/*
 * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
 * and is available at http://www.eclipse.org/legal/epl-v10.html
 */
package org.opendaylight.controller.md.sal.dom.broker.impl;

import com.google.common.base.Optional;
import com.google.common.base.Preconditions;
import com.google.common.util.concurrent.CheckedFuture;
import com.google.common.util.concurrent.FutureCallback;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.ListenableFuture;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import javax.annotation.Nonnull;
import javax.annotation.concurrent.GuardedBy;
import org.opendaylight.controller.md.sal.common.api.TransactionStatus;
import org.opendaylight.controller.md.sal.common.api.data.AsyncTransaction;
import org.opendaylight.controller.md.sal.common.api.data.LogicalDatastoreType;
import org.opendaylight.controller.md.sal.common.api.data.ReadFailedException;
import org.opendaylight.controller.md.sal.common.api.data.TransactionChain;
import org.opendaylight.controller.md.sal.common.api.data.TransactionChainListener;
import org.opendaylight.controller.md.sal.common.api.data.TransactionCommitFailedException;
import org.opendaylight.controller.md.sal.dom.api.DOMDataBroker;
import org.opendaylight.controller.md.sal.dom.api.DOMDataReadOnlyTransaction;
import org.opendaylight.controller.md.sal.dom.api.DOMDataReadWriteTransaction;
import org.opendaylight.controller.md.sal.dom.api.DOMDataWriteTransaction;
import org.opendaylight.controller.md.sal.dom.api.DOMTransactionChain;
import org.opendaylight.controller.md.sal.dom.spi.ForwardingDOMDataReadWriteTransaction;
import org.opendaylight.yangtools.yang.common.RpcResult;
import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier;
import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNode;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * An implementation of {@link DOMTransactionChain}, which has a very specific
 * behavior, which some users may find surprising. If keeps the general
 * intent of the contract, but it makes sure there are never more than two
 * transactions allocated at any given time: one of them is being committed,
 * and while that is happening, the other one acts as the scratch pad. Once
 * the committing transaction completes successfully, the scratch transaction
 * is enqueued as soon as it is ready.
 *
 * This mode of operation means that there is no inherent isolation between
 * the front-end transactions and transactions cannot be reasonably cancelled.
 *
 * It furthermore means that the transactions returned by {@link #newReadOnlyTransaction()}
 * counts as an outstanding transaction and the user may not allocate multiple
 * read-only transactions at the same time.
 */
public final class PingPongTransactionChain implements DOMTransactionChain {
    private static final Logger LOG = LoggerFactory.getLogger(PingPongTransactionChain.class);
    private final DOMTransactionChain delegate;

    @GuardedBy("this")
    private boolean failed;

    /**
     * This updater is used to manipulate the "ready" transaction. We perform only atomic
     * get-and-set on it.
     */
    private static final AtomicReferenceFieldUpdater<PingPongTransactionChain, PingPongTransaction> READY_UPDATER =
            AtomicReferenceFieldUpdater.newUpdater(PingPongTransactionChain.class, PingPongTransaction.class, "readyTx");
    @SuppressWarnings("unused") // Accessed via READY_UPDATER
    private volatile PingPongTransaction readyTx;

    /**
     * This updater is used to manipulate the "locked" transaction. A locked transaction
     * means we know that the user still holds a transaction and should at some point call
     * us. We perform on compare-and-swap to ensure we properly detect when a user is
     * attempting to allocated multiple transactions concurrently.
     */
    private static final AtomicReferenceFieldUpdater<PingPongTransactionChain, PingPongTransaction> LOCKED_UPDATER =
            AtomicReferenceFieldUpdater.newUpdater(PingPongTransactionChain.class, PingPongTransaction.class, "lockedTx");
    private volatile PingPongTransaction lockedTx;

    /**
     * This updater is used to manipulate the "inflight" transaction. There can be at most
     * one of these at any given time. We perform only compare-and-swap on these.
     */
    private static final AtomicReferenceFieldUpdater<PingPongTransactionChain, PingPongTransaction> INFLIGHT_UPDATER =
            AtomicReferenceFieldUpdater.newUpdater(PingPongTransactionChain.class, PingPongTransaction.class, "inflightTx");
    private volatile PingPongTransaction inflightTx;

    PingPongTransactionChain(final DOMDataBroker broker, final TransactionChainListener listener) {
        this.delegate = broker.createTransactionChain(new TransactionChainListener() {
            @Override
            public void onTransactionChainFailed(final TransactionChain<?, ?> chain, final AsyncTransaction<?, ?> transaction, final Throwable cause) {
                LOG.debug("Delegate chain {} reported failure in {}", chain, transaction, cause);

                final DOMDataReadWriteTransaction frontend;
                final PingPongTransaction tx = inflightTx;
                if (tx == null) {
                    LOG.warn("Transaction chain {} failed with no pending transactions", chain);
                    frontend = null;
                } else {
                    frontend = tx.getFrontendTransaction();
                }

                listener.onTransactionChainFailed(PingPongTransactionChain.this, frontend, cause);
                delegateFailed();
            }

            @Override
            public void onTransactionChainSuccessful(final TransactionChain<?, ?> chain) {
                listener.onTransactionChainSuccessful(PingPongTransactionChain.this);
            }
        });
    }

    private synchronized void delegateFailed() {
        failed = true;

        /*
         * If we do not have a locked transaction, we need to ensure that
         * the backend transaction is cancelled. Otherwise we can defer
         * until the user calls us.
         */
        if (lockedTx == null) {
            processIfReady();
        }
    }

    private synchronized PingPongTransaction slowAllocateTransaction() {
        final DOMDataReadWriteTransaction delegateTx = delegate.newReadWriteTransaction();
        final PingPongTransaction newTx = new PingPongTransaction(delegateTx);

        if (!LOCKED_UPDATER.compareAndSet(this, null, newTx)) {
            delegateTx.cancel();
            throw new IllegalStateException(String.format("New transaction %s raced with transacion %s", newTx, lockedTx));
        }

        return newTx;
    }

    private PingPongTransaction allocateTransaction() {
        // Step 1: acquire current state
        final PingPongTransaction oldTx = READY_UPDATER.getAndSet(this, null);

        // Slow path: allocate a delegate transaction
        if (oldTx == null) {
            return slowAllocateTransaction();
        }

        // Fast path: reuse current transaction. We will check
        //            failures and similar on submit().
        if (!LOCKED_UPDATER.compareAndSet(this, null, oldTx)) {
            // Ouch. Delegate chain has not detected a duplicate
            // transaction allocation. This is the best we can do.
            oldTx.getTransaction().cancel();
            throw new IllegalStateException(String.format("Reusable transaction %s raced with transaction %s", oldTx, lockedTx));
        }

        return oldTx;
    }

    // This forces allocateTransaction() on a slow path
    @GuardedBy("this")
    private void processIfReady() {
        final PingPongTransaction tx = READY_UPDATER.getAndSet(this, null);
        if (tx != null) {
            processTransaction(tx);
        }
    }

    /**
     * Process a ready transaction. The caller needs to ensure that
     * each transaction is seen only once by this method.
     *
     * @param tx Transaction which needs processing.
     */
    @GuardedBy("this")
    private void processTransaction(final @Nonnull PingPongTransaction tx) {
        if (failed) {
            LOG.debug("Cancelling transaction {}", tx);
            tx.getTransaction().cancel();
            return;
        }

        LOG.debug("Submitting transaction {}", tx);
        if (!INFLIGHT_UPDATER.compareAndSet(this, null, tx)) {
            LOG.warn("Submitting transaction {} while {} is still running", tx, inflightTx);
        }

        Futures.addCallback(tx.getTransaction().submit(), new FutureCallback<Void>() {
            @Override
            public void onSuccess(final Void result) {
                transactionSuccessful(tx, result);
            }

            @Override
            public void onFailure(final Throwable t) {
                transactionFailed(tx, t);
            }
        });
    }

    private void transactionSuccessful(final PingPongTransaction tx, final Void result) {
        LOG.debug("Transaction {} completed successfully", tx);

        final boolean success = INFLIGHT_UPDATER.compareAndSet(this, tx, null);
        Preconditions.checkState(success, "Successful transaction %s while %s was submitted", tx, inflightTx);

        synchronized (this) {
            processIfReady();
        }

        // Can run unsynchronized
        tx.onSuccess(result);
    }

    private void transactionFailed(final PingPongTransaction tx, final Throwable t) {
        LOG.debug("Transaction {} failed", tx, t);

        final boolean success = INFLIGHT_UPDATER.compareAndSet(this, tx, null);
        Preconditions.checkState(success, "Failed transaction %s while %s was submitted", tx, inflightTx);

        tx.onFailure(t);
    }

    private void readyTransaction(final @Nonnull PingPongTransaction tx) {
        final boolean lockedMatch = LOCKED_UPDATER.compareAndSet(this, tx, null);
        Preconditions.checkState(lockedMatch, "Attempted to submit transaction %s while we have %s", tx, lockedTx);

        LOG.debug("Transaction {} unlocked", tx);

        if (inflightTx == null) {
            synchronized (this) {
                processTransaction(tx);
            }
        }
    }

    @Override
    public void close() {
        final PingPongTransaction notLocked = lockedTx;
        Preconditions.checkState(notLocked == null, "Attempted to close chain with outstanding transaction %s", notLocked);

        synchronized (this) {
            processIfReady();
            delegate.close();
        }
    }

    @Override
    public DOMDataReadOnlyTransaction newReadOnlyTransaction() {
        final PingPongTransaction tx = allocateTransaction();

        return new DOMDataReadOnlyTransaction() {
            @Override
            public CheckedFuture<Optional<NormalizedNode<?, ?>>, ReadFailedException> read(final LogicalDatastoreType store,
                    final YangInstanceIdentifier path) {
                return tx.getTransaction().read(store, path);
            }

            @Override
            public CheckedFuture<Boolean, ReadFailedException> exists(final LogicalDatastoreType store,
                    final YangInstanceIdentifier path) {
                return tx.getTransaction().exists(store, path);
            }

            @Override
            public Object getIdentifier() {
                return tx.getTransaction().getIdentifier();
            }

            @Override
            public void close() {
                readyTransaction(tx);
            }
        };
    }

    @Override
    public DOMDataReadWriteTransaction newReadWriteTransaction() {
        final PingPongTransaction tx = allocateTransaction();
        final DOMDataReadWriteTransaction ret = new ForwardingDOMDataReadWriteTransaction() {
            @Override
            protected DOMDataReadWriteTransaction delegate() {
                return tx.getTransaction();
            }

            @Override
            public CheckedFuture<Void, TransactionCommitFailedException> submit() {
                readyTransaction(tx);
                return tx.getSubmitFuture();
            }

            @Override
            public ListenableFuture<RpcResult<TransactionStatus>> commit() {
                readyTransaction(tx);
                return tx.getCommitFuture();
            }

            @Override
            public boolean cancel() {
                throw new UnsupportedOperationException("Transaction cancellation is not supported");
            }
        };

        tx.recordFrontendTransaction(ret);
        return ret;
    }

    @Override
    public DOMDataWriteTransaction newWriteOnlyTransaction() {
        return newReadWriteTransaction();
    }
}