Issue #1652, #3456 - Replace 's_asyncWebBrowser2' with thread-safe facilities

src/plugin/Instances.h

Index: src/plugin/Instances.h
===================================================================
--- a/src/plugin/Instances.h
+++ b/src/plugin/Instances.h
@@ -15,11 +15,12 @@
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef _INSTANCES_H_
+#ifndef _INSTANCES_H_
 #define _INSTANCES_H_
 
+#include <map>
 #include <mutex>
-#include <map>
+#include <set>
 
 /**
  * A base class for a synchronized map from threads to BHO instances.
@@ -96,4 +97,92 @@
   }
 };
 
+/**
+ * Template class for a synchronized set of BHO instances
+ */
+template<class T, T nullValue>
+class SyncSet
+{
+  typedef std::lock_guard<std::mutex> SentryType;
+
+  /**
+   * Underlying set container
+   */
+  std::set<T> set;
+
+  /**
+   * Synchronization primitive
+   */
+  mutable std::mutex mutex;
+
+public:
+  /**
+   * \return
+   *   true if (as expected) the argument was not already a member of the set
+   *   false otherwise
+   *
+   * \par Postcondition
+   *   - argument is a member of the set
+   */
+  bool InsertIfAbsent(T p)
+  {
+    SentryType sentry(mutex);
+    auto it = set.find(p);
+    if (it != set.end())
+    {
+      return false;
+    }
+    set.insert(p);
+    return true;
+  }
+
+  /**
+   * \return
+   *   true if (as expected) the argument was already a member of the set
+   *   false otherwise
+   *
+   * \par Postcondition
+   *   - argument is not a member of the set
+   */
+  bool EraseWithCheck(T p)
+  {
+    SentryType sentry(mutex);
+    auto it = set.find(p);
+    if (it == set.end())
+    {
+      return false;
+    }
+    set.erase(it);
+    return true;
+  }
+
+  /**
+   * \param f
+   *   Search criterion is a simple predicate function 
+   *
+   * \return
+   *   - If no member of the set meets the search criterion, the null value
+   *   - Otherwise some member of the set that meets the search criterion
+   */
+  T LinearSearch(std::function<bool(T)> f)
+  {
+    SentryType sentry(mutex);
+    for (auto member : set)
+    {
+      bool b = f(member);
+      if (b)
+      {
+        return member;
+      }
+    }
+    return nullValue;
+  }
+
+  bool Empty() const
+  {
+    SentryType sentry(mutex);
+    return set.empty();
+  }
+};
+
 #endif