1/*
2 * Copyright (c) 2001, 2012, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.  Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26package sun.nio.ch;
27
28import java.nio.channels.spi.SelectorProvider;
29import java.security.AccessController;
30import sun.security.action.GetPropertyAction;
31
32/**
33 * Creates this platform's default SelectorProvider
34 */
35
36public class DefaultSelectorProvider {
37
38    /**
39     * Prevent instantiation.
40     */
41    private DefaultSelectorProvider() { }
42
43    // Android-removed: Dead code: We always use PollSelectorProvider.
44    /*
45    @SuppressWarnings("unchecked")
46    private static SelectorProvider createProvider(String cn) {
47        Class<SelectorProvider> c;
48        try {
49            c = (Class<SelectorProvider>)Class.forName(cn);
50        } catch (ClassNotFoundException x) {
51            throw new AssertionError(x);
52        }
53        try {
54            return c.newInstance();
55        } catch (IllegalAccessException | InstantiationException x) {
56            throw new AssertionError(x);
57        }
58
59    }
60    */
61
62    /**
63     * Returns the default SelectorProvider.
64     */
65    public static SelectorProvider create() {
66        // Android-note: Explain why we always use of PollSelectorProvider.
67        /*
68        The OpenJDK epoll based selector suffers from a serious bug where it
69        can never successfully deregister keys from closed channels.
70
71        The root cause of this bug is the sequence of operations that occur when
72        a channel that's registered with a selector is closed :
73
74        (0) Application code calls Channel.close().
75
76        (1) The channel is "preClosed" - We dup2(2) /dev/null into the channel's
77        file descriptor and the channel is marked as closed at the Java level.
78
79        (2) All keys associated with the channel are cancelled. Cancels are
80        lazy, which means that the Selectors involved won't necessarily
81        deregister these keys until an ongoing call to select() (if any) returns
82        or until the next call to select() on that selector.
83
84        (3) Once all selectors associated with the channel deregister these
85        cancelled keys, the channel FD is properly closed (via close(2)). Note
86        that an arbitrary length of time might elapse between Step 0 and this step.
87        This isn't a resource leak because the channel's FD is now a reference
88        to "/dev/null".
89
90        THE PROBLEM :
91        -------------
92        The default Selector implementation on Linux 2.6 and higher uses epoll(7).
93        epoll can scale better than poll(2) because a lot of the state related
94        to the interest set (the set of descriptors we're polling on) is
95        maintained by the kernel. One of the side-effects of this design is that
96        callers must call into the kernel to make changes to the interest set
97        via epoll_ctl(7), for eg., by using EPOLL_CTL_ADD to add descriptors or
98        EPOLL_CTL_DEL to remove descriptors from the interest set. A call to
99        epoll_ctl with op = EPOLL_CTL_DEL is made when the selector attempts to
100        deregister an FD associated with a channel from the interest set (see
101        Step 2, above). These calls will *always fail* because the channel has
102        been preClosed (see Step 1). They fail because the kernel uses its own
103        internal file structure to maintain state, and rejects the command
104        because the descriptor we're passing in describes a different file
105        (/dev/null) that isn't selectable and isn't registered with the epoll
106        instance.
107
108        This is an issue in upstream OpenJDK as well and various select
109        implementations (such as netty) have hacks to work around it. Outside
110        of Android, things will work OK in most cases because the kernel has its
111        own internal cleanup logic to deregister files from epoll instances
112        whenever the last *non epoll* reference to the file has been closed -
113        and usually this happens at the point at which the dup2(2) from Step 1
114        is called. However, on Android, sockets tagged with the SocketTagger
115        will never hit this code path because the socket tagging implementation
116        (qtaguid) keeps a reference to the internal file until the socket
117        has been untagged. In cases where sockets are closed without being
118        untagged, the tagger keeps a reference to it until the process dies.
119
120        THE SOLUTION :
121        --------------
122        We switch over to using poll(2) instead of epoll(7). One of the
123        advantages of poll(2) is that there's less state maintained by the
124        kernel. We don't need to make a syscall (analogous to epoll_ctl)
125        whenever we want to remove an FD from the interest set; we merely
126        remove it from the list of FDs passed in the next time we call
127        through to poll. Poll is also slightly more efficient and less
128        overhead to set up when the number of FDs being polled is small
129        (which is the common case on Android).
130
131        ALTERNATE APPROACHES :
132        ----------------------
133        For completeness, I'm listing a couple of other approaches that were
134        considered but discarded.
135
136        - Removing preClose: This has the disadvantage of increasing the amount
137        of time (Delta between Step 0 and Step 3) a channel's descriptor is
138        kept alive. This also opens up races in the rare case where a
139        closed FD number is reused on a different thread while we have reads
140        pending.
141
142        - A Synchronous call to EPOLL_CTL_DEL when a channel is removed: This is a
143        non-starter because of the specified order of events in
144        AbstractSelectableChannel; implCloseSelectableChannel must be called
145        */
146
147
148        // Android-changed: Always use PollSelectorProvider.
149        /*
150        String osname = AccessController
151            .doPrivileged(new GetPropertyAction("os.name"));
152        if (osname.equals("SunOS"))
153            return createProvider("sun.nio.ch.DevPollSelectorProvider");
154        if (osname.equals("Linux"))
155            return createProvider("sun.nio.ch.EPollSelectorProvider");
156        */
157        return new sun.nio.ch.PollSelectorProvider();
158    }
159
160}
161