(4);
+  for (int i = 0; i < 100; ++i) {
+    SWFHM* m2 = new SWFHM(4, *m);
+    delete m;
+    ASSERT_LT(m2->used(), m2->capacity());
+    m2->insert(i, i);
+    m2->erase(i);
+    m = m2;
+  }
+  delete m;
+}
+
+TEST(SingleWriterFixedHashMap, copy_tombstones) {
+  copy_tombstones_test();
+}
+
 // Benchmarks
 
 template <typename Func>