[bugfix] Avoid deadlock with NativeBroker#getXMLResource(XmldbURI, LockMode) when <document use-path-locks=true/> is set in conf.xml

Author: adamretter

exist-core/src/main/java/org/exist/storage/NativeBroker.java

@@ -2209,7 +2209,8 @@ public LockedDocument getXMLResource(XmldbURI fileName, final LockMode lockMode)
         //TODO : resolve URIs !
         final XmldbURI collUri = fileName.removeLastSegment();
         final XmldbURI docUri = fileName.lastSegment();
-        try(final Collection collection = openCollection(collUri, LockMode.READ_LOCK)) {
+        final LockMode collectionLockMode = lockManager.relativeCollectionLockMode(LockMode.READ_LOCK, lockMode);
+        try(final Collection collection = openCollection(collUri, collectionLockMode)) {
             if (collection == null) {
                 LOG.debug("Collection '{}' not found!", collUri);
                 return null;

exist-core/src/main/java/org/exist/storage/lock/LockManager.java

@@ -602,6 +602,48 @@ public boolean isDocumentLockedForRead(final XmldbURI documentPath) {
         return existingLock.getReadLockCount() > 0;
     }
 
+    /**
+     * Returns the LockMode that should be used for accessing
+     * a Collection when that access is just for the purposes
+     * or accessing a document(s) within the Collection.
+     *
+     * When Path Locks are enabled for Documents, both Collections and
+     * Documents share the same lock domain, a path based tree hierarchy.
+     * With a shared lock-hierarchy, if we were to READ_LOCK a Collection
+     * and then request a WRITE_LOCK for a Document in that Collection we
+     * would reach a dead-lock situation. To avoid this, if this method is
+     * called like
+     * {@code relativeCollectionLockMode(LockMode.READ_LOCK, LockMode.WRITE_LOCK)}
+     * it will return a WRITE_LOCK. That is to say that to aid dealock-avoidance,
+     * this function may return a stricter locking mode than the {@code desiredCollectionLockMode}.
+     *
+     * When Path Locks are disabled (the default) for Documents, Collection and Documents
+     * have independent locking domains. In this case this function will always return
+     * the {@code desiredCollectionLockMode}.
+     *
+     * @param desiredCollectionLockMode The desired lock mode for the Collection.
+     * @param documentLockMode The lock mode that will be used for subsequent document operations in the Collection.
+     *
+     * @return The lock mode that should be used for accessing the Collection.
+     */
+    public Lock.LockMode relativeCollectionLockMode(final Lock.LockMode desiredCollectionLockMode,
+            final Lock.LockMode documentLockMode) {
+        if (!usePathLocksForDocuments) {
+            return desiredCollectionLockMode;
+
+        } else {
+            switch (documentLockMode) {
+                case NO_LOCK:
+                case INTENTION_READ:
+                case READ_LOCK:
+                    return Lock.LockMode.READ_LOCK;
+
+                default:
+                    return Lock.LockMode.WRITE_LOCK;
+            }
+        }
+    }
+
     /**
      * Retrieves a lock for a {@link org.exist.storage.dom.DOMFile}
      *

exist-core/src/main/java/org/exist/xmlrpc/RpcConnection.java

@@ -275,9 +275,11 @@ protected LockedDocumentMap beginProtected(final DBBroker broker, final Map<Stri
         do {
             MutableDocumentSet docs = null;
             final LockedDocumentMap lockedDocuments = new LockedDocumentMap();
-            try(final Collection coll = broker.openCollection(XmldbURI.createInternal(protectColl), LockMode.READ_LOCK)) {
+            final LockMode documentLockMode = LockMode.WRITE_LOCK;
+            final LockMode collectionLockMode = broker.getBrokerPool().getLockManager().relativeCollectionLockMode(LockMode.READ_LOCK, documentLockMode);
+            try (final Collection coll = broker.openCollection(XmldbURI.createInternal(protectColl), collectionLockMode)) {
                 docs = new DefaultDocumentSet();
-                coll.allDocs(broker, docs, true, lockedDocuments, LockMode.WRITE_LOCK);
+                coll.allDocs(broker, docs, true, lockedDocuments, documentLockMode);
                 return lockedDocuments;
             } catch (final LockException e) {
                 LOG.warn("Deadlock detected. Starting over again. Docs: {}; locked: {}. Cause: {}", docs.getDocumentCount(), lockedDocuments.size(), e.getMessage());