From f41fc4eb25ade48a358511564f3911a4605c1c31 Mon Sep 17 00:00:00 2001
From: Justin Clark-Casey (justincc)
Date: Wed, 5 Jun 2013 22:20:48 +0100
Subject: Avoid a deadlock where a script can attempt to take a
 ScriptInstance.m_Scripts lock then a lock on SP.m_attachments whilst
 SP.MakeRootAgent() attempts to take in the opposite order.

This is because scripts (at least on XEngine) start unsuspended - deceptively the ResumeScripts() calls in various places in the code are actually completely redundant (and useless).
The solution chosen here is to use a copy of the SP attachments and not have the list locked whilst creating the scripts when an avatar enters the region.
This looks to address http://opensimulator.org/mantis/view.php?id=6557
---
 OpenSim/Region/Framework/Scenes/ScenePresence.cs | 32 ++++++++++++++++--------
 1 file changed, 21 insertions(+), 11 deletions(-)

(limited to 'OpenSim/Region/Framework')

diff --git a/OpenSim/Region/Framework/Scenes/ScenePresence.cs b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
index b8ff7f7..bab14dd 100644
--- a/OpenSim/Region/Framework/Scenes/ScenePresence.cs
+++ b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
@@ -121,6 +121,8 @@ namespace OpenSim.Region.Framework.Scenes
         /// <remarks>
         /// TODO: For some reason, we effectively have a list both here and in Appearance.  Need to work out if this is
         /// necessary.
+        /// NOTE: To avoid deadlocks, do not lock m_attachments and then perform other tasks under that lock.  Take a copy
+        /// of the list and act on that instead.
         /// </remarks>
         private List<SceneObjectGroup> m_attachments = new List<SceneObjectGroup>();
 
@@ -971,19 +973,27 @@ namespace OpenSim.Region.Framework.Scenes
                 // and CHANGED_REGION) when the attachments have been rezzed in the new region.  This cannot currently
                 // be done in AttachmentsModule.CopyAttachments(AgentData ad, IScenePresence sp) itself since we are
                 // not transporting the required data.
-                lock (m_attachments)
+                //
+                // We must take a copy of the attachments list here (rather than locking) to avoid a deadlock where a script in one of 
+                // the attachments may start processing an event (which locks ScriptInstance.m_Script) that then calls a method here
+                // which needs to lock m_attachments.  ResumeScripts() needs to take a ScriptInstance.m_Script lock to try to unset the Suspend status.
+                //
+                // FIXME: In theory, this deadlock should not arise since scripts should not be processing events until ResumeScripts().
+                // But XEngine starts all scripts unsuspended.  Starting them suspended will not currently work because script rezzing
+                // is placed in an asynchronous queue in XEngine and so the ResumeScripts() call will almost certainly execute before the 
+                // script is rezzed.  This means the ResumeScripts() does absolutely nothing when using XEngine.
+                List<SceneObjectGroup> attachments = GetAttachments();
+
+                if (attachments.Count > 0)
                 {
-                    if (HasAttachments())
-                    {
-                        m_log.DebugFormat(
-                            "[SCENE PRESENCE]: Restarting scripts in attachments for {0} in {1}", Name, Scene.Name);
+                    m_log.DebugFormat(
+                        "[SCENE PRESENCE]: Restarting scripts in attachments for {0} in {1}", Name, Scene.Name);
 
-                        // Resume scripts
-                        foreach (SceneObjectGroup sog in m_attachments)
-                        {
-                            sog.RootPart.ParentGroup.CreateScriptInstances(0, false, m_scene.DefaultScriptEngine, GetStateSource());
-                            sog.ResumeScripts();
-                        }
+                    // Resume scripts
+                    foreach (SceneObjectGroup sog in attachments)
+                    {
+                        sog.RootPart.ParentGroup.CreateScriptInstances(0, false, m_scene.DefaultScriptEngine, GetStateSource());
+                        sog.ResumeScripts();
                     }
                 }
             }
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