关于hadoop搭建的问题org.apache.hadoop.io.nativeio.NativeIO.java

/**
 * 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.hadoop.io.nativeio;
 
import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.reflect.Field;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
 
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.CommonConfigurationKeys;
import org.apache.hadoop.io.SecureIOUtils.AlreadyExistsException;
import org.apache.hadoop.util.NativeCodeLoader;
import org.apache.hadoop.util.Shell;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
 
import sun.misc.Unsafe;
 
import com.google.common.annotations.VisibleForTesting;
 
/**
 * JNI wrappers for various native IO-related calls not available in Java. These
 * functions should generally be used alongside a fallback to another more
 * portable mechanism.
 */
@InterfaceAudience.Private
@InterfaceStability.Unstable
public class NativeIO {
  public static class POSIX {
    // Flags for open() call from bits/fcntl.h
    public static final int O_RDONLY = 00;
    public static final int O_WRONLY = 01;
    public static final int O_RDWR = 02;
    public static final int O_CREAT = 0100;
    public static final int O_EXCL = 0200;
    public static final int O_NOCTTY = 0400;
    public static final int O_TRUNC = 01000;
    public static final int O_APPEND = 02000;
    public static final int O_NONBLOCK = 04000;
    public static final int O_SYNC = 010000;
    public static final int O_ASYNC = 020000;
    public static final int O_FSYNC = O_SYNC;
    public static final int O_NDELAY = O_NONBLOCK;
 
    // Flags for posix_fadvise() from bits/fcntl.h
    /* No further special treatment. */
    public static final int POSIX_FADV_NORMAL = 0;
    /* Expect random page references. */
    public static final int POSIX_FADV_RANDOM = 1;
    /* Expect sequential page references. */
    public static final int POSIX_FADV_SEQUENTIAL = 2;
    /* Will need these pages. */
    public static final int POSIX_FADV_WILLNEED = 3;
    /* Don't need these pages. */
    public static final int POSIX_FADV_DONTNEED = 4;
    /* Data will be accessed once. */
    public static final int POSIX_FADV_NOREUSE = 5;
 
    /*
     * Wait upon writeout of all pages in the range before performing the
     * write.
     */
    public static final int SYNC_FILE_RANGE_WAIT_BEFORE = 1;
    /*
     * Initiate writeout of all those dirty pages in the range which are not
     * presently under writeback.
     */
    public static final int SYNC_FILE_RANGE_WRITE = 2;
 
    /*
     * Wait upon writeout of all pages in the range after performing the
     * write.
     */
    public static final int SYNC_FILE_RANGE_WAIT_AFTER = 4;
 
    private static final Log LOG = LogFactory.getLog(NativeIO.class);
 
    private static boolean nativeLoaded = false;
    private static boolean fadvisePossible = true;
    private static boolean syncFileRangePossible = true;
 
    static final String WORKAROUND_NON_THREADSAFE_CALLS_KEY = "hadoop.workaround.non.threadsafe.getpwuid";
    static final boolean WORKAROUND_NON_THREADSAFE_CALLS_DEFAULT = true;
 
    private static long cacheTimeout = -1;
 
    private static CacheManipulator cacheManipulator = new CacheManipulator();
 
    public static CacheManipulator getCacheManipulator() {
      return cacheManipulator;
    }
 
    public static void setCacheManipulator(CacheManipulator cacheManipulator) {
      POSIX.cacheManipulator = cacheManipulator;
    }
 
    /**
     * Used to manipulate the operating system cache.
     */
    @VisibleForTesting
    public static class CacheManipulator {
      public void mlock(String identifier, ByteBuffer buffer, long len) throws IOException {
        POSIX.mlock(buffer, len);
      }
 
      public long getMemlockLimit() {
        return NativeIO.getMemlockLimit();
      }
 
      public long getOperatingSystemPageSize() {
        return NativeIO.getOperatingSystemPageSize();
      }
 
      public void posixFadviseIfPossible(String identifier, FileDescriptor fd, long offset, long len, int flags)
          throws NativeIOException {
        NativeIO.POSIX.posixFadviseIfPossible(identifier, fd, offset, len, flags);
      }
 
      public boolean verifyCanMlock() {
        return NativeIO.isAvailable();
      }
    }
 
    /**
     * A CacheManipulator used for testing which does not actually call
     * mlock. This allows many tests to be run even when the operating
     * system does not allow mlock, or only allows limited mlocking.
     */
    @VisibleForTesting
    public static class NoMlockCacheManipulator extends CacheManipulator {
      public void mlock(String identifier, ByteBuffer buffer, long len) throws IOException {
        LOG.info("mlocking " + identifier);
      }
 
      public long getMemlockLimit() {
        return 1125899906842624L;
      }
 
      public long getOperatingSystemPageSize() {
        return 4096;
      }
 
      public boolean verifyCanMlock() {
        return true;
      }
    }
 
    static {
      if (NativeCodeLoader.isNativeCodeLoaded()) {
        try {
          Configuration conf = new Configuration();
          workaroundNonThreadSafePasswdCalls = conf.getBoolean(WORKAROUND_NON_THREADSAFE_CALLS_KEY,
              WORKAROUND_NON_THREADSAFE_CALLS_DEFAULT);
 
          initNative();
          nativeLoaded = true;
 
          cacheTimeout = conf.getLong(CommonConfigurationKeys.HADOOP_SECURITY_UID_NAME_CACHE_TIMEOUT_KEY,
              CommonConfigurationKeys.HADOOP_SECURITY_UID_NAME_CACHE_TIMEOUT_DEFAULT) * 1000;
          LOG.debug("Initialized cache for IDs to User/Group mapping with a " + " cache timeout of "
              + cacheTimeout / 1000 + " seconds.");
 
        } catch (Throwable t) {
          // This can happen if the user has an older version of
          // libhadoop.so
          // installed - in this case we can continue without native
          // IO
          // after warning
          LOG.error("Unable to initialize NativeIO libraries", t);
        }
      }
    }
 
    /**
     * Return true if the JNI-based native IO extensions are available.
     */
    public static boolean isAvailable() {
      return NativeCodeLoader.isNativeCodeLoaded() && nativeLoaded;
    }
 
    private static void assertCodeLoaded() throws IOException {
      if (!isAvailable()) {
        throw new IOException("NativeIO was not loaded");
      }
    }
 
    /** Wrapper around open(2) */
    public static native FileDescriptor open(String path, int flags, int mode) throws IOException;
 
    /** Wrapper around fstat(2) */
    private static native Stat fstat(FileDescriptor fd) throws IOException;
 
    /**
     * Native chmod implementation. On UNIX, it is a wrapper around chmod(2)
     */
    private static native void chmodImpl(String path, int mode) throws IOException;
 
    public static void chmod(String path, int mode) throws IOException {
      if (!Shell.WINDOWS) {
        chmodImpl(path, mode);
      } else {
        try {
          chmodImpl(path, mode);
        } catch (NativeIOException nioe) {
          if (nioe.getErrorCode() == 3) {
            throw new NativeIOException("No such file or directory", Errno.ENOENT);
          } else {
            LOG.warn(
                String.format("NativeIO.chmod error (%d): %s", nioe.getErrorCode(), nioe.getMessage()));
            throw new NativeIOException("Unknown error", Errno.UNKNOWN);
          }
        }
      }
    }
 
    /** Wrapper around posix_fadvise(2) */
    static native void posix_fadvise(FileDescriptor fd, long offset, long len, int flags) throws NativeIOException;
 
    /** Wrapper around sync_file_range(2) */
    static native void sync_file_range(FileDescriptor fd, long offset, long nbytes, int flags)
        throws NativeIOException;
 
    /**
     * Call posix_fadvise on the given file descriptor. See the manpage for
     * this syscall for more information. On systems where this call is not
     * available, does nothing.
     *
     * @throws NativeIOException
     *             if there is an error with the syscall
     */
    static void posixFadviseIfPossible(String identifier, FileDescriptor fd, long offset, long len, int flags)
        throws NativeIOException {
      if (nativeLoaded && fadvisePossible) {
        try {
          posix_fadvise(fd, offset, len, flags);
        } catch (UnsupportedOperationException uoe) {
          fadvisePossible = false;
        } catch (UnsatisfiedLinkError ule) {
          fadvisePossible = false;
        }
      }
    }
 
    /**
     * Call sync_file_range on the given file descriptor. See the manpage
     * for this syscall for more information. On systems where this call is
     * not available, does nothing.
     *
     * @throws NativeIOException
     *             if there is an error with the syscall
     */
    public static void syncFileRangeIfPossible(FileDescriptor fd, long offset, long nbytes, int flags)
        throws NativeIOException {
      if (nativeLoaded && syncFileRangePossible) {
        try {
          sync_file_range(fd, offset, nbytes, flags);
        } catch (UnsupportedOperationException uoe) {
          syncFileRangePossible = false;
        } catch (UnsatisfiedLinkError ule) {
          syncFileRangePossible = false;
        }
      }
    }
 
    static native void mlock_native(ByteBuffer buffer, long len) throws NativeIOException;
 
    static native void munlock_native(ByteBuffer buffer, long len) throws NativeIOException;
 
    /**
     * Locks the provided direct ByteBuffer into memory, preventing it from
     * swapping out. After a buffer is locked, future accesses will not
     * incur a page fault.
     * 
     * See the mlock(2) man page for more information.
     * 
     * @throws NativeIOException
     */
    static void mlock(ByteBuffer buffer, long len) throws IOException {
      assertCodeLoaded();
      if (!buffer.isDirect()) {
        throw new IOException("Cannot mlock a non-direct ByteBuffer");
      }
      mlock_native(buffer, len);
    }
 
    /**
     * Unlocks a locked direct ByteBuffer, allowing it to swap out of
     * memory. This is a no-op if the ByteBuffer was not previously locked.
     * 
     * See the munlock(2) man page for more information.
     * 
     * @throws NativeIOException
     */
    public static void munlock(ByteBuffer buffer, long len) throws IOException {
      assertCodeLoaded();
      if (!buffer.isDirect()) {
        throw new IOException("Cannot munlock a non-direct ByteBuffer");
      }
      munlock_native(buffer, len);
    }
 
    /**
     * Unmaps the block from memory. See munmap(2).
     *
     * There isn't any portable way to unmap a memory region in Java. So we
     * use the sun.nio method here. Note that unmapping a memory region
     * could cause crashes if code continues to reference the unmapped code.
     * However, if we don't manually unmap the memory, we are dependent on
     * the finalizer to do it, and we have no idea when the finalizer will
     * run.
     *
     * @param buffer
     *            The buffer to unmap.
     */
    public static void munmap(MappedByteBuffer buffer) {
      if (buffer instanceof sun.nio.ch.DirectBuffer) {
        sun.misc.Cleaner cleaner = ((sun.nio.ch.DirectBuffer) buffer).cleaner();
        cleaner.clean();
      }
    }
 
    /** Linux only methods used for getOwner() implementation */
    private static native long getUIDforFDOwnerforOwner(FileDescriptor fd) throws IOException;
 
    private static native String getUserName(long uid) throws IOException;
 
    /**
     * Result type of the fstat call
     */
    public static class Stat {
      private int ownerId, groupId;
      private String owner, group;
      private int mode;
 
      // Mode constants
      public static final int S_IFMT = 0170000; /* type of file */
      public static final int S_IFIFO = 0010000; /* named pipe (fifo) */
      public static final int S_IFCHR = 0020000; /* character special */
      public static final int S_IFDIR = 0040000; /* directory */
      public static final int S_IFBLK = 0060000; /* block special */
      public static final int S_IFREG = 0100000; /* regular */
      public static final int S_IFLNK = 0120000; /* symbolic link */
      public static final int S_IFSOCK = 0140000; /* socket */
      public static final int S_IFWHT = 0160000; /* whiteout */
      public static final int S_ISUID = 0004000; /*
                             * set user id on
                             * execution
                             */
      public static final int S_ISGID = 0002000; /*
                             * set group id on
                             * execution
                             */
      public static final int S_ISVTX = 0001000; /*
                             * save swapped text even
                             * after use
                             */
      public static final int S_IRUSR = 0000400; /*
                             * read permission, owner
                             */
      public static final int S_IWUSR = 0000200; /*
                             * write permission,
                             * owner
                             */
      public static final int S_IXUSR = 0000100; /*
                             * execute/search
                             * permission, owner
                             */
 
      Stat(int ownerId, int groupId, int mode) {
        this.ownerId = ownerId;
        this.groupId = groupId;
        this.mode = mode;
      }
 
      Stat(String owner, String group, int mode) {
        if (!Shell.WINDOWS) {
          this.owner = owner;
        } else {
          this.owner = stripDomain(owner);
        }
        if (!Shell.WINDOWS) {
          this.group = group;
        } else {
          this.group = stripDomain(group);
        }
        this.mode = mode;
      }
 
      @Override
      public String toString() {
        return "Stat(owner='" + owner + "', group='" + group + "'" + ", mode=" + mode + ")";
      }
 
      public String getOwner() {
        return owner;
      }
 
      public String getGroup() {
        return group;
      }
 
      public int getMode() {
        return mode;
      }
    }
 
    /**
     * Returns the file stat for a file descriptor.
     *
     * @param fd
     *            file descriptor.
     * @return the file descriptor file stat.
     * @throws IOException
     *             thrown if there was an IO error while obtaining the file
     *             stat.
     */
    public static Stat getFstat(FileDescriptor fd) throws IOException {
      Stat stat = null;
      if (!Shell.WINDOWS) {
        stat = fstat(fd);
        stat.owner = getName(IdCache.USER, stat.ownerId);
        stat.group = getName(IdCache.GROUP, stat.groupId);
      } else {
        try {
          stat = fstat(fd);
        } catch (NativeIOException nioe) {
          if (nioe.getErrorCode() == 6) {
            throw new NativeIOException("The handle is invalid.", Errno.EBADF);
          } else {
            LOG.warn(String.format("NativeIO.getFstat error (%d): %s", nioe.getErrorCode(),
                nioe.getMessage()));
            throw new NativeIOException("Unknown error", Errno.UNKNOWN);
          }
        }
      }
      return stat;
    }
 
    private static String getName(IdCache domain, int id) throws IOException {
      Map<Integer, CachedName> idNameCache = (domain == IdCache.USER) ? USER_ID_NAME_CACHE : GROUP_ID_NAME_CACHE;
      String name;
      CachedName cachedName = idNameCache.get(id);
      long now = System.currentTimeMillis();
      if (cachedName != null && (cachedName.timestamp + cacheTimeout) > now) {
        name = cachedName.name;
      } else {
        name = (domain == IdCache.USER) ? getUserName(id) : getGroupName(id);
        if (LOG.isDebugEnabled()) {
          String type = (domain == IdCache.USER) ? "UserName" : "GroupName";
          LOG.debug("Got " + type + " " + name + " for ID " + id + " from the native implementation");
        }
        cachedName = new CachedName(name, now);
        idNameCache.put(id, cachedName);
      }
      return name;
    }
 
    static native String getUserName(int uid) throws IOException;
 
    static native String getGroupName(int uid) throws IOException;
 
    private static class CachedName {
      final long timestamp;
      final String name;
 
      public CachedName(String name, long timestamp) {
        this.name = name;
        this.timestamp = timestamp;
      }
    }
 
    private static final Map<Integer, CachedName> USER_ID_NAME_CACHE = new ConcurrentHashMap<Integer, CachedName>();
 
    private static final Map<Integer, CachedName> GROUP_ID_NAME_CACHE = new ConcurrentHashMap<Integer, CachedName>();
 
    private enum IdCache {
      USER, GROUP
    }
 
    public final static int MMAP_PROT_READ = 0x1;
    public final static int MMAP_PROT_WRITE = 0x2;
    public final static int MMAP_PROT_EXEC = 0x4;
 
    public static native long mmap(FileDescriptor fd, int prot, boolean shared, long length) throws IOException;
 
    public static native void munmap(long addr, long length) throws IOException;
  }
 
  private static boolean workaroundNonThreadSafePasswdCalls = false;
 
  public static class Windows {
    // Flags for CreateFile() call on Windows
    public static final long GENERIC_READ = 0x80000000L;
    public static final long GENERIC_WRITE = 0x40000000L;
 
    public static final long FILE_SHARE_READ = 0x00000001L;
    public static final long FILE_SHARE_WRITE = 0x00000002L;
    public static final long FILE_SHARE_DELETE = 0x00000004L;
 
    public static final long CREATE_NEW = 1;
    public static final long CREATE_ALWAYS = 2;
    public static final long OPEN_EXISTING = 3;
    public static final long OPEN_ALWAYS = 4;
    public static final long TRUNCATE_EXISTING = 5;
 
    public static final long FILE_BEGIN = 0;
    public static final long FILE_CURRENT = 1;
    public static final long FILE_END = 2;
 
    /** Wrapper around CreateFile() on Windows */
    public static native FileDescriptor createFile(String path, long desiredAccess, long shareMode,
        long creationDisposition) throws IOException;
 
    /** Wrapper around SetFilePointer() on Windows */
    public static native long setFilePointer(FileDescriptor fd, long distanceToMove, long moveMethod)
        throws IOException;
 
    /** Windows only methods used for getOwner() implementation */
    private static native String getOwner(FileDescriptor fd) throws IOException;
 
    /** Supported list of Windows access right flags */
    public static enum AccessRight {
      ACCESS_READ(0x0001), // FILE_READ_DATA
      ACCESS_WRITE(0x0002), // FILE_WRITE_DATA
      ACCESS_EXECUTE(0x0020); // FILE_EXECUTE
 
      private final int accessRight;
 
      AccessRight(int access) {
        accessRight = access;
      }
 
      public int accessRight() {
        return accessRight;
      }
    };
 
    /**
     * Windows only method used to check if the current process has
     * requested access rights on the given path.
     */
    private static native boolean access0(String path, int requestedAccess);
 
    /**
     * Checks whether the current process has desired access rights on the
     * given path.
     * 
     * Longer term this native function can be substituted with JDK7
     * function Files#isReadable, isWritable, isExecutable.
     *
     * @param path
     *            input path
     * @param desiredAccess
     *            ACCESS_READ, ACCESS_WRITE or ACCESS_EXECUTE
     * @return true if access is allowed
     * @throws IOException
     *             I/O exception on error
     */
    public static boolean access(String path, AccessRight desiredAccess) throws IOException {
      return true;
      // return access0(path, desiredAccess.accessRight());
    }
 
    static {
      if (NativeCodeLoader.isNativeCodeLoaded()) {
        try {
          initNative();
          nativeLoaded = true;
        } catch (Throwable t) {
          // This can happen if the user has an older version of
          // libhadoop.so
          // installed - in this case we can continue without native
          // IO
          // after warning
          LOG.error("Unable to initialize NativeIO libraries", t);
        }
      }
    }
  }
 
  private static final Log LOG = LogFactory.getLog(NativeIO.class);
 
  private static boolean nativeLoaded = false;
 
  static {
    if (NativeCodeLoader.isNativeCodeLoaded()) {
      try {
        initNative();
        nativeLoaded = true;
      } catch (Throwable t) {
        // This can happen if the user has an older version of
        // libhadoop.so
        // installed - in this case we can continue without native IO
        // after warning
        LOG.error("Unable to initialize NativeIO libraries", t);
      }
    }
  }
 
  /**
   * Return true if the JNI-based native IO extensions are available.
   */
  public static boolean isAvailable() {
    return NativeCodeLoader.isNativeCodeLoaded() && nativeLoaded;
  }
 
  /** Initialize the JNI method ID and class ID cache */
  private static native void initNative();
 
  /**
   * Get the maximum number of bytes that can be locked into memory at any
   * given point.
   *
   * @return 0 if no bytes can be locked into memory; Long.MAX_VALUE if there
   *         is no limit; The number of bytes that can be locked into memory
   *         otherwise.
   */
  static long getMemlockLimit() {
    return isAvailable() ? getMemlockLimit0() : 0;
  }
 
  private static native long getMemlockLimit0();
 
  /**
   * @return the operating system's page size.
   */
  static long getOperatingSystemPageSize() {
    try {
      Field f = Unsafe.class.getDeclaredField("theUnsafe");
      f.setAccessible(true);
      Unsafe unsafe = (Unsafe) f.get(null);
      return unsafe.pageSize();
    } catch (Throwable e) {
      LOG.warn("Unable to get operating system page size.  Guessing 4096.", e);
      return 4096;
    }
  }
 
  private static class CachedUid {
    final long timestamp;
    final String username;
 
    public CachedUid(String username, long timestamp) {
      this.timestamp = timestamp;
      this.username = username;
    }
  }
 
  private static final Map<Long, CachedUid> uidCache = new ConcurrentHashMap<Long, CachedUid>();
  private static long cacheTimeout;
  private static boolean initialized = false;
 
  /**
   * The Windows logon name has two part, NetBIOS domain name and user account
   * name, of the format DOMAIN\UserName. This method will remove the domain
   * part of the full logon name.
   *
   * @param the
   *            full principal name containing the domain
   * @return name with domain removed
   */
  private static String stripDomain(String name) {
    int i = name.indexOf('\\');
    if (i != -1)
      name = name.substring(i + 1);
    return name;
  }
 
  public static String getOwner(FileDescriptor fd) throws IOException {
    ensureInitialized();
    if (Shell.WINDOWS) {
      String owner = Windows.getOwner(fd);
      owner = stripDomain(owner);
      return owner;
    } else {
      long uid = POSIX.getUIDforFDOwnerforOwner(fd);
      CachedUid cUid = uidCache.get(uid);
      long now = System.currentTimeMillis();
      if (cUid != null && (cUid.timestamp + cacheTimeout) > now) {
        return cUid.username;
      }
      String user = POSIX.getUserName(uid);
      LOG.info("Got UserName " + user + " for UID " + uid + " from the native implementation");
      cUid = new CachedUid(user, now);
      uidCache.put(uid, cUid);
      return user;
    }
  }
 
  /**
   * Create a FileInputStream that shares delete permission on the file
   * opened, i.e. other process can delete the file the FileInputStream is
   * reading. Only Windows implementation uses the native interface.
   */
  public static FileInputStream getShareDeleteFileInputStream(File f) throws IOException {
    if (!Shell.WINDOWS) {
      // On Linux the default FileInputStream shares delete permission
      // on the file opened.
      //
      return new FileInputStream(f);
    } else {
      // Use Windows native interface to create a FileInputStream that
      // shares delete permission on the file opened.
      //
      FileDescriptor fd = Windows.createFile(f.getAbsolutePath(), Windows.GENERIC_READ,
          Windows.FILE_SHARE_READ | Windows.FILE_SHARE_WRITE | Windows.FILE_SHARE_DELETE,
          Windows.OPEN_EXISTING);
      return new FileInputStream(fd);
    }
  }
 
  /**
   * Create a FileInputStream that shares delete permission on the file opened
   * at a given offset, i.e. other process can delete the file the
   * FileInputStream is reading. Only Windows implementation uses the native
   * interface.
   */
  public static FileInputStream getShareDeleteFileInputStream(File f, long seekOffset) throws IOException {
    if (!Shell.WINDOWS) {
      RandomAccessFile rf = new RandomAccessFile(f, "r");
      if (seekOffset > 0) {
        rf.seek(seekOffset);
      }
      return new FileInputStream(rf.getFD());
    } else {
      // Use Windows native interface to create a FileInputStream that
      // shares delete permission on the file opened, and set it to the
      // given offset.
      //
      FileDescriptor fd = NativeIO.Windows.createFile(
          f.getAbsolutePath(), NativeIO.Windows.GENERIC_READ, NativeIO.Windows.FILE_SHARE_READ
              | NativeIO.Windows.FILE_SHARE_WRITE | NativeIO.Windows.FILE_SHARE_DELETE,
          NativeIO.Windows.OPEN_EXISTING);
      if (seekOffset > 0)
        NativeIO.Windows.setFilePointer(fd, seekOffset, NativeIO.Windows.FILE_BEGIN);
      return new FileInputStream(fd);
    }
  }
 
  /**
   * Create the specified File for write access, ensuring that it does not
   * exist.
   * 
   * @param f
   *            the file that we want to create
   * @param permissions
   *            we want to have on the file (if security is enabled)
   *
   * @throws AlreadyExistsException
   *             if the file already exists
   * @throws IOException
   *             if any other error occurred
   */
  public static FileOutputStream getCreateForWriteFileOutputStream(File f, int permissions) throws IOException {
    if (!Shell.WINDOWS) {
      // Use the native wrapper around open(2)
      try {
        FileDescriptor fd = NativeIO.POSIX.open(f.getAbsolutePath(),
            NativeIO.POSIX.O_WRONLY | NativeIO.POSIX.O_CREAT | NativeIO.POSIX.O_EXCL, permissions);
        return new FileOutputStream(fd);
      } catch (NativeIOException nioe) {
        if (nioe.getErrno() == Errno.EEXIST) {
          throw new AlreadyExistsException(nioe);
        }
        throw nioe;
      }
    } else {
      // Use the Windows native APIs to create equivalent FileOutputStream
      try {
        FileDescriptor fd = NativeIO.Windows.createFile(f.getCanonicalPath(),
            NativeIO.Windows.GENERIC_WRITE, NativeIO.Windows.FILE_SHARE_DELETE
                | NativeIO.Windows.FILE_SHARE_READ | NativeIO.Windows.FILE_SHARE_WRITE,
            NativeIO.Windows.CREATE_NEW);
        NativeIO.POSIX.chmod(f.getCanonicalPath(), permissions);
        return new FileOutputStream(fd);
      } catch (NativeIOException nioe) {
        if (nioe.getErrorCode() == 80) {
          // ERROR_FILE_EXISTS
          // 80 (0x50)
          // The file exists
          throw new AlreadyExistsException(nioe);
        }
        throw nioe;
      }
    }
  }
 
  private synchronized static void ensureInitialized() {
    if (!initialized) {
      cacheTimeout = new Configuration().getLong("hadoop.security.uid.cache.secs", 4 * 60 * 60) * 1000;
      LOG.info("Initialized cache for UID to User mapping with a cache" + " timeout of " + cacheTimeout / 1000
          + " seconds.");
      initialized = true;
    }
  }
 
  /**
   * A version of renameTo that throws a descriptive exception when it fails.
   *
   * @param src
   *            The source path
   * @param dst
   *            The destination path
   * 
   * @throws NativeIOException
   *             On failure.
   */
  public static void renameTo(File src, File dst) throws IOException {
    if (!nativeLoaded) {
      if (!src.renameTo(dst)) {
        throw new IOException("renameTo(src=" + src + ", dst=" + dst + ") failed.");
      }
    } else {
      renameTo0(src.getAbsolutePath(), dst.getAbsolutePath());
    }
  }
 
  /**
   * A version of renameTo that throws a descriptive exception when it fails.
   *
   * @param src
   *            The source path
   * @param dst
   *            The destination path
   * 
   * @throws NativeIOException
   *             On failure.
   */
  private static native void renameTo0(String src, String dst) throws NativeIOException;
}