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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.ch; import java.nio.channels.spi.SelectorProvider; import java.security.AccessController; import java.security.PrivilegedAction; import sun.security.action.GetPropertyAction; /** * Creates this platform's default SelectorProvider */ public class DefaultSelectorProvider { /** * Prevent instantiation. */ private DefaultSelectorProvider() { } /** * Returns the default SelectorProvider. */ public static SelectorProvider create() { /* The OpenJDK epoll based selector suffers from a serious bug where it can never successfully deregister keys from closed channels. The root cause of this bug is the sequence of operations that occur when a channel that's registered with a selector is closed : (0) Application code calls Channel.close(). (1) The channel is "preClosed" - We dup2(2) /dev/null into the channel's file descriptor and the channel is marked as closed at the Java level. (2) All keys associated with the channel are cancelled. Cancels are lazy, which means that the Selectors involved won't necessarily deregister these keys until an ongoing call to select() (if any) returns or until the next call to select() on that selector. (3) Once all selectors associated with the channel deregister these cancelled keys, the channel FD is properly closed (via close(2)). Note that an arbitrary length of time might elapse between Step 0 and this step. This isn't a resource leak because the channel's FD is now a reference to "/dev/null". THE PROBLEM : ------------- The default Selector implementation on Linux 2.6 and higher uses epoll(7). epoll can scale better than poll(2) because a lot of the state related to the interest set (the set of descriptors we're polling on) is maintained by the kernel. One of the side-effects of this design is that callers must call into the kernel to make changes to the interest set via epoll_ctl(7), for eg., by using EPOLL_CTL_ADD to add descriptors or EPOLL_CTL_DEL to remove descriptors from the interest set. A call to epoll_ctl with op = EPOLL_CTL_DEL is made when the selector attempts to deregister an FD associated with a channel from the interest set (see Step 2, above). These calls will *always fail* because the channel has been preClosed (see Step 1). They fail because the kernel uses its own internal file structure to maintain state, and rejects the command because the descriptor we're passing in describes a different file (/dev/null) that isn't selectable and isn't registered with the epoll instance. This is an issue in upstream OpenJDK as well and various select implementations (such as netty) have hacks to work around it. Outside of Android, things will work OK in most cases because the kernel has its own internal cleanup logic to deregister files from epoll instances whenever the last *non epoll* reference to the file has been closed - and usually this happens at the point at which the dup2(2) from Step 1 is called. However, on Android, sockets tagged with the SocketTagger will never hit this code path because the socket tagging implementation (qtaguid) keeps a reference to the internal file until the socket has been untagged. In cases where sockets are closed without being untagged, the tagger keeps a reference to it until the process dies. THE SOLUTION : -------------- We switch over to using poll(2) instead of epoll(7). One of the advantages of poll(2) is that there's less state maintained by the kernel. We don't need to make a syscall (analogous to epoll_ctl) whenever we want to remove an FD from the interest set; we merely remove it from the list of FDs passed in the next time we call through to poll. Poll is also slightly more efficient and less overhead to set up when the number of FDs being polled is small (which is the common case on Android). We also need to make sure that all tagged sockets are untagged before they're preclosed at the platform level. However, there's nothing we can do about applications that abuse public api (android.net.TrafficStats). ALTERNATE APPROACHES : ---------------------- For completeness, I'm listing a couple of other approaches that were considered but discarded. - Removing preClose: This has the disadvantage of increasing the amount of time (Delta between Step 0 and Step 3) a channel's descriptor is kept alive. This also opens up races in the rare case where a closed FD number is reused on a different thread while we have reads pending. - A Synchronous call to EPOLL_CTL_DEL when a channel is removed: This is a non-starter because of the specified order of events in AbstractSelectableChannel; implCloseSelectableChannel must be called */ // Android-changed: Always use PollSelectorProvider. return new sun.nio.ch.PollSelectorProvider(); } }