EMSUSD-1896 avoid crash when shader has no uvLink

The uvLink command can return nothing if it cannot determine the UV link
of the shader node. This would break assumptions in the export code and
lead to a crash when accessing missing data. Handle the case where the
uvLink command returns nothing to prevent that crash.

Add a unit test that reproduced the crash and now no longer crashes.

lib/mayaUsd/fileio/shading/shadingModeExporterContext.cpp

@@ -612,19 +612,26 @@ class _UVMappingManager
         // See lib\usd\translators\shading\usdFileTextureWriter.cpp for an example of an exporter
         // declaring UV inputs that require linking.
         for (const UsdShadeInput& input : material.GetInputs()) {
+            // Split the input name along ':' (that is what SplitName does)
+            // and if the last element is "varname" or "varnameStr" then
+            // the Maya node name is added to _nodesWithUVInput. (The Maya
+            // node name is the middle part of the split name.)
             const UsdAttribute&      usdAttr = input.GetAttr();
             std::vector<std::string> splitName = usdAttr.SplitName();
             if (splitName.back() != _tokens->varname.GetString()
                 && splitName.back() != _tokens->varnameStr.GetString()) {
                 continue;
             }
 
+            // Maya node names can be inside a Maya namespace. The Maya namespace
+            // separator is *also* ':'. So we need to check if the split name
+            // has more than 3 parts, and if so, we need to concatenate the parts.
             switch (splitName.size()) {
-            case 3: _nodesWithUVInput.push_back(TfToken(splitName[1])); break;
-            default
-                : // NOTE: (yliangsiew) Means that we have a Maya node with a namespace/multiple
-                  // namespaces.
-            {
+            case 3: {
+                _nodesWithUVInput.push_back(TfToken(splitName[1]));
+                break;
+            }
+            default: {
                 std::string mayaNodeName = splitName[1];
                 for (size_t i = 2; i < splitName.size() - 1; ++i) {
                     mayaNodeName += ":" + splitName[i];
@@ -645,12 +652,26 @@ class _UVMappingManager
         }
 
         // Ask Maya about UV linkage:
-        for (const TfToken& nodeName : _nodesWithUVInput) {
-            MString uvLinkCmd;
+
+        for (size_t i = 0; i < _nodesWithUVInput.size(); ++i) {
+            const TfToken& nodeName = _nodesWithUVInput[i];
+            MString        uvLinkCmd;
             uvLinkCmd.format(
                 "stringArrayToString(`uvLink -q -t \"^1s\"`, \" \");", nodeName.GetText());
             std::string uvLinkResult = MGlobal::executeCommandStringResult(uvLinkCmd).asChar();
-            for (std::string uvSetRef : TfStringTokenize(uvLinkResult)) {
+            const std::vector<std::string> uvSets = TfStringTokenize(uvLinkResult);
+            // If there are not UV sets, then the node has no valid UV input we must remove it.
+            // Otherwise, assumptions in the code below won't be satisfied.
+            if (uvSets.empty()) {
+                TF_WARN(
+                    "Could not determine the UV link of the UV input of the node \"%s\".",
+                    nodeName.GetText());
+                _nodesWithUVInput.erase(_nodesWithUVInput.begin() + i);
+                --i;
+                continue;
+            }
+
+            for (std::string uvSetRef : uvSets) {
                 // NOTE: If the mesh shape has the same name as the transform, then we will get a
                 //       complete path like
                 //           |mesh|mesh.uvSet[0].uvSetName
@@ -706,6 +727,8 @@ class _UVMappingManager
 
         // Update the original material with the most common mapping:
         if (largestSize) {
+            // TODO: why is it assumed that the largest set has the same size as the nodes?
+            //       Could the UV sets not be disjoint?
             TfTokenVector::const_iterator itNode = _nodesWithUVInput.cbegin();
             TfTokenVector::const_iterator itName = largestSet.cbegin();
             for (; itNode != _nodesWithUVInput.cend(); ++itNode, ++itName) {

test/lib/usd/translators/CMakeLists.txt

@@ -28,6 +28,7 @@ set(TEST_SCRIPT_FILES
     testUsdExportTypes.py
     testUsdExportUnits.py
     testUsdExportUpAxis.py
+    testUsdExportUVSetsSwitch.py
 
     # To investigate: following test asserts in MFnParticleSystem, but passes.
     # PPT, 17-Jun-20.