[bugfix] eXist-db was previously doing more work than needed by

evaluating steps upon an empty context sequence. This was also not
XQuery specification compliant which led to incorrect results.
Closes #4466

Author: adamretter

exist-core/src/main/java/org/exist/xquery/PathExpr.java

@@ -229,7 +229,7 @@ public Sequence eval(Sequence contextSequence, final Item contextItem) throws XP
             //TODO : let the parser do it ? -pb
             boolean gotAtomicResult = false;
             Expression prev = null;
-            for (Expression step : steps) {
+            for (final Expression step : steps) {
                 prev = expr;
                 expr = step;
                 context.getWatchDog().proceed(expr);
@@ -299,7 +299,22 @@ public Sequence eval(Sequence contextSequence, final Item contextItem) throws XP
                         // expression with more than one step
                         result.removeDuplicates();
                     }
+
+                    /**
+                     * If the result is an Empty Sequence, we don't need to process
+                     * any more steps as there is no Context Item for the next step!
+                     *
+                     * There is one exception to this rule, which is a TextConstructor,
+                     * that only contains whitespace but has been configured
+                     * to strip-whitespace. In this instance the TextConstructor will
+                     * return an {@link Sequence.EMPTY_SEQUENCE_VALID} to indicate
+                     * that.
+                     */
+                    if (result != Sequence.EMPTY_SEQUENCE_VALID && result.isEmpty()) {
+                        break;
+                    }
                 }
+
                 if (!staticContext) {
                     currentContext = result;
                 }

exist-core/src/main/java/org/exist/xquery/TextConstructor.java

@@ -54,7 +54,7 @@ public TextConstructor(XQueryContext context, String text) throws XPathException
      */
     public Sequence eval(Sequence contextSequence, Item contextItem) throws XPathException {
         if (isWhitespaceOnly && context.stripWhitespace())
-            {return Sequence.EMPTY_SEQUENCE;}
+            {return Sequence.EMPTY_SEQUENCE_VALID;}
         if (newDocumentContext)
             {context.pushDocumentContext();}
         try {

exist-core/src/main/java/org/exist/xquery/value/Sequence.java

@@ -54,6 +54,8 @@ public interface Sequence {
      */
     Sequence EMPTY_SEQUENCE = new EmptySequence();
 
+    Sequence EMPTY_SEQUENCE_VALID = new EmptySequence();
+
     /**
      * The purpose of ordered and unordered flag is to set the ordering mode
      * in the static context to ordered or unordered for a certain region in a query.