[bugfix] fn:transform() now logs xsl:message output

exist-core/src/main/java/org/exist/xquery/functions/fn/transform/Transform.java

@@ -53,6 +53,7 @@
 import net.sf.saxon.s9api.*;
 import net.sf.saxon.serialize.SerializationProperties;
 import net.sf.saxon.trans.UncheckedXPathException;
+import org.apache.commons.io.output.StringBuilderWriter;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.exist.dom.QName;
@@ -162,6 +163,34 @@ public Sequence eval(final Sequence[] args, final Sequence contextSequence) thro
 
                 final Xslt30Transformer xslt30Transformer = xsltExecutable.load30();
 
+                xslt30Transformer.setMessageListener((content, terminate, locator) ->{
+                    try {
+                        final StringBuilderWriter writer = new StringBuilderWriter();
+                        final Serializer serializer = context.getBroker().getBrokerPool().getSaxonProcessor().newSerializer();
+                        serializer.setOutputProperty(Serializer.Property.OMIT_XML_DECLARATION, "yes");
+                        serializer.setOutputWriter(writer);
+                        serializer.serializeNode(content);
+
+                        final String source;
+                        final int sourceLine;
+                        final int sourceColumn;
+                        if (locator != null) {
+                            source = locator.getSystemId();
+                            sourceLine = locator.getLineNumber();
+                            sourceColumn = locator.getColumnNumber();
+                        } else {
+                            source = null;
+                            sourceLine = -1;
+                            sourceColumn = -1;
+                        }
+
+                        LOGGER.info("<xsl:message terminate=\"{}\" source=\"{}\" sourceLine=\"{}\" sourceColumn=\"{}\">{}</xsl:message>",
+                                terminate, source, sourceLine, sourceColumn, writer.toString());
+                    } catch (final SaxonApiException e) {
+                        LOGGER.error("Unable to serialize xsl:message content", e);
+                    }
+                });
+
                 options.initialMode.ifPresent(qNameValue -> xslt30Transformer.setInitialMode(Convert.ToSaxon.of(qNameValue.getQName())));
                 xslt30Transformer.setInitialTemplateParameters(options.templateParams, false);
                 xslt30Transformer.setInitialTemplateParameters(options.tunnelParams, true);

exist-core/src/test/java/org/exist/xquery/functions/fn/transform/FunTransformITTest.java

@@ -21,6 +21,11 @@
 package org.exist.xquery.functions.fn.transform;
 
 import com.evolvedbinary.j8fu.tuple.Tuple2;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.appender.AbstractAppender;
+import org.apache.logging.log4j.core.config.Property;
+import org.apache.logging.log4j.core.layout.PatternLayout;
 import org.exist.EXistException;
 import org.exist.collections.Collection;
 import org.exist.security.PermissionDeniedException;
@@ -49,7 +54,9 @@
 
 import javax.xml.transform.Source;
 import java.io.IOException;
+import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import static com.evolvedbinary.j8fu.tuple.Tuple.Tuple;
 import static org.junit.Assert.*;
@@ -247,6 +254,46 @@ public void identityMixedMemoryAndPersistentDom() throws XPathException, Permiss
         expectQuery(IDENTITY_MIXED_XSLT_QUERY_5, expected);
     }
 
+    @Test
+    public void xslMessageIsLogged() throws EXistException, PermissionDeniedException, IOException {
+        final CapturingAppender appender = new CapturingAppender();
+        appender.start();
+
+        final org.apache.logging.log4j.core.Logger transformLogger =
+                (org.apache.logging.log4j.core.Logger) LogManager.getLogger(Transform.class);
+        transformLogger.addAppender(appender);
+
+        try {
+            final String query =
+                "fn:transform(map {\n" +
+                "  \"stylesheet-text\": '<xsl:stylesheet xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\" version=\"3.0\">\n" +
+                "    <xsl:template match=\"/\">\n" +
+                "      <xsl:message>Hello from XSLT</xsl:message>\n" +
+                "      <out/>\n" +
+                "    </xsl:template>\n" +
+                "  </xsl:stylesheet>',\n" +
+                "  \"source-node\": document { <in/> }\n" +
+                "})?output";
+
+            final BrokerPool pool = existEmbeddedServer.getBrokerPool();
+            try (final DBBroker broker = pool.get(Optional.of(pool.getSecurityManager().getSystemSubject()));
+                 final XQueryUtil.QueryResult queryResult = XQueryUtil.query(broker, new StringSource(query), false, null, null, null, null, null)) {
+                assertNotNull(queryResult.result);
+            } catch (final XPathException e) {
+                fail("Transform should have succeeded: " + e.getMessage());
+            }
+
+            final Optional<String> logged = appender.getMessages().stream()
+                    .filter(message -> message.contains("<xsl:message"))
+                    .findFirst();
+
+            assertTrue("Expected an xsl:message log entry", logged.isPresent());
+            assertTrue(logged.get().contains("Hello from XSLT"));
+        } finally {
+            transformLogger.removeAppender(appender);
+        }
+    }
+
     private static void expectQuery(final String query, final Source expected) throws EXistException, XPathException, PermissionDeniedException, IOException {
         final BrokerPool pool = existEmbeddedServer.getBrokerPool();
         try(final DBBroker broker = pool.get(Optional.of(pool.getSecurityManager().getSystemSubject()));
@@ -298,4 +345,26 @@ private static void createCollection(final DBBroker broker, final Txn transactio
             }
         }
     }
+
+    /**
+     * A minimal in-memory Log4j2 appender that just remembers every
+     * formatted message it receives, so a test can inspect what was logged.
+     */
+    private static class CapturingAppender extends AbstractAppender {
+
+        private final List<String> messages = new CopyOnWriteArrayList<>();
+
+        CapturingAppender() {
+            super("capturing-appender", null, PatternLayout.createDefaultLayout(), false, Property.EMPTY_ARRAY);
+        }
+
+        @Override
+        public void append(final LogEvent event) {
+            messages.add(event.getMessage().getFormattedMessage());
+        }
+
+        List<String> getMessages() {
+            return messages;
+        }
+    }
 }