[refactor] Generalised the solution so that it isn't just about TextConstructor

Author: adamretter

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

@@ -183,4 +183,9 @@ public Expression simplify() {
     public Expression getParent() {
         return null;
     }
+
+    @Override
+    public boolean evalNextExpressionOnEmptyContextSequence() {
+        return false;
+    }
 }

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

@@ -194,4 +194,9 @@ public Expression simplify() {
     public Expression getParent() {
         return null;
     }
+
+    @Override
+    public boolean evalNextExpressionOnEmptyContextSequence() {
+        return false;
+    }
 }

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

@@ -255,4 +255,13 @@ public interface Expression {
     public boolean allowMixedNodesInReturn();
 
     public Expression getParent();
+
+    /**
+     * Return true only if the next expression within a path expression
+     * should be evaluated even when this expression returns an
+     * empty sequence.
+     *
+     * @return true if the next expression should be evaluated, false otherwise.
+     */
+    boolean evalNextExpressionOnEmptyContextSequence();
 }

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

@@ -302,15 +302,15 @@ public Sequence eval(Sequence contextSequence, final Item contextItem) throws XP
 
                     /**
                      * 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!
+                     * any more steps as there is no Context Sequence for the next step!
                      *
-                     * There is one exception to this rule, which is a TextConstructor,
-                     * that only contains whitespace but has been configured
+                     * There is one exception to this rule, which is a TextConstructor
+                     * expression 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.
+                     * return true when {@link Expression#evalNextExpressionOnEmptyContextSequence()}
+                     * is called to indicate that.
                      */
-                    if (result != Sequence.EMPTY_SEQUENCE_VALID && result.isEmpty()) {
+                    if (result.isEmpty() && !step.evalNextExpressionOnEmptyContextSequence()) {
                         break;
                     }
                 }

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_VALID;}
+            {return Sequence.EMPTY_SEQUENCE;}
         if (newDocumentContext)
             {context.pushDocumentContext();}
         try {
@@ -96,4 +96,13 @@ public String getText() {
     public boolean allowMixedNodesInReturn() {
         return true;
     }
+
+    @Override
+    public boolean evalNextExpressionOnEmptyContextSequence() {
+        /*
+        When this TextConstructor only contains whitespace but has been configured
+        to strip-whitespace, return true so that the next expression is processed correctly.
+         */
+        return isWhitespaceOnly && context.stripWhitespace();
+    }
 }

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

@@ -54,8 +54,6 @@ 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.