From 1cbc489081c1467abb5081904b8e46270007df46 Mon Sep 17 00:00:00 2001
From: "Matias N. Goldberg" <dark_sylinc@yahoo.com.ar>
Date: Sat, 28 Aug 2021 20:01:00 -0300
Subject: [PATCH] [Regression] Fix Terra crash if trying to assign a datablock
 more than once

---
 .../Tutorial_Terrain/src/Terra/Terra.cpp      | 26 +++++++++++--------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/Samples/2.0/Tutorials/Tutorial_Terrain/src/Terra/Terra.cpp b/Samples/2.0/Tutorials/Tutorial_Terrain/src/Terra/Terra.cpp
index 9b6ae56e66f..32ec504101d 100644
--- a/Samples/2.0/Tutorials/Tutorial_Terrain/src/Terra/Terra.cpp
+++ b/Samples/2.0/Tutorials/Tutorial_Terrain/src/Terra/Terra.cpp
@@ -669,14 +669,6 @@ namespace Ogre
             accumDim += maxPixelDimension * (1u << iteration);
             ++iteration;
 
-            if( !m_terrainCells[0].empty() && m_terrainCells[0].back().getDatablock() )
-            {
-                HlmsDatablock *datablock = m_terrainCells[0].back().getDatablock();
-                OGRE_ASSERT_HIGH( dynamic_cast<HlmsTerra *>( datablock->getCreator() ) );
-                HlmsTerra *hlms = static_cast<HlmsTerra *>( datablock->getCreator() );
-                hlms->_unlinkTerra( this );
-            }
-
             for( size_t i = 0u; i < 2u; ++i )
             {
                 m_terrainCells[i].clear();
@@ -755,6 +747,15 @@ namespace Ogre
     //-----------------------------------------------------------------------------------
     void Terra::setDatablock( HlmsDatablock *datablock )
     {
+        if( !datablock && !m_terrainCells[0].empty() && m_terrainCells[0].back().getDatablock() )
+        {
+            // Unsetting the datablock. We have no way of unlinking later on. Do it now
+            HlmsDatablock *datablock = m_terrainCells[0].back().getDatablock();
+            OGRE_ASSERT_HIGH( dynamic_cast<HlmsTerra *>( datablock->getCreator() ) );
+            HlmsTerra *hlms = static_cast<HlmsTerra *>( datablock->getCreator() );
+            hlms->_unlinkTerra( this );
+        }
+
         for( size_t i = 0u; i < 2u; ++i )
         {
             std::vector<TerrainCell>::iterator itor = m_terrainCells[i].begin();
@@ -767,9 +768,12 @@ namespace Ogre
             }
         }
 
-        OGRE_ASSERT_HIGH( dynamic_cast<HlmsTerra *>( datablock->getCreator() ) );
-        HlmsTerra *hlms = static_cast<HlmsTerra *>( datablock->getCreator() );
-        hlms->_linkTerra( this );
+        if( mHlmsTerraIndex != std::numeric_limits<uint32>::max() )
+        {
+            OGRE_ASSERT_HIGH( dynamic_cast<HlmsTerra *>( datablock->getCreator() ) );
+            HlmsTerra *hlms = static_cast<HlmsTerra *>( datablock->getCreator() );
+            hlms->_linkTerra( this );
+        }
     }
     //-----------------------------------------------------------------------------------
     Ogre::TextureGpu* Terra::_getShadowMapTex(void) const