From f573927d48a7ceb155cf681e7cbad1e5176b2c0b Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 6 Mar 2012 11:34:57 +0200
Subject: Detect situations where session is required but not used

Throw session_required.
---
 NEWS                      |  5 +++++
 doc/manual.xhtml          | 56 +++++++++++++++++++++++++++++++++++++++++++++++
 odb/relational/source.hxx | 18 +++++++++++++++
 3 files changed, 79 insertions(+)

diff --git a/NEWS b/NEWS
index 4363f64..06879eb 100644
--- a/NEWS
+++ b/NEWS
@@ -38,6 +38,11 @@ Version 1.9.0
     namespace. For more information, refer to Section 12.5.1, "pointer" and
     Section 12.5.2, "table" in the ODB manual.
 
+  * New exception, odb::session_required, is thrown when ODB detects that
+    correctly loading a bidirectional object relationship requires a session
+    but one is not used. For more information, refer to Section 6.2,
+    "Bidirectional Relationships" in the ODB manual.
+
 Version 1.8.0
 
   * Support for the Microsoft SQL Server database. The provided connection
diff --git a/doc/manual.xhtml b/doc/manual.xhtml
index a36481d..6257cf7 100644
--- a/doc/manual.xhtml
+++ b/doc/manual.xhtml
@@ -3648,6 +3648,12 @@ namespace odb
     what () const throw ();
   };
 
+  struct session_required: exception
+  {
+    virtual const char*
+    what () const throw ();
+  };
+
   // Database operations exceptions.
   //
   struct recoverable: exception
@@ -3729,6 +3735,12 @@ namespace odb
      <code>not_in_session</code>) are thrown by the <code>odb::session</code>
      class and are discussed in <a href="#10">Chapter 10, "Session"</a>.</p>
 
+  <p>The <code>session_required</code> exception is thrown when ODB detects
+     that correctly loading a bidirectional object relationship requires a
+     session but one is not used. See <a href="#6.2">Section 6.2,
+     "Bidirectional Relationships"</a> for more information on this
+     exception.</p>
+
   <p>The <code>recoverable</code> exception serves as a common base
      for all the recoverable exceptions, which are: <code>connection_lost</code>,
      <code>timeout</code>, and <code>deadlock</code>. The
@@ -5143,6 +5155,50 @@ private:
 };
   </pre>
 
+  <p>At the beginning of this chapter we examined how to use a session
+     to make sure a single object is shared among all other objects pointing
+     to it. With bidirectional relationships involving weak pointers the
+     use of a session becomes even more crucial. Consider the following
+     transaction that tries to load the <code>position</code> object
+     from the above example without using a session:</p>
+
+  <pre class="c++">
+transaction t (db.begin ())
+shared_ptr&lt;position> p (db.load&lt;position> (1));
+...
+t.commit ();
+  </pre>
+
+  <p>When we load the <code>position</code> object, the <code>employee</code>
+     object, which it points to, is also loaded. While <code>employee</code>
+     is initially stored as <code>shared_ptr</code>, it is then assigned to
+     the <code>employee_</code> member which is <code>weak_ptr</code>. Once
+     the assignment is complete, the shared pointer goes out of scope
+     and the only pointer that points to the newly loaded
+     <code>employee</code> object is the <code>employee_</code> weak
+     pointer. And that means the <code>employee</code> object is deleted
+     immediately after being loaded. To help avoid such pathological
+     situations ODB detects cases where a newly loaded object will
+     immediately be deleted and throws the <code>odb::session_required</code>
+     exception.</p>
+
+  <p>As the exception name suggests, the easiest way to resolve this
+     problem is to use a session:</p>
+
+  <pre class="c++">
+session s;
+transaction t (db.begin ())
+shared_ptr&lt;position> p (db.load&lt;position> (1));
+...
+t.commit ();
+  </pre>
+
+  <p>In our example, the session will maintain a shared pointer to the
+     loaded <code>employee</code> object preventing its immediate
+     deletion. Another way to resolve this problem is to avoid
+     immediate loading of the pointed-to objects using lazy weak
+     pointers. Lazy pointers are discussed in <a href="#6.3">Section 6.3,
+     "Lazy Pointers"</a> later in this chapter.</p>
 
   <p>Above, to model a bidirectional relationship in persistent classes,
      we used two pointers, one in each object. While this is a natural
diff --git a/odb/relational/source.hxx b/odb/relational/source.hxx
index fb13ade..a8edcf3 100644
--- a/odb/relational/source.hxx
+++ b/odb/relational/source.hxx
@@ -1195,6 +1195,7 @@ namespace relational
           if (lazy_pointer (pt))
             os << member << " = ptr_traits::pointer_type (*db, id);";
           else
+          {
             os << "// If a compiler error points to the line below, then" << endl
                << "// it most likely means that a pointer used in a member" << endl
                << "// cannot be initialized from an object pointer." << endl
@@ -1202,6 +1203,23 @@ namespace relational
                << member << " = ptr_traits::pointer_type (" << endl
                << "db->load< obj_traits::object_type > (id));";
 
+            // If we are loading into an eager weak pointer, make sure there
+            // is someone else holding a strong pointer to it (normally a
+            // session). Otherwise, the object will be loaded and immediately
+            // deleted. Besides not making much sense, this also breaks the
+            // delayed loading machinery which expects the object to be around
+            // at least until the top-level load() returns.
+            //
+            if (weak_pointer (pt))
+            {
+              os << endl
+                 << "if (pointer_traits< ptr_traits::strong_pointer_type >" <<
+                "::null_ptr (" << endl
+                 << "ptr_traits::lock (" << member << ")))" << endl
+                 << "throw session_required ();";
+            }
+          }
+
           // @@ Composite value currently cannot be NULL.
           //
           if (!composite (mi.t))
-- 
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