Initial C++ fuzz tests

This changelist adds initial C++ fuzz tests to the XmlIo module of the MaterialX test suite.  In these new tests, pseudo-random edits are applied to a set of source documents before import, in order to uncover edge cases that aren't yet handled in MaterialXCore and MaterialXFormat.

Also:

- Add missing type casts to invalidNameChar and hasWindowDriveSpecifier.
- Make random numbers deterministic in shaderGenPerformanceTest.

source/MaterialXCore/Util.cpp

@@ -24,7 +24,7 @@ const std::tuple<int, int, int> LIBRARY_VERSION_TUPLE(MATERIALX_MAJOR_VERSION,
 
 bool invalidNameChar(char c)
 {
-    return !isalnum(c) && c != '_' && c != ':';
+    return !isalnum((unsigned char) c) && c != '_' && c != ':';
 }
 
 } // anonymous namespace

source/MaterialXFormat/File.cpp

@@ -51,7 +51,7 @@ const string MATERIALX_SEARCH_PATH_ENV_VAR = "MATERIALX_SEARCH_PATH";
 
 inline bool hasWindowsDriveSpecifier(const string& val)
 {
-    return (val.length() > 1 && std::isalpha(val[0]) && (val[1] == ':'));
+    return (val.length() > 1 && std::isalpha((unsigned char) val[0]) && (val[1] == ':'));
 }
 
 //

source/MaterialXTest/MaterialXFormat/XmlIo.cpp

@@ -6,7 +6,6 @@
 #include <MaterialXTest/External/Catch/catch.hpp>
 
 #include <MaterialXFormat/Environ.h>
-#include <MaterialXFormat/File.h>
 #include <MaterialXFormat/Util.h>
 #include <MaterialXFormat/XmlIo.h>
 
@@ -262,6 +261,51 @@ TEST_CASE("Comments and newlines", "[xmlio]")
     REQUIRE(origXml == newXml);
 }
 
+TEST_CASE("Fuzz testing", "[xmlio]")
+{
+    mx::FileSearchPath searchPath = mx::getDefaultDataSearchPath();
+    mx::FilePath examplesPath = searchPath.find("resources/Materials/Examples/StandardSurface");
+
+    std::mt19937 rng(0);
+    std::uniform_int_distribution<size_t> randChar(0, 255);
+
+    for (const mx::FilePath& filename : examplesPath.getFilesInDirectory(mx::MTLX_EXTENSION))
+    {
+        // Read the example file into an XML string buffer.
+        const std::string origString = mx::readFile(examplesPath / filename);
+        REQUIRE(origString.size() > 0);
+        std::uniform_int_distribution<size_t> randPos(0, origString.size() - 1);
+
+        // Iterate over test runs.
+        for (size_t testRun = 0; testRun < 256; testRun++)
+        {
+            std::string editString = origString;
+
+            // Iterate over string edits.
+            for (size_t editIndex = 0; editIndex < 32; editIndex++)
+            {
+                // Randomly alter one character in the document.
+                size_t charIndex = randPos(rng);
+                size_t newChar = randChar(rng);
+                editString[charIndex] = (char) newChar;
+
+                // Attempt to interpret the edited string as a document, allowing only MaterialX exceptions.
+                mx::DocumentPtr doc = mx::createDocument();
+                try
+                {
+                    mx::readFromXmlString(doc, editString, searchPath);
+                    doc->validate();
+                }
+                catch (const mx::Exception&)
+                {
+                    // On a MaterialX exception, proceed to the next test run.
+                    break;
+                }
+            }
+        }
+    }
+}
+
 TEST_CASE("Locale region testing", "[xmlio]")
 {
     // In the United States, the thousands separator is a comma, while in Germany it is a period.

source/MaterialXTest/MaterialXGenShader/GenShaderUtil.cpp

@@ -370,9 +370,8 @@ void shaderGenPerformanceTest(mx::GenContext& context)
     REQUIRE(loadedDocuments.size() == documentsPaths.size());
 
     // Shuffle the order of documents and perform document library import validatation and shadergen
-    std::random_device random_dev;
-    std::mt19937 generator(random_dev());
-    std::shuffle(loadedDocuments.begin(), loadedDocuments.end(), generator);
+    std::mt19937 rng(0);
+    std::shuffle(loadedDocuments.begin(), loadedDocuments.end(), rng);
     for (const auto& doc : loadedDocuments)
     {
         doc->importLibrary(nodeLibrary);