Display an informative error message when a SCH file needs conversion to XSLT before being used

Author: costas80

xmlvalidator-common/src/main/java/eu/europa/ec/itb/xml/validation/PureSchematronErrorHandler.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2025 European Union
+ *
+ * Licensed under the EUPL, Version 1.2 or - as soon they will be approved by the European Commission - subsequent
+ * versions of the EUPL (the "Licence"); You may not use this work except in compliance with the Licence.
+ *
+ * You may obtain a copy of the Licence at:
+ *
+ * https://interoperable-europe.ec.europa.eu/collection/eupl/eupl-text-eupl-12
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the Licence is distributed on an
+ * "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the Licence for
+ * the specific language governing permissions and limitations under the Licence.
+ */
+
+package eu.europa.ec.itb.xml.validation;
+
+import com.helger.commons.error.IError;
+import com.helger.schematron.pure.errorhandler.LoggingPSErrorHandler;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Custom error handler that is used in the case of pure schematron resources to detect whether errors are due to
+ * the presence of external functions.
+ * <p/>
+ * In this case, the pure Schematron file should first be converted to XSLT format to avoid doing this on the fly for
+ * each validation.
+ */
+public class PureSchematronErrorHandler extends LoggingPSErrorHandler {
+
+    private boolean dueToExternalFunctionCall = false;
+
+    @Override
+    protected void handleInternally(@Nonnull IError aError) {
+        super.handleInternally(aError);
+        if (!dueToExternalFunctionCall && aError.getLinkedException() != null && aError.getLinkedException().getMessage() != null) {
+            dueToExternalFunctionCall = aError.getLinkedException().getMessage().contains("External function calls have been disabled");
+        }
+    }
+
+    public boolean isDueToExternalFunctionCall() {
+        return dueToExternalFunctionCall;
+    }
+
+}

xmlvalidator-common/src/main/java/eu/europa/ec/itb/xml/validation/XMLValidator.java

@@ -473,28 +473,45 @@ private SchematronOutputType applySchematron(ISchematronResource schematron) thr
     private TAR validateSchematron(File schematronFile, boolean supportPureValidationApproach) {
         SchematronOutputType svrlOutput;
         boolean convertXPathExpressions = false;
+        boolean retryAsXslt = false;
         String schematronFileName = schematronFile.getName().toLowerCase();
+        ISchematronResource schematronResource = null;
         try {
             if (schematronFileName.endsWith("xslt") || schematronFileName.endsWith("xsl")) {
                 // Validate as XSLT.
-                svrlOutput = applySchematron(schematronAsXSLT(schematronFile));
+                schematronResource = schematronAsXSLT(schematronFile);
             } else if (schematronFileName.endsWith("sch")) {
                 // Validate as raw schematron.
                 convertXPathExpressions = supportPureValidationApproach;
-                svrlOutput = applySchematron(schematronAsRaw(schematronFile, supportPureValidationApproach));
+                schematronResource = schematronAsRaw(schematronFile, supportPureValidationApproach);
             } else {
                 // We're not certain - validate as raw and if that fails validate as XSLT.
-                try {
-                    convertXPathExpressions = supportPureValidationApproach;
-                    svrlOutput = applySchematron(schematronAsRaw(schematronFile, supportPureValidationApproach));
-                } catch (Exception e) {
+                convertXPathExpressions = supportPureValidationApproach;
+                schematronResource = schematronAsRaw(schematronFile, supportPureValidationApproach);
+                retryAsXslt = true;
+            }
+            try {
+                if (schematronResource instanceof SchematronResourcePure pureSchematron) {
+                    pureSchematron.setErrorHandler(new PureSchematronErrorHandler());
+                }
+                svrlOutput = applySchematron(schematronResource);
+            } catch (Exception e) {
+                if (retryAsXslt) {
                     // Try also as XSLT.
                     convertXPathExpressions = false;
                     svrlOutput = applySchematron(schematronAsXSLT(schematronFile));
+                } else {
+                    throw e;
                 }
             }
         } catch (Exception e) {
-            throw new IllegalStateException("Schematron file ["+schematronFile.getName()+"] is invalid", e);
+            if (schematronResource instanceof SchematronResourcePure pureSchematron
+                    && pureSchematron.getErrorHandler() instanceof PureSchematronErrorHandler errorHandler
+                    && errorHandler.isDueToExternalFunctionCall()) {
+                throw new IllegalStateException("Schematron file ["+schematronFile.getName()+"] is provided in pure Schematron format (as a .sch file) and contains references to functions (built-in or external). To be able to use functions you must convert the Schematron file to its XSLT representation and use the XSLT file instead", e);
+            } else {
+                throw new IllegalStateException("Schematron file ["+schematronFile.getName()+"] is invalid", e);
+            }
         }
         SchematronReportHandler handler = new SchematronReportHandler(specs.inputAsDocumentForSchematronValidation(), svrlOutput, convertXPathExpressions, specs.getDomainConfig().isIncludeTestDefinition(), specs.getDomainConfig().isIncludeAssertionID(), specs.isLocationAsPath(), specs.isShowLocationPaths(), specs.getLocalisationHelper());
         return handler.createReport();