[Runtime] Fix ConcurrentReadableArray's handling of FreeList to not double-free items. Also implement a destructor so it can be used for non-globals.

rdar://problem/40484362

Author: mikeash

include/swift/Runtime/Concurrent.h

@@ -458,6 +458,13 @@ template <class ElemTy> struct ConcurrentReadableArray {
     ReaderCount.fetch_sub(1, std::memory_order_release);
   }
 
+  void deallocateFreeList() {
+    for (Storage *storage : FreeList)
+      storage->deallocate();
+    FreeList.clear();
+    FreeList.shrink_to_fit();
+  }
+  
 public:
   struct Snapshot {
     ConcurrentReadableArray *Array;
@@ -488,6 +495,12 @@ template <class ElemTy> struct ConcurrentReadableArray {
 
   ConcurrentReadableArray() : Capacity(0), ReaderCount(0), Elements(nullptr) {}
 
+  ~ConcurrentReadableArray() {
+    assert(ReaderCount.load(std::memory_order_acquire) == 0 &&
+           "deallocating ConcurrentReadableArray with outstanding snapshots");
+    deallocateFreeList();
+  }
+  
   void push_back(const ElemTy &elem) {
     ScopedLock guard(WriterLock);
 
@@ -511,8 +524,7 @@ template <class ElemTy> struct ConcurrentReadableArray {
     storage->Count.store(count + 1, std::memory_order_release);
 
     if (ReaderCount.load(std::memory_order_acquire) == 0)
-      for (Storage *storage : FreeList)
-        storage->deallocate();
+      deallocateFreeList();
   }
 
   Snapshot snapshot() {

unittests/runtime/CMakeLists.txt

@@ -34,6 +34,7 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
   add_swift_unittest(SwiftRuntimeTests
     Array.cpp
     CompatibilityOverride.cpp
+    Concurrent.cpp
     Exclusivity.cpp
     Metadata.cpp
     Mutex.cpp

unittests/runtime/Concurrent.cpp

@@ -0,0 +1,44 @@
+//===--- Concurrent.cpp - Concurrent data structure tests -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Runtime/Concurrent.h"
+#include "gtest/gtest.h"
+
+using namespace swift;
+
+TEST(ConcurrentReadableArrayTest, SingleThreaded) {
+  ConcurrentReadableArray<size_t> array;
+  
+  auto add = [&](size_t limit) {
+    for (size_t i = array.snapshot().count(); i < limit; i++)
+      array.push_back(i);
+  };
+  auto check = [&]{
+    size_t i = 0;
+    for (auto element : array.snapshot()) {
+      ASSERT_EQ(element, i);
+      i++;
+    }
+  };
+  
+  check();
+  add(1);
+  check();
+  add(16);
+  check();
+  add(100);
+  check();
+  add(1000);
+  check();
+  add(1000000);
+  check();
+}