From 1e78bdc724e95898c04a3409b0b192aa7f77780b Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 15 Oct 2012 13:17:30 +0200
Subject: Implement early connection release

---
 odb/prepared-query.hxx | 96 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 89 insertions(+), 7 deletions(-)

(limited to 'odb/prepared-query.hxx')

diff --git a/odb/prepared-query.hxx b/odb/prepared-query.hxx
index e4c9c2b..53d8110 100644
--- a/odb/prepared-query.hxx
+++ b/odb/prepared-query.hxx
@@ -17,24 +17,56 @@
 
 namespace odb
 {
-  struct LIBODB_EXPORT prepared_query_impl: details::shared_base
+  class LIBODB_EXPORT prepared_query_impl: public details::shared_base
   {
+  public:
     virtual
     ~prepared_query_impl ();
 
+    prepared_query_impl (connection&);
+
+    bool cached;
+    connection& conn;
     const char* name;
     details::shared_ptr<statement> stmt;
     details::shared_ptr<result_impl> (*execute) (prepared_query_impl&);
+
+  private:
+    prepared_query_impl (const prepared_query_impl&);
+    prepared_query_impl& operator= (const prepared_query_impl&);
+
+    // Doubly-linked list of results.
+    //
+    // prev_ ==    0 means we are the first element.
+    // next_ ==    0 means we are the last element.
+    // next_ == this means we are not on the list.
+    //
+  protected:
+    friend class connection;
+
+    void
+    list_remove ();
+
+    prepared_query_impl* prev_;
+    prepared_query_impl* next_;
   };
 
   template <typename T>
   struct prepared_query
   {
-    prepared_query () {}
+    // Cached version.
+    //
+    explicit
+    prepared_query (prepared_query_impl* impl = 0): impl_ (impl) {}
 
+    // Uncached version.
+    //
     explicit
-    prepared_query (details::shared_ptr<prepared_query_impl> impl)
-        : impl_ (impl) {}
+    prepared_query (const details::shared_ptr<prepared_query_impl>& impl)
+        : impl_ (impl.get ())
+    {
+      impl_->_inc_ref ();
+    }
 
     result<T>
     execute (bool cache = true)
@@ -68,16 +100,66 @@ namespace odb
       return *impl_->stmt;
     }
 
-    typedef details::shared_ptr<prepared_query_impl>
-    prepared_query::*unspecified_bool_type;
+    typedef prepared_query_impl* prepared_query::*unspecified_bool_type;
     operator unspecified_bool_type () const
     {
       return impl_ ? &prepared_query::impl_ : 0;
     }
 
+  public:
+    ~prepared_query ()
+    {
+      if (impl_ != 0 && !impl_->cached && impl_->_dec_ref ())
+        delete impl_;
+    }
+
+    prepared_query (const prepared_query& x)
+        : impl_ (x.impl_)
+    {
+      if (!impl_->cached)
+        impl_->_inc_ref ();
+    }
+
+    prepared_query&
+    operator= (const prepared_query& x)
+    {
+      if (impl_ != x.impl_)
+      {
+        if (impl_ != 0 && !impl_->cached && impl_->_dec_ref ())
+          delete impl_;
+
+        impl_ = x.impl_;
+
+        if (!impl_->cached)
+          impl_->_inc_ref ();
+      }
+
+      return *this;
+    }
+
   private:
+    // Ideally, we would just use shared_ptr to manage the impl object.
+    // However, there is a problem if the prepared query is cached on
+    // the connection and the connection is released early when the
+    // transaction is committed or rolled back. In this case, the
+    // prepared_query object might still be around pointing to impl. If
+    // this connection and the prepared query are then used by another
+    // thread while we release the impl object, then we have a race
+    // condition.
+    //
+    // To work around this problem we will simply "reference" the impl
+    // object without counting if the prepared query is cached. For
+    // transition from pointer to reference, see cache_query_() in
+    // connection.cxx.
+    //
+    // You may also observe that in order to know whether this is a
+    // cached prepared query or not, we have to read the cached data
+    // member in the impl object. This does not cause a race because,
+    // unlike the reference count, this member is immutable once set
+    // to true.
+    //
     friend class connection;
-    details::shared_ptr<prepared_query_impl> impl_;
+    prepared_query_impl* impl_;
   };
 
   namespace core
-- 
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