Fix corrupted scene primMap with invalid prim (USDU-60, #158)

package/com.unity.formats.usd/CHANGELOG.md

@@ -5,6 +5,9 @@
 - Set material name in Unity to match the material name in the USD file on import.
 - Fixed USDPayload's "Is Loaded" field in the inspector staying at false even when the payload has been loaded.
 
+### Bug Fixes
+- Fix the scene primMap that was corrupted with invalid prim making the write fail.
+
 ## [1.0.4-preview.1] - 2020-07-24
 ### Changed
 - Updated USD version to 20.08

src/Tests/Cases/BasicTests.cs

@@ -566,5 +566,33 @@ public static void GetUsdObjectsTest() {
       AssertTrue(scene.GetRelationshipAtPath("/Foo.number") == null);
       AssertTrue(scene.GetAttributeAtPath("/Foo.rel") == null);
     }
+
+    public static void ReadNonExistingPrimsTest()
+    {
+      var scene = USD.NET.Scene.Create();
+
+      var sample = new MinimalSample();
+      // Read non existing prim, not in the prim map
+      scene.Read<MinimalSample>("/Foo", sample);
+      AssertTrue(sample.number == 0);
+      AssertTrue(sample.rel == null);
+
+      sample.number = 45;
+      sample.rel = new USD.NET.Relationship("/Foo");      
+      scene.Write("/Foo", sample);
+
+      //// Reading the prim adds it the internal prim map
+      var sample2 = new MinimalSample();
+      scene.Read<MinimalSample>("/Foo", sample2);
+      AssertTrue(sample.number == sample2.number);
+      AssertTrue(sample.rel != null);
+
+      // Reading a non existing prim that is still in the prim map
+      scene.Stage.RemovePrim(new pxr.SdfPath("/Foo"));
+      var sample3 = new MinimalSample();
+      scene.Read<MinimalSample>("/Foo", sample3);
+      AssertTrue(sample3.number == 0);
+      AssertTrue(sample3.rel == null);
+    }
   }
 }

src/Tests/UnitTestDriver.cs

@@ -58,6 +58,7 @@ static void RunTestCases() {
       Cases.BasicTests.PrimvarsTest();
       Cases.BasicTests.SampleBaseTest();
       Cases.BasicTests.GetUsdObjectsTest();
+      Cases.BasicTests.ReadNonExistingPrimsTest();
 
       Cases.UnityIoTests.VectorsTest();
       Cases.UnityIoTests.QuaternionTest();

src/USD.NET/serialization/Scene.cs

@@ -745,10 +745,12 @@ private UsdPrim GetUsdPrim(string path) {
     private pxr.UsdPrim GetUsdPrim(SdfPath path) {
       UsdPrim prim;
       lock(m_stageLock) {
-      if (!m_primMap.TryGetValue(path, out prim) || !prim.IsValid()) {
-        prim = Stage.GetPrimAtPath(path);
-        m_primMap[path] = prim;
-      }
+        if (!m_primMap.TryGetValue(path, out prim) || !prim.IsValid()) {
+          prim = Stage.GetPrimAtPath(path);
+          if (prim.IsValid()) {
+            m_primMap[path] = prim;
+          }
+        }
       }
       return prim;
     }