[bugfix] PathExpr: iterate atomic, context-independent RHS per-item (closes #798)

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

@@ -259,7 +259,38 @@ public Sequence eval(Sequence contextSequence, final Item contextItem) throws XP
                         !currentContext.isPersistentSet();
                 //DESIGN : first test the dependency then the result
                 final int exprDeps = expr.getDependencies();
-                if (inMemProcessing ||
+                // XPath 3.1 §3.3.5 (Path Operator) requires E2 in E1/E2 to be evaluated
+                // for each item in E1's result. The "single eval" else-branch below is a
+                // performance shortcut that only preserves the correct multiplicity when
+                // E2 returns nodes (the subsequent removeDuplicates() call absorbs any
+                // missing iterations). When E2 is a context-INdependent step that
+                // returns atomic values, the shortcut collapses the result — every
+                // iteration of E1 would produce the same atomic value, so the missing
+                // iterations matter. See #798.
+                //
+                // This is narrowly the "atomic literal RHS" case (`//b/3`,
+                // `//x/'name'`, etc.). Context-dependent atomic steps already
+                // declare CONTEXT_ITEM/POSITION/SET and take the existing iterate
+                // branch — or, in the index-optimised Predicate.selectByNodeSet
+                // path, are intentionally evaluated once against the full node-set.
+                // We must not force iteration in those cases.
+                //
+                // Restricted to non-first steps: the first step is the path's
+                // starting point, not a "RHS of /". This also keeps us out of the
+                // function-argument-wrapper case, where a single-step PathExpr
+                // wraps an atomic-returning argument (e.g. the literal pattern
+                // `'^HAM.*'` of matches() inside a predicate) that must NOT be
+                // iterated over the surrounding context.
+                final boolean stepReturnsNonNode = !Type.subTypeOf(expr.returnsType(), Type.NODE);
+                final boolean stepIsContextIndependent =
+                        !Dependency.dependsOn(exprDeps, Dependency.CONTEXT_ITEM)
+                        && !Dependency.dependsOn(exprDeps, Dependency.CONTEXT_POSITION)
+                        && !Dependency.dependsOn(exprDeps, Dependency.CONTEXT_SET);
+                final boolean atomicRhsMustIterate = stepReturnsNonNode
+                        && stepIsContextIndependent
+                        && stepIdx > 0
+                        && currentContext != null && currentContext.hasMany();
+                if (inMemProcessing || atomicRhsMustIterate ||
                         ((Dependency.dependsOn(exprDeps, Dependency.CONTEXT_ITEM) ||
                                 Dependency.dependsOn(exprDeps, Dependency.CONTEXT_POSITION)) &&
                                 //A positional predicate will be evaluated one time

exist-core/src/test/java/org/exist/xquery/PathExprAtomicRhsTest.java

@@ -0,0 +1,103 @@
+/*
+ * eXist-db Open Source Native XML Database
+ * Copyright (C) 2001 The eXist-db Authors
+ *
+ * info@exist-db.org
+ * http://www.exist-db.org
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package org.exist.xquery;
+
+import org.exist.test.ExistXmldbEmbeddedServer;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.ClassRule;
+import org.junit.Test;
+import org.xmldb.api.base.ResourceSet;
+import org.xmldb.api.base.XMLDBException;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Regression test for PathExpr per-item iteration over atomic-returning steps.
+ *
+ * <p>XPath 3.1 §3.3.5 mandates that in {@code E1/E2}, E2 is evaluated once
+ * <em>for each</em> item produced by E1. eXist's PathExpr.eval previously
+ * short-circuited to a single E2 evaluation when E1 was a persistent node-set
+ * and E2 did not declare a context dependency — collapsing multiplicity for
+ * atomic-returning E2 (literals, etc.).</p>
+ *
+ * @see <a href="https://github.com/eXist-db/exist/issues/798">Issue #798</a>
+ */
+public class PathExprAtomicRhsTest {
+
+    @ClassRule
+    public static final ExistXmldbEmbeddedServer embedded =
+            new ExistXmldbEmbeddedServer(false, true, true);
+
+    @BeforeClass
+    public static void store() throws XMLDBException {
+        embedded.executeQuery(
+                "xmldb:store('/db', 'pathexpr-issue798.xml', <a><b/><b/></a>)");
+    }
+
+    @AfterClass
+    public static void cleanup() throws XMLDBException {
+        try {
+            embedded.executeQuery("xmldb:remove('/db', 'pathexpr-issue798.xml')");
+        } catch (final XMLDBException ignored) {
+            // best-effort cleanup
+        }
+    }
+
+    /** In-memory baseline: //b/3 returns one 3 per b element. */
+    @Test
+    public void inMemoryAtomicRhsIteratesPerItem() throws XMLDBException {
+        final ResourceSet rs = embedded.executeQuery(
+                "count((<a><b/><b/></a>)//b/3)");
+        assertEquals("2", rs.getResource(0).getContent());
+    }
+
+    /** Persistent input must iterate identically to in-memory. The bug. */
+    @Test
+    public void persistentAtomicRhsIteratesPerItem() throws XMLDBException {
+        final ResourceSet rs = embedded.executeQuery(
+                "count(doc('/db/pathexpr-issue798.xml')//b/3)");
+        assertEquals("2", rs.getResource(0).getContent());
+    }
+
+    /** Both forms must return the same sequence content. */
+    @Test
+    public void inMemoryAndPersistentAgree() throws XMLDBException {
+        final ResourceSet inMem = embedded.executeQuery(
+                "string-join((for $x in (<a><b/><b/></a>)//b/3 return string($x)), ',')");
+        final ResourceSet stored = embedded.executeQuery(
+                "string-join((for $x in doc('/db/pathexpr-issue798.xml')//b/3 return string($x)), ',')");
+        assertEquals(inMem.getResource(0).getContent(), stored.getResource(0).getContent());
+        assertEquals("3,3", stored.getResource(0).getContent());
+    }
+
+    /**
+     * Sanity: node-axis RHS still de-duplicates as before. Two b elements
+     * with the same parent: //b/.. should de-dup to one a element.
+     */
+    @Test
+    public void nodeRhsStillDedupes() throws XMLDBException {
+        final ResourceSet rs = embedded.executeQuery(
+                "count(doc('/db/pathexpr-issue798.xml')//b/..)");
+        assertEquals("1", rs.getResource(0).getContent());
+    }
+}