/*
 * Copyright (c) 2008, 2016, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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 General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package sun.nio.fs;

import java.nio.file.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.*;
import java.io.IOException;
import jdk.internal.misc.Unsafe;

import static sun.nio.fs.UnixNativeDispatcher.*;
import static sun.nio.fs.UnixConstants.*;

Linux implementation of WatchService based on inotify.
In summary a background thread polls inotify plus a socket used for the wakeup
mechanism. Requests to add or remove a watch, or close the watch service,
cause the thread to wakeup and process the request. Events are processed
by the thread which causes it to signal/queue the corresponding watch keys.
/**
 * Linux implementation of WatchService based on inotify.
 *
 * In summary a background thread polls inotify plus a socket used for the wakeup
 * mechanism. Requests to add or remove a watch, or close the watch service,
 * cause the thread to wakeup and process the request. Events are processed
 * by the thread which causes it to signal/queue the corresponding watch keys.
 */

class LinuxWatchService
    extends AbstractWatchService
{
    private static final Unsafe unsafe = Unsafe.getUnsafe();

    // background thread to read change events
    private final Poller poller;

    LinuxWatchService(UnixFileSystem fs) throws IOException {
        // initialize inotify
        int ifd = - 1;
        try {
            ifd = inotifyInit();
        } catch (UnixException x) {
            String msg = (x.errno() == EMFILE) ?
                "User limit of inotify instances reached or too many open files" :
                x.errorString();
            throw new IOException(msg);
        }

        // configure inotify to be non-blocking
        // create socketpair used in the close mechanism
        int sp[] = new int[2];
        try {
            configureBlocking(ifd, false);
            socketpair(sp);
            configureBlocking(sp[0], false);
        } catch (UnixException x) {
            UnixNativeDispatcher.close(ifd);
            throw new IOException(x.errorString());
        }

        this.poller = new Poller(fs, this, ifd, sp);
        this.poller.start();
    }

    @Override
    WatchKey register(Path dir,
                      WatchEvent.Kind<?>[] events,
                      WatchEvent.Modifier... modifiers)
         throws IOException
    {
        // delegate to poller
        return poller.register(dir, events, modifiers);
    }

    @Override
    void implClose() throws IOException {
        // delegate to poller
        poller.close();
    }

    
WatchKey implementation
/**
     * WatchKey implementation
     */
    private static class LinuxWatchKey extends AbstractWatchKey {
        // inotify descriptor
        private final int ifd;
        // watch descriptor
        private volatile int wd;

        LinuxWatchKey(UnixPath dir, LinuxWatchService watcher, int ifd, int wd) {
            super(dir, watcher);
            this.ifd = ifd;
            this.wd = wd;
        }

        int descriptor() {
            return wd;
        }

        void invalidate(boolean remove) {
            if (remove) {
                try {
                    inotifyRmWatch(ifd, wd);
                } catch (UnixException x) {
                    // ignore
                }
            }
            wd = -1;
        }

        @Override
        public boolean isValid() {
            return (wd != -1);
        }

        @Override
        public void cancel() {
            if (isValid()) {
                // delegate to poller
                ((LinuxWatchService)watcher()).poller.cancel(this);
            }
        }
    }

    
Background thread to read from inotify
/**
     * Background thread to read from inotify
     */
    private static class Poller extends AbstractPoller {
        
struct inotify_event {
    int          wd;
    uint32_t     mask;
    uint32_t     len;
    char name    __flexarr;  // present if len > 0
} act_t;
/**
         * struct inotify_event {
         *     int          wd;
         *     uint32_t     mask;
         *     uint32_t     len;
         *     char name    __flexarr;  // present if len > 0
         * } act_t;
         */
        private static final int SIZEOF_INOTIFY_EVENT  = eventSize();
        private static final int[] offsets             = eventOffsets();
        private static final int OFFSETOF_WD           = offsets[0];
        private static final int OFFSETOF_MASK         = offsets[1];
        private static final int OFFSETOF_LEN          = offsets[3];
        private static final int OFFSETOF_NAME         = offsets[4];

        private static final int IN_MODIFY          = 0x00000002;
        private static final int IN_ATTRIB          = 0x00000004;
        private static final int IN_MOVED_FROM      = 0x00000040;
        private static final int IN_MOVED_TO        = 0x00000080;
        private static final int IN_CREATE          = 0x00000100;
        private static final int IN_DELETE          = 0x00000200;

        private static final int IN_UNMOUNT         = 0x00002000;
        private static final int IN_Q_OVERFLOW      = 0x00004000;
        private static final int IN_IGNORED         = 0x00008000;

        // sizeof buffer for when polling inotify
        private static final int BUFFER_SIZE = 8192;

        private final UnixFileSystem fs;
        private final LinuxWatchService watcher;

        // inotify file descriptor
        private final int ifd;
        // socketpair used to shutdown polling thread
        private final int socketpair[];
        // maps watch descriptor to Key
        private final Map<Integer,LinuxWatchKey> wdToKey;
        // address of read buffer
        private final long address;

        Poller(UnixFileSystem fs, LinuxWatchService watcher, int ifd, int[] sp) {
            this.fs = fs;
            this.watcher = watcher;
            this.ifd = ifd;
            this.socketpair = sp;
            this.wdToKey = new HashMap<>();
            this.address = unsafe.allocateMemory(BUFFER_SIZE);
        }

        @Override
        void wakeup() throws IOException {
            // write to socketpair to wakeup polling thread
            try {
                write(socketpair[1], address, 1);
            } catch (UnixException x) {
                throw new IOException(x.errorString());
            }
        }

        @Override
        Object implRegister(Path obj,
                            Set<? extends WatchEvent.Kind<?>> events,
                            WatchEvent.Modifier... modifiers)
        {
            UnixPath dir = (UnixPath)obj;

            int mask = 0;
            for (WatchEvent.Kind<?> event: events) {
                if (event == StandardWatchEventKinds.ENTRY_CREATE) {
                    mask |= IN_CREATE | IN_MOVED_TO;
                    continue;
                }
                if (event == StandardWatchEventKinds.ENTRY_DELETE) {
                    mask |= IN_DELETE | IN_MOVED_FROM;
                    continue;
                }
                if (event == StandardWatchEventKinds.ENTRY_MODIFY) {
                    mask |= IN_MODIFY | IN_ATTRIB;
                    continue;
                }
            }

            // no modifiers supported at this time
            if (modifiers.length > 0) {
                for (WatchEvent.Modifier modifier: modifiers) {
                    if (modifier == null)
                        return new NullPointerException();
                    if (!ExtendedOptions.SENSITIVITY_HIGH.matches(modifier) &&
                            !ExtendedOptions.SENSITIVITY_MEDIUM.matches(modifier) &&
                            !ExtendedOptions.SENSITIVITY_LOW.matches(modifier)) {
                        return new UnsupportedOperationException("Modifier not supported");
                    }
                }
            }

            // check file is directory
            UnixFileAttributes attrs = null;
            try {
                attrs = UnixFileAttributes.get(dir, true);
            } catch (UnixException x) {
                return x.asIOException(dir);
            }
            if (!attrs.isDirectory()) {
                return new NotDirectoryException(dir.getPathForExceptionMessage());
            }

            // register with inotify (replaces existing mask if already registered)
            int wd = -1;
            try {
                NativeBuffer buffer =
                    NativeBuffers.asNativeBuffer(dir.getByteArrayForSysCalls());
                try {
                    wd = inotifyAddWatch(ifd, buffer.address(), mask);
                } finally {
                    buffer.release();
                }
            } catch (UnixException x) {
                if (x.errno() == ENOSPC) {
                    return new IOException("User limit of inotify watches reached");
                }
                return x.asIOException(dir);
            }

            // ensure watch descriptor is in map
            LinuxWatchKey key = wdToKey.get(wd);
            if (key == null) {
                key = new LinuxWatchKey(dir, watcher, ifd, wd);
                wdToKey.put(wd, key);
            }
            return key;
        }

        // cancel single key
        @Override
        void implCancelKey(WatchKey obj) {
            LinuxWatchKey key = (LinuxWatchKey)obj;
            if (key.isValid()) {
                wdToKey.remove(key.descriptor());
                key.invalidate(true);
            }
        }

        // close watch service
        @Override
        void implCloseAll() {
            // invalidate all keys
            for (Map.Entry<Integer,LinuxWatchKey> entry: wdToKey.entrySet()) {
                entry.getValue().invalidate(true);
            }
            wdToKey.clear();

            // free resources
            unsafe.freeMemory(address);
            UnixNativeDispatcher.close(socketpair[0]);
            UnixNativeDispatcher.close(socketpair[1]);
            UnixNativeDispatcher.close(ifd);
        }

        
Poller main loop
/**
         * Poller main loop
         */
        @Override
        public void run() {
            try {
                for (;;) {
                    int nReady, bytesRead;

                    // wait for close or inotify event
                    nReady = poll(ifd, socketpair[0]);

                    // read from inotify
                    try {
                        bytesRead = read(ifd, address, BUFFER_SIZE);
                    } catch (UnixException x) {
                        if (x.errno() != EAGAIN)
                            throw x;
                        bytesRead = 0;
                    }

                    // iterate over buffer to decode events
                    int offset = 0;
                    while (offset < bytesRead) {
                        long event = address + offset;
                        int wd = unsafe.getInt(event + OFFSETOF_WD);
                        int mask = unsafe.getInt(event + OFFSETOF_MASK);
                        int len = unsafe.getInt(event + OFFSETOF_LEN);

                        // file name
                        UnixPath name = null;
                        if (len > 0) {
                            int actual = len;

                            // null-terminated and maybe additional null bytes to
                            // align the next event
                            while (actual > 0) {
                                long last = event + OFFSETOF_NAME + actual - 1;
                                if (unsafe.getByte(last) != 0)
                                    break;
                                actual--;
                            }
                            if (actual > 0) {
                                byte[] buf = new byte[actual];
                                unsafe.copyMemory(null, event + OFFSETOF_NAME,
                                    buf, Unsafe.ARRAY_BYTE_BASE_OFFSET, actual);
                                name = new UnixPath(fs, buf);
                            }
                        }

                        // process event
                        processEvent(wd, mask, name);

                        offset += (SIZEOF_INOTIFY_EVENT + len);
                    }

                    // process any pending requests
                    if ((nReady > 1) || (nReady == 1 && bytesRead == 0)) {
                        try {
                            read(socketpair[0], address, BUFFER_SIZE);
                            boolean shutdown = processRequests();
                            if (shutdown)
                                break;
                        } catch (UnixException x) {
                            if (x.errno() != UnixConstants.EAGAIN)
                                throw x;
                        }
                    }
                }
            } catch (UnixException x) {
                x.printStackTrace();
            }
        }


        
map inotify event to WatchEvent.Kind
/**
         * map inotify event to WatchEvent.Kind
         */
        private WatchEvent.Kind<?> maskToEventKind(int mask) {
            if ((mask & IN_MODIFY) > 0)
                return StandardWatchEventKinds.ENTRY_MODIFY;
            if ((mask & IN_ATTRIB) > 0)
                return StandardWatchEventKinds.ENTRY_MODIFY;
            if ((mask & IN_CREATE) > 0)
                return StandardWatchEventKinds.ENTRY_CREATE;
            if ((mask & IN_MOVED_TO) > 0)
                return StandardWatchEventKinds.ENTRY_CREATE;
            if ((mask & IN_DELETE) > 0)
                return StandardWatchEventKinds.ENTRY_DELETE;
            if ((mask & IN_MOVED_FROM) > 0)
                return StandardWatchEventKinds.ENTRY_DELETE;
            return null;
        }

        
Process event from inotify
/**
         * Process event from inotify
         */
        private void processEvent(int wd, int mask, final UnixPath name) {
            // overflow - signal all keys
            if ((mask & IN_Q_OVERFLOW) > 0) {
                for (Map.Entry<Integer,LinuxWatchKey> entry: wdToKey.entrySet()) {
                    entry.getValue()
                        .signalEvent(StandardWatchEventKinds.OVERFLOW, null);
                }
                return;
            }

            // lookup wd to get key
            LinuxWatchKey key = wdToKey.get(wd);
            if (key == null)
                return; // should not happen

            // file deleted
            if ((mask & IN_IGNORED) > 0) {
                wdToKey.remove(wd);
                key.invalidate(false);
                key.signal();
                return;
            }

            // event for directory itself
            if (name == null)
                return;

            // map to event and queue to key
            WatchEvent.Kind<?> kind = maskToEventKind(mask);
            if (kind != null) {
                key.signalEvent(kind, name);
            }
        }
    }

    // -- native methods --

    // sizeof inotify_event
    private static native int eventSize();

    // offsets of inotify_event
    private static native int[] eventOffsets();

    private static native int inotifyInit() throws UnixException;

    private static native int inotifyAddWatch(int fd, long pathAddress, int mask)
        throws UnixException;

    private static native void inotifyRmWatch(int fd, int wd)
        throws UnixException;

    private static native void configureBlocking(int fd, boolean blocking)
        throws UnixException;

    private static native void socketpair(int[] sv) throws UnixException;

    private static native int poll(int fd1, int fd2) throws UnixException;

    static {
        AccessController.doPrivileged(new PrivilegedAction<>() {
            public Void run() {
                System.loadLibrary("nio");
                return null;
        }});
    }
}