Gui: Escape XML special characters in attributes for XHTML

Add escapeXmlAttribute() helper function to properly escape special
characters in XML attribute values:
- Double quotes (") --> "
- Ampersands (&) --> &
- Less-than (<) --> &lt;
- Greater-than (>) --> &gt;
- Single quotes (') --> &apos;

src/Gui/SoFCDB.cpp

@@ -497,6 +497,43 @@ bool Gui::SoFCDB::writeToX3D(SoNode* node, bool exportViewpoints, std::string& b
     return true;
 }
 
+namespace
+{
+std::string escapeXmlAttribute(const char* str)
+{
+    if (!str) {
+        return std::string();
+    }
+
+    std::string result;
+    result.reserve(strlen(str));
+
+    for (const char* p = str; *p; ++p) {
+        switch (*p) {
+            case '"':
+                result.append(""");
+                break;
+            case '&':
+                result.append("&");
+                break;
+            case '<':
+                result.append("&lt;");
+                break;
+            case '>':
+                result.append("&gt;");
+                break;
+            case '\'':
+                result.append("&apos;");
+                break;
+            default:
+                result.push_back(*p);
+                break;
+        }
+    }
+    return result;
+}
+}  // namespace
+
 void Gui::SoFCDB::writeX3DFields(
     SoNode* node,
     std::map<SoNode*, std::string>& nodeMap,
@@ -522,7 +559,8 @@ void Gui::SoFCDB::writeX3DFields(
         }
 
         nodeMap[node] = str.str();
-        out << " DEF=\"" << str.str() << "\"";
+        std::string escapedName = escapeXmlAttribute(str.str().c_str());
+        out << " DEF=\"" << escapedName << "\"";
     }
 
     const SoFieldData* fielddata = node->getFieldData();
@@ -545,9 +583,12 @@ void Gui::SoFCDB::writeX3DFields(
                         ba = ba.simplified();
                     }
 
+                    // escape XML special characters in attribute value
+                    std::string escapedValue = escapeXmlAttribute(ba.data());
+
                     out << '\n'
                         << Base::blanks(spaces + 2) << fielddata->getFieldName(i).getString()
-                        << "=\"" << ba.data() << "\" ";
+                        << "=\"" << escapedValue << "\" ";
                 }
                 else {
                     numFieldNodes++;
@@ -606,7 +647,8 @@ void Gui::SoFCDB::writeX3DChild(
         // remove the VRML prefix from the type name
         std::string sftype(node->getTypeId().getName().getString());
         sftype = sftype.substr(4);
-        out << Base::blanks(spaces) << "<" << sftype << " USE=\"" << mapIt->second << "\" />\n";
+        std::string escapedRef = escapeXmlAttribute(mapIt->second.c_str());
+        out << Base::blanks(spaces) << "<" << sftype << " USE=\"" << escapedRef << "\" />\n";
     }
 }