Fix some obscure swizzle upgrade bugs (AcademySoftwareFoundation#2637)

Was working on upgrading a MaterialX material when it is written using UsdShade data model, and that required reviewing the current upgrade code to convert it to Python/USD. Also was working on a unit test, which forced me to put on my QA hat and start devising devious ways to fail the code.

This resulted in the following two fixes for extremely rare corner cases.

Author: JGamache-autodesk

source/MaterialXCore/Version.cpp

@@ -1166,36 +1166,60 @@ void Document::upgradeVersion()
             else if (nodeCategory == "swizzle")
             {
                 InputPtr inInput = node->getInput("in");
-                InputPtr channelsInput = node->getInput("channels");
-                if (inInput &&
-                    CHANNEL_COUNT_MAP.count(inInput->getType()) &&
+                const string sourceType = inInput ? inInput->getType() : "float";
+                if (CHANNEL_COUNT_MAP.count(sourceType) &&
                     CHANNEL_COUNT_MAP.count(node->getType()))
                 {
+                    InputPtr channelsInput = node->getInput("channels");
                     string channelString = channelsInput ? channelsInput->getValueString() : EMPTY_STRING;
-                    string sourceType = inInput->getType();
                     string destType = node->getType();
                     size_t sourceChannelCount = CHANNEL_COUNT_MAP.at(sourceType);
                     size_t destChannelCount = CHANNEL_COUNT_MAP.at(destType);
 
                     // Resolve the invalid case of having both a connection and a value
                     // by removing the value attribute.
-                    if (inInput->hasValue())
+                    if (inInput && inInput->hasValue())
                     {
                         if (inInput->hasNodeName() || inInput->hasNodeGraphString() || inInput->hasInterfaceName())
                         {
                             inInput->removeAttribute(ValueElement::VALUE_ATTRIBUTE);
                         }
                     }
 
-                    if (inInput->hasValue())
+                    // We convert to a constant node if "in" input is a constant value or does not exist:
+                    bool convertToConstantNode = !inInput || inInput->hasValue();
+                    // We also convert to a constant node if every destination
+                    // channel is constant:
+                    //   eg: "ND_swizzle_color3_color3" node with
+                    //       "010" in the "channels" input.
+                    if (!convertToConstantNode)
+                    {
+                        convertToConstantNode = true;
+                        for (size_t i = 0; i < destChannelCount; i++)
+                        {
+                            if (i < channelString.size())
+                            {
+                                if (CHANNEL_CONSTANT_MAP.count(channelString[i]))
+                                {
+                                    // Still in constant territory:
+                                    continue;
+                                }
+                            }
+                            // Every other scenario: not constant
+                            convertToConstantNode = false;
+                            break;
+                        }
+                    }
+
+                    if (convertToConstantNode)
                     {
                         // Replace swizzle with constant.
                         node->setCategory("constant");
                         if (node->hasNodeDefString())
                         {
                             node->setNodeDefString("ND_constant_" + node->getType());
                         }
-                        string valueString = inInput->getValueString();
+                        string valueString = inInput ? inInput->getValueString() : "0";
                         StringVec origValueTokens = splitString(valueString, ARRAY_VALID_SEPARATORS);
                         StringVec newValueTokens;
                         for (size_t i = 0; i < destChannelCount; i++)
@@ -1208,25 +1232,25 @@ void Document::upgradeVersion()
                                     if (index < origValueTokens.size())
                                     {
                                         newValueTokens.push_back(origValueTokens[index]);
+                                        continue;
                                     }
                                 }
                                 else if (CHANNEL_CONSTANT_MAP.count(channelString[i]))
                                 {
                                     newValueTokens.push_back(std::to_string(CHANNEL_CONSTANT_MAP.at(channelString[i])));
+                                    continue;
                                 }
-                                else
-                                {
-                                    newValueTokens.push_back(origValueTokens[0]);
-                                }
-                            }
-                            else
-                            {
-                                newValueTokens.push_back(origValueTokens[0]);
                             }
+                            // Invalid channel name, or missing channel name:
+                            newValueTokens.push_back(origValueTokens[0]);
                         }
                         InputPtr valueInput = node->addInput("value", node->getType());
                         valueInput->setValueString(joinStrings(newValueTokens, ", "));
-                        node->removeInput(inInput->getName());
+                        // This is the last place we need to check for nullptr for inInput.
+                        if (inInput)
+                        {
+                            node->removeInput(inInput->getName());
+                        }
                     }
                     else if (destChannelCount == 1)
                     {
@@ -1298,17 +1322,17 @@ void Document::upgradeVersion()
                                 {
                                     combineInInput->setConnectedNode(separateNode);
                                     combineInInput->setOutputString(std::string("out") + channelString[i]);
+                                    continue;
                                 }
                                 else if (CHANNEL_CONSTANT_MAP.count(channelString[i]))
                                 {
                                     combineInInput->setValue(CHANNEL_CONSTANT_MAP.at(channelString[i]));
+                                    continue;
                                 }
                             }
-                            else
-                            {
-                                combineInInput->setConnectedNode(separateNode);
-                                combineInInput->setOutputString(combineInInput->isColorType() ? "outr" : "outx");
-                            }
+                            // Invalid channel name, or missing channel name:
+                            combineInInput->setConnectedNode(separateNode);
+                            combineInInput->setOutputString(inInput->isColorType() ? "outr" : "outx");
                         }
                         copyInputWithBindings(node, inInput->getName(), separateNode, "in");
                         node->removeInput(inInput->getName());