Github user ajs6f commented on a diff in the pull request:
https://github.com/apache/jena/pull/153#discussion_r70181662
--- Diff: jena-arq/src/main/java/org/apache/jena/system/ThreadTxn.java ---
@@ -0,0 +1,148 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.jena.system;
+
+import java.util.Objects ;
+import java.util.concurrent.Executor ;
+import java.util.concurrent.Executors ;
+import java.util.concurrent.Semaphore ;
+import java.util.concurrent.atomic.AtomicReference ;
+
+import org.apache.jena.query.ReadWrite ;
+import org.apache.jena.sparql.core.Transactional ;
+
+/**
+ * An action that will happen on a different thread later when {@link
#run} is
+ * called. A thread is created and the transaction started during a call
to the
+ * creation operations {@link #threadTxnRead} or {@link #threadTxnWrite}.
+ * The associated Runnable is called and the transaction completed when
+ * {@link #run} is called. Being on a thread, the state of the world the
+ * forked transaction sees is outside the creating thread which may itself
be in a
+ * transaction. Warning: creating a write transaction inside a write
transaction
+ * will cause deadlock.
+ */
+public class ThreadTxn {
+ // ---- Thread
+
+ /** Create a thread-backed delayed READ transaction action.
+ * Call {@link ThreadTxn#run} to perform the read transaction.
+ */
+ public static ThreadTxn threadTxnRead(Transactional trans, Runnable
action) {
+ return ThreadTxn.create(trans, ReadWrite.READ, action, false) ;
+ }
+
+ /** Create a thread-backed delayed WRITE action.
+ * Call {@link ThreadTxn#run} to perform the write transaction.
+ * (If called from inside a write transaction on the {@code trans},
+ * this will deadlock.)
+ */
+ public static ThreadTxn threadTxnWrite(Transactional trans, Runnable
action) {
+ return ThreadTxn.create(trans, ReadWrite.WRITE, action, true) ;
+ }
+
+ /** Create a thread-backed delayed WRITE-abort action (mainly for
testing). */
+ public static ThreadTxn threadTxnWriteAbort(Transactional trans,
Runnable action) {
+ return ThreadTxn.create(trans, ReadWrite.WRITE, action, false) ;
+ }
+
+ private final Semaphore semaStart ;
+ private final Semaphore semaFinish ;
+ private final AtomicReference<RuntimeException> thrownRuntimeException
= new AtomicReference<>(null) ;
+ private final AtomicReference<Error> thrownError = new
AtomicReference<>(null) ;
+ private final Runnable action ;
+
+ private ThreadTxn(Runnable action) {
+ this.action = action ;
+ this.semaStart = new Semaphore(0, true) ;
--- End diff --
I'm a little confused about the way these semaphores work. E.g. how do they
function with 0 permits? Below, in `::run()` you release the start semaphore,
but I can't see where you acquire itâ¦Â on the theory (possibly a bad one)
that other newbies will also find this opaque, can you comment this up a bit to
explain the concurrency control at work here?
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
