Fixed an issue causing Render Graph execution errors after a random amount of time. (#6329)

* Fixed an issue causing Render Graph execution errors after a random amount of time.

* Made sure of no collision when going for fallback validity bits

* Update CHANGELOG.md

Author: JulienIgnace-Unity

com.unity.render-pipelines.core/CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 - Fixed issue when changing volume profiles at runtime with a script (case 1364256).
+- Fixed an issue causing Render Graph execution errors after a random amount of time.
 
 ## [13.1.1] - 2021-10-04
 

com.unity.render-pipelines.core/Runtime/RenderGraph/RenderGraphResources.cs

@@ -48,14 +48,21 @@ public bool IsValid()
 
         static public void NewFrame(int executionIndex)
         {
+            uint previousValidBit = s_CurrentValidBit;
             // Scramble frame count to avoid collision when wrapping around.
             s_CurrentValidBit = (uint)(((executionIndex >> 16) ^ (executionIndex & 0xffff) * 58546883) << 16);
             // In case the current valid bit is 0, even though perfectly valid, 0 represents an invalid handle, hence we'll
             // trigger an invalid state incorrectly. To account for this, we actually skip 0 as a viable s_CurrentValidBit and
             // start from 1 again.
-            if (s_CurrentValidBit == 0)
+            // In the same spirit, s_SharedResourceValidBit is reserved for shared textures so we should never use it otherwise
+            // resources could be considered valid at frame N+1 (because shared) even though they aren't.
+            if (s_CurrentValidBit == 0 || s_CurrentValidBit == s_SharedResourceValidBit)
             {
-                s_CurrentValidBit = 1 << 16;
+                // We need to make sure we don't pick the same value twice.
+                uint value = 1;
+                while (previousValidBit == (value << 16))
+                    value++;
+                s_CurrentValidBit = (value << 16);
             }
         }
     }