/*
 * Copyright (c) 2003, 2021, 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 java.lang;

import java.lang.ProcessBuilder.Redirect;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.ByteArrayInputStream;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Arrays;
import java.util.EnumSet;
import java.util.Locale;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Properties;
import jdk.internal.access.JavaIOFileDescriptorAccess;
import jdk.internal.access.SharedSecrets;
import jdk.internal.util.StaticProperty;
import sun.security.action.GetPropertyAction;

java.lang.Process subclass in the UNIX environment.
Author:
Mario Wolczko and Ross Knippel., Konstantin Kladko (ported to Linux and Bsd), Martin Buchholz, Volker Simonis (ported to AIX)
Since:
  1.5/**
 * java.lang.Process subclass in the UNIX environment.
 *
 * @author Mario Wolczko and Ross Knippel.
 * @author Konstantin Kladko (ported to Linux and Bsd)
 * @author Martin Buchholz
 * @author Volker Simonis (ported to AIX)
 * @since   1.5
 */
final class ProcessImpl extends Process {
    private static final JavaIOFileDescriptorAccess fdAccess
        = SharedSecrets.getJavaIOFileDescriptorAccess();

    // Linux platforms support a normal (non-forcible) kill signal.
    static final boolean SUPPORTS_NORMAL_TERMINATION = true;

    private final int pid;
    private final ProcessHandleImpl processHandle;
    private int exitcode;
    private boolean hasExited;

    private /* final */ OutputStream stdin;
    private /* final */ InputStream  stdout;
    private /* final */ InputStream  stderr;

    private static enum LaunchMechanism {
        // order IS important!
        FORK,
        POSIX_SPAWN,
        VFORK
    }

    private static enum Platform {

        LINUX(LaunchMechanism.POSIX_SPAWN, LaunchMechanism.VFORK, LaunchMechanism.FORK),

        BSD(LaunchMechanism.POSIX_SPAWN, LaunchMechanism.FORK),

        AIX(LaunchMechanism.POSIX_SPAWN, LaunchMechanism.FORK);

        final LaunchMechanism defaultLaunchMechanism;
        final Set<LaunchMechanism> validLaunchMechanisms;

        Platform(LaunchMechanism ... launchMechanisms) {
            this.defaultLaunchMechanism = launchMechanisms[0];
            this.validLaunchMechanisms =
                EnumSet.copyOf(Arrays.asList(launchMechanisms));
        }

        @SuppressWarnings("removal")
        LaunchMechanism launchMechanism() {
            return AccessController.doPrivileged(
                (PrivilegedAction<LaunchMechanism>) () -> {
                    String s = System.getProperty(
                        "jdk.lang.Process.launchMechanism");
                    LaunchMechanism lm;
                    if (s == null) {
                        lm = defaultLaunchMechanism;
                        s = lm.name().toLowerCase(Locale.ENGLISH);
                    } else {
                        try {
                            lm = LaunchMechanism.valueOf(
                                s.toUpperCase(Locale.ENGLISH));
                        } catch (IllegalArgumentException e) {
                            lm = null;
                        }
                    }
                    if (lm == null || !validLaunchMechanisms.contains(lm)) {
                        throw new Error(
                            s + " is not a supported " +
                            "process launch mechanism on this platform."
                        );
                    }
                    return lm;
                }
            );
        }

        static Platform get() {
            String osName = GetPropertyAction.privilegedGetProperty("os.name");

            if (osName.equals("Linux")) { return LINUX; }
            if (osName.contains("OS X")) { return BSD; }
            if (osName.equals("AIX")) { return AIX; }

            throw new Error(osName + " is not a supported OS platform.");
        }
    }

    private static final Platform platform = Platform.get();
    private static final LaunchMechanism launchMechanism = platform.launchMechanism();
    private static final byte[] helperpath = toCString(StaticProperty.javaHome() + "/lib/jspawnhelper");

    private static byte[] toCString(String s) {
        if (s == null)
            return null;
        byte[] bytes = s.getBytes();
        byte[] result = new byte[bytes.length + 1];
        System.arraycopy(bytes, 0,
                         result, 0,
                         bytes.length);
        result[result.length-1] = (byte)0;
        return result;
    }

    // Only for use by ProcessBuilder.start()
    static Process start(String[] cmdarray,
                         java.util.Map<String,String> environment,
                         String dir,
                         ProcessBuilder.Redirect[] redirects,
                         boolean redirectErrorStream)
            throws IOException
    {
        assert cmdarray != null && cmdarray.length > 0;

        // Convert arguments to a contiguous block; it's easier to do
        // memory management in Java than in C.
        byte[][] args = new byte[cmdarray.length-1][];
        int size = args.length; // For added NUL bytes
        for (int i = 0; i < args.length; i++) {
            args[i] = cmdarray[i+1].getBytes();
            size += args[i].length;
        }
        byte[] argBlock = new byte[size];
        int i = 0;
        for (byte[] arg : args) {
            System.arraycopy(arg, 0, argBlock, i, arg.length);
            i += arg.length + 1;
            // No need to write NUL bytes explicitly
        }

        int[] envc = new int[1];
        byte[] envBlock = ProcessEnvironment.toEnvironmentBlock(environment, envc);

        int[] std_fds;

        FileInputStream  f0 = null;
        FileOutputStream f1 = null;
        FileOutputStream f2 = null;

        try {
            boolean forceNullOutputStream = false;
            if (redirects == null) {
                std_fds = new int[] { -1, -1, -1 };
            } else {
                std_fds = new int[3];

                if (redirects[0] == Redirect.PIPE) {
                    std_fds[0] = -1;
                } else if (redirects[0] == Redirect.INHERIT) {
                    std_fds[0] = 0;
                } else if (redirects[0] instanceof ProcessBuilder.RedirectPipeImpl) {
                    std_fds[0] = fdAccess.get(((ProcessBuilder.RedirectPipeImpl) redirects[0]).getFd());
                } else {
                    f0 = new FileInputStream(redirects[0].file());
                    std_fds[0] = fdAccess.get(f0.getFD());
                }

                if (redirects[1] == Redirect.PIPE) {
                    std_fds[1] = -1;
                } else if (redirects[1] == Redirect.INHERIT) {
                    std_fds[1] = 1;
                } else if (redirects[1] instanceof ProcessBuilder.RedirectPipeImpl) {
                    std_fds[1] = fdAccess.get(((ProcessBuilder.RedirectPipeImpl) redirects[1]).getFd());
                    // Force getInputStream to return a null stream,
                    // the fd is directly assigned to the next process.
                    forceNullOutputStream = true;
                } else {
                    f1 = new FileOutputStream(redirects[1].file(),
                            redirects[1].append());
                    std_fds[1] = fdAccess.get(f1.getFD());
                }

                if (redirects[2] == Redirect.PIPE) {
                    std_fds[2] = -1;
                } else if (redirects[2] == Redirect.INHERIT) {
                    std_fds[2] = 2;
                } else if (redirects[2] instanceof ProcessBuilder.RedirectPipeImpl) {
                    std_fds[2] = fdAccess.get(((ProcessBuilder.RedirectPipeImpl) redirects[2]).getFd());
                } else {
                    f2 = new FileOutputStream(redirects[2].file(),
                            redirects[2].append());
                    std_fds[2] = fdAccess.get(f2.getFD());
                }
            }

            Process p = new ProcessImpl
                    (toCString(cmdarray[0]),
                            argBlock, args.length,
                            envBlock, envc[0],
                            toCString(dir),
                            std_fds,
                            forceNullOutputStream,
                            redirectErrorStream);
            if (redirects != null) {
                // Copy the fd's if they are to be redirected to another process
                if (std_fds[0] >= 0 &&
                        redirects[0] instanceof ProcessBuilder.RedirectPipeImpl) {
                    fdAccess.set(((ProcessBuilder.RedirectPipeImpl) redirects[0]).getFd(), std_fds[0]);
                }
                if (std_fds[1] >= 0 &&
                        redirects[1] instanceof ProcessBuilder.RedirectPipeImpl) {
                    fdAccess.set(((ProcessBuilder.RedirectPipeImpl) redirects[1]).getFd(), std_fds[1]);
                }
                if (std_fds[2] >= 0 &&
                        redirects[2] instanceof ProcessBuilder.RedirectPipeImpl) {
                    fdAccess.set(((ProcessBuilder.RedirectPipeImpl) redirects[2]).getFd(), std_fds[2]);
                }
            }
            return p;
        } finally {
            // In theory, close() can throw IOException
            // (although it is rather unlikely to happen here)
            try { if (f0 != null) f0.close(); }
            finally {
                try { if (f1 != null) f1.close(); }
                finally { if (f2 != null) f2.close(); }
            }
        }
    }


    
Creates a process. Depending on the mode flag, this is done by one of the following mechanisms: 
  1 - fork(2) and exec(2)
  2 - posix_spawn(3P)
  3 - vfork(2) and exec(2)

Params:
fds – an array of three file descriptors.
       Indexes 0, 1, and 2 correspond to standard input,
       standard output and standard error, respectively.  On
       input, a value of -1 means to create a pipe to connect
       child and parent processes.  On output, a value which
       is not -1 is the parent pipe fd corresponding to the
       pipe which has been created.  An element of this array
       is -1 on input if and only if it is not -1 on
       output.
Returns:
the pid of the subprocess/**
     * Creates a process. Depending on the {@code mode} flag, this is done by
     * one of the following mechanisms:
     * <pre>
     *   1 - fork(2) and exec(2)
     *   2 - posix_spawn(3P)
     *   3 - vfork(2) and exec(2)
     * </pre>
     * @param fds an array of three file descriptors.
     *        Indexes 0, 1, and 2 correspond to standard input,
     *        standard output and standard error, respectively.  On
     *        input, a value of -1 means to create a pipe to connect
     *        child and parent processes.  On output, a value which
     *        is not -1 is the parent pipe fd corresponding to the
     *        pipe which has been created.  An element of this array
     *        is -1 on input if and only if it is <em>not</em> -1 on
     *        output.
     * @return the pid of the subprocess
     */
    private native int forkAndExec(int mode, byte[] helperpath,
                                   byte[] prog,
                                   byte[] argBlock, int argc,
                                   byte[] envBlock, int envc,
                                   byte[] dir,
                                   int[] fds,
                                   boolean redirectErrorStream)
        throws IOException;

    @SuppressWarnings("removal")
    private ProcessImpl(final byte[] prog,
                final byte[] argBlock, final int argc,
                final byte[] envBlock, final int envc,
                final byte[] dir,
                final int[] fds,
                final boolean forceNullOutputStream,
                final boolean redirectErrorStream)
            throws IOException {

        pid = forkAndExec(launchMechanism.ordinal() + 1,
                          helperpath,
                          prog,
                          argBlock, argc,
                          envBlock, envc,
                          dir,
                          fds,
                          redirectErrorStream);
        processHandle = ProcessHandleImpl.getInternal(pid);

        try {
            AccessController.doPrivileged((PrivilegedExceptionAction<Void>) () -> {
                initStreams(fds, forceNullOutputStream);
                return null;
            });
        } catch (PrivilegedActionException ex) {
            throw (IOException) ex.getCause();
        }
    }

    static FileDescriptor newFileDescriptor(int fd) {
        FileDescriptor fileDescriptor = new FileDescriptor();
        fdAccess.set(fileDescriptor, fd);
        return fileDescriptor;
    }

    
Initialize the streams from the file descriptors.
Params:
fds – array of stdin, stdout, stderr fds
forceNullOutputStream – true if the stdout is being directed to
       a subsequent process. The stdout stream should be a null output stream .
Throws:
IOException – /**
     * Initialize the streams from the file descriptors.
     * @param fds array of stdin, stdout, stderr fds
     * @param forceNullOutputStream true if the stdout is being directed to
     *        a subsequent process. The stdout stream should be a null output stream .
     * @throws IOException
     */
    void initStreams(int[] fds, boolean forceNullOutputStream) throws IOException {
        switch (platform) {
            case LINUX:
            case BSD:
                stdin = (fds[0] == -1) ?
                        ProcessBuilder.NullOutputStream.INSTANCE :
                        new ProcessPipeOutputStream(fds[0]);

                stdout = (fds[1] == -1 || forceNullOutputStream) ?
                         ProcessBuilder.NullInputStream.INSTANCE :
                         new ProcessPipeInputStream(fds[1]);

                stderr = (fds[2] == -1) ?
                         ProcessBuilder.NullInputStream.INSTANCE :
                         new ProcessPipeInputStream(fds[2]);

                ProcessHandleImpl.completion(pid, true).handle((exitcode, throwable) -> {
                    synchronized (this) {
                        this.exitcode = (exitcode == null) ? -1 : exitcode.intValue();
                        this.hasExited = true;
                        this.notifyAll();
                    }

                    if (stdout instanceof ProcessPipeInputStream)
                        ((ProcessPipeInputStream) stdout).processExited();

                    if (stderr instanceof ProcessPipeInputStream)
                        ((ProcessPipeInputStream) stderr).processExited();

                    if (stdin instanceof ProcessPipeOutputStream)
                        ((ProcessPipeOutputStream) stdin).processExited();

                    return null;
                });
                break;

            case AIX:
                stdin = (fds[0] == -1) ?
                        ProcessBuilder.NullOutputStream.INSTANCE :
                        new ProcessPipeOutputStream(fds[0]);

                stdout = (fds[1] == -1 || forceNullOutputStream) ?
                         ProcessBuilder.NullInputStream.INSTANCE :
                         new DeferredCloseProcessPipeInputStream(fds[1]);

                stderr = (fds[2] == -1) ?
                         ProcessBuilder.NullInputStream.INSTANCE :
                         new DeferredCloseProcessPipeInputStream(fds[2]);

                ProcessHandleImpl.completion(pid, true).handle((exitcode, throwable) -> {
                    synchronized (this) {
                        this.exitcode = (exitcode == null) ? -1 : exitcode.intValue();
                        this.hasExited = true;
                        this.notifyAll();
                    }

                    if (stdout instanceof DeferredCloseProcessPipeInputStream)
                        ((DeferredCloseProcessPipeInputStream) stdout).processExited();

                    if (stderr instanceof DeferredCloseProcessPipeInputStream)
                        ((DeferredCloseProcessPipeInputStream) stderr).processExited();

                    if (stdin instanceof ProcessPipeOutputStream)
                        ((ProcessPipeOutputStream) stdin).processExited();

                    return null;
                });
                break;

            default: throw new AssertionError("Unsupported platform: " + platform);
        }
    }

    public OutputStream getOutputStream() {
        return stdin;
    }

    public InputStream getInputStream() {
        return stdout;
    }

    public InputStream getErrorStream() {
        return stderr;
    }

    public synchronized int waitFor() throws InterruptedException {
        while (!hasExited) {
            wait();
        }
        return exitcode;
    }

    @Override
    public synchronized boolean waitFor(long timeout, TimeUnit unit)
        throws InterruptedException
    {
        long remainingNanos = unit.toNanos(timeout);    // throw NPE before other conditions
        if (hasExited) return true;
        if (timeout <= 0) return false;

        long deadline = System.nanoTime() + remainingNanos;
        do {
            TimeUnit.NANOSECONDS.timedWait(this, remainingNanos);
            if (hasExited) {
                return true;
            }
            remainingNanos = deadline - System.nanoTime();
        } while (remainingNanos > 0);
        return hasExited;
    }

    public synchronized int exitValue() {
        if (!hasExited) {
            throw new IllegalThreadStateException("process hasn't exited");
        }
        return exitcode;
    }

    private void destroy(boolean force) {
        switch (platform) {
            case LINUX:
            case BSD:
            case AIX:
                // There is a risk that pid will be recycled, causing us to
                // kill the wrong process!  So we only terminate processes
                // that appear to still be running.  Even with this check,
                // there is an unavoidable race condition here, but the window
                // is very small, and OSes try hard to not recycle pids too
                // soon, so this is quite safe.
                synchronized (this) {
                    if (!hasExited)
                        processHandle.destroyProcess(force);
                }
                try { stdin.close();  } catch (IOException ignored) {}
                try { stdout.close(); } catch (IOException ignored) {}
                try { stderr.close(); } catch (IOException ignored) {}
                break;

            default: throw new AssertionError("Unsupported platform: " + platform);
        }
    }

    @Override
    public CompletableFuture<Process> onExit() {
        return ProcessHandleImpl.completion(pid, false)
                .handleAsync((unusedExitStatus, unusedThrowable) -> {
                    boolean interrupted = false;
                    while (true) {
                        // Ensure that the concurrent task setting the exit status has completed
                        try {
                            waitFor();
                            break;
                        } catch (InterruptedException ie) {
                            interrupted = true;
                        }
                    }
                    if (interrupted) {
                        Thread.currentThread().interrupt();
                    }
                    return this;
                });
    }

    @Override
    public ProcessHandle toHandle() {
        @SuppressWarnings("removal")
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("manageProcess"));
        }
        return processHandle;
    }

    @Override
    public boolean supportsNormalTermination() {
        return ProcessImpl.SUPPORTS_NORMAL_TERMINATION;
    }

    @Override
    public void destroy() {
        destroy(false);
    }

    @Override
    public Process destroyForcibly() {
        destroy(true);
        return this;
    }

    @Override
    public long pid() {
        return pid;
    }

    @Override
    public synchronized boolean isAlive() {
        return !hasExited;
    }

    
The toString method returns a string consisting of the native process ID of the process and the exit value of the process. 
Returns:
a string representation of the object./**
     * The {@code toString} method returns a string consisting of
     * the native process ID of the process and the exit value of the process.
     *
     * @return a string representation of the object.
     */
    @Override
    public String toString() {
        return new StringBuilder("Process[pid=").append(pid)
                .append(", exitValue=").append(hasExited ? exitcode : "\"not exited\"")
                .append("]").toString();
    }

    private static native void init();

    static {
        init();
    }

    
A buffered input stream for a subprocess pipe file descriptor
that allows the underlying file descriptor to be reclaimed when
the process exits, via the processExited hook.
This is tricky because we do not want the user-level InputStream to be
closed until the user invokes close(), and we need to continue to be
able to read any buffered data lingering in the OS pipe buffer.
/**
     * A buffered input stream for a subprocess pipe file descriptor
     * that allows the underlying file descriptor to be reclaimed when
     * the process exits, via the processExited hook.
     *
     * This is tricky because we do not want the user-level InputStream to be
     * closed until the user invokes close(), and we need to continue to be
     * able to read any buffered data lingering in the OS pipe buffer.
     */
    private static class ProcessPipeInputStream extends BufferedInputStream {
        private final Object closeLock = new Object();

        ProcessPipeInputStream(int fd) {
            super(new PipeInputStream(newFileDescriptor(fd)));
        }
        private static byte[] drainInputStream(InputStream in)
                throws IOException {
            int n = 0;
            int j;
            byte[] a = null;
            while ((j = in.available()) > 0) {
                a = (a == null) ? new byte[j] : Arrays.copyOf(a, n + j);
                n += in.read(a, n, j);
            }
            return (a == null || n == a.length) ? a : Arrays.copyOf(a, n);
        }

        
Called by the process reaper thread when the process exits. /** Called by the process reaper thread when the process exits. */
        synchronized void processExited() {
            synchronized (closeLock) {
                try {
                    InputStream in = this.in;
                    // this stream is closed if and only if: in == null
                    if (in != null) {
                        byte[] stragglers = drainInputStream(in);
                        in.close();
                        this.in = (stragglers == null) ?
                            ProcessBuilder.NullInputStream.INSTANCE :
                            new ByteArrayInputStream(stragglers);
                    }
                } catch (IOException ignored) {}
            }
        }

        @Override
        public void close() throws IOException {
            // BufferedInputStream#close() is not synchronized unlike most other
            // methods. Synchronizing helps avoid race with processExited().
            synchronized (closeLock) {
                super.close();
            }
        }
    }

    
A buffered output stream for a subprocess pipe file descriptor
that allows the underlying file descriptor to be reclaimed when
the process exits, via the processExited hook.
/**
     * A buffered output stream for a subprocess pipe file descriptor
     * that allows the underlying file descriptor to be reclaimed when
     * the process exits, via the processExited hook.
     */
    private static class ProcessPipeOutputStream extends BufferedOutputStream {
        ProcessPipeOutputStream(int fd) {
            super(new FileOutputStream(newFileDescriptor(fd)));
        }

        
Called by the process reaper thread when the process exits. /** Called by the process reaper thread when the process exits. */
        synchronized void processExited() {
            OutputStream out = this.out;
            if (out != null) {
                try {
                    out.close();
                } catch (IOException ignored) {
                    // We know of no reason to get an IOException, but if
                    // we do, there's nothing else to do but carry on.
                }
                this.out = ProcessBuilder.NullOutputStream.INSTANCE;
            }
        }
    }

    // A FileInputStream that supports the deferment of the actual close
    // operation until the last pending I/O operation on the stream has
    // finished.  This is required on Solaris because we must close the stdin
    // and stdout streams in the destroy method in order to reclaim the
    // underlying file descriptors.  Doing so, however, causes any thread
    // currently blocked in a read on one of those streams to receive an
    // IOException("Bad file number"), which is incompatible with historical
    // behavior.  By deferring the close we allow any pending reads to see -1
    // (EOF) as they did before.
    //
    private static class DeferredCloseInputStream extends PipeInputStream {
        DeferredCloseInputStream(FileDescriptor fd) {
            super(fd);
        }

        private Object lock = new Object();     // For the following fields
        private boolean closePending = false;
        private int useCount = 0;
        private InputStream streamToClose;

        private void raise() {
            synchronized (lock) {
                useCount++;
            }
        }

        private void lower() throws IOException {
            synchronized (lock) {
                useCount--;
                if (useCount == 0 && closePending) {
                    streamToClose.close();
                }
            }
        }

        // stc is the actual stream to be closed; it might be this object, or
        // it might be an upstream object for which this object is downstream.
        //
        private void closeDeferred(InputStream stc) throws IOException {
            synchronized (lock) {
                if (useCount == 0) {
                    stc.close();
                } else {
                    closePending = true;
                    streamToClose = stc;
                }
            }
        }

        public void close() throws IOException {
            synchronized (lock) {
                useCount = 0;
                closePending = false;
            }
            super.close();
        }

        public int read() throws IOException {
            raise();
            try {
                return super.read();
            } finally {
                lower();
            }
        }

        public int read(byte[] b) throws IOException {
            raise();
            try {
                return super.read(b);
            } finally {
                lower();
            }
        }

        public int read(byte[] b, int off, int len) throws IOException {
            raise();
            try {
                return super.read(b, off, len);
            } finally {
                lower();
            }
        }

        public long skip(long n) throws IOException {
            raise();
            try {
                return super.skip(n);
            } finally {
                lower();
            }
        }

        public int available() throws IOException {
            raise();
            try {
                return super.available();
            } finally {
                lower();
            }
        }
    }

    
A buffered input stream for a subprocess pipe file descriptor
that allows the underlying file descriptor to be reclaimed when
the process exits, via the processExited hook.
This is tricky because we do not want the user-level InputStream to be
closed until the user invokes close(), and we need to continue to be
able to read any buffered data lingering in the OS pipe buffer.
On AIX this is especially tricky, because the 'close()' system call
will block if another thread is at the same time blocked in a file
operation (e.g. 'read()') on the same file descriptor. We therefore
combine 'ProcessPipeInputStream' approach used on Linux and Bsd
with the DeferredCloseInputStream approach used on Solaris. This means
that every potentially blocking operation on the file descriptor
increments a counter before it is executed and decrements it once it
finishes. The 'close()' operation will only be executed if there are
no pending operations. Otherwise it is deferred after the last pending
operation has finished.
/**
     * A buffered input stream for a subprocess pipe file descriptor
     * that allows the underlying file descriptor to be reclaimed when
     * the process exits, via the processExited hook.
     *
     * This is tricky because we do not want the user-level InputStream to be
     * closed until the user invokes close(), and we need to continue to be
     * able to read any buffered data lingering in the OS pipe buffer.
     *
     * On AIX this is especially tricky, because the 'close()' system call
     * will block if another thread is at the same time blocked in a file
     * operation (e.g. 'read()') on the same file descriptor. We therefore
     * combine 'ProcessPipeInputStream' approach used on Linux and Bsd
     * with the DeferredCloseInputStream approach used on Solaris. This means
     * that every potentially blocking operation on the file descriptor
     * increments a counter before it is executed and decrements it once it
     * finishes. The 'close()' operation will only be executed if there are
     * no pending operations. Otherwise it is deferred after the last pending
     * operation has finished.
     *
     */
    private static class DeferredCloseProcessPipeInputStream
        extends BufferedInputStream {

        private final Object closeLock = new Object();
        private int useCount = 0;
        private boolean closePending = false;

        DeferredCloseProcessPipeInputStream(int fd) {
            super(new PipeInputStream(newFileDescriptor(fd)));
        }

        private InputStream drainInputStream(InputStream in)
                throws IOException {
            int n = 0;
            int j;
            byte[] a = null;
            synchronized (closeLock) {
                if (buf == null) // asynchronous close()?
                    return null; // discard
                j = in.available();
            }
            while (j > 0) {
                a = (a == null) ? new byte[j] : Arrays.copyOf(a, n + j);
                synchronized (closeLock) {
                    if (buf == null) // asynchronous close()?
                        return null; // discard
                    n += in.read(a, n, j);
                    j = in.available();
                }
            }
            return (a == null) ?
                    ProcessBuilder.NullInputStream.INSTANCE :
                    new ByteArrayInputStream(n == a.length ? a : Arrays.copyOf(a, n));
        }

        
Called by the process reaper thread when the process exits. /** Called by the process reaper thread when the process exits. */
        synchronized void processExited() {
            try {
                InputStream in = this.in;
                if (in != null) {
                    InputStream stragglers = drainInputStream(in);
                    in.close();
                    this.in = stragglers;
                }
            } catch (IOException ignored) { }
        }

        private void raise() {
            synchronized (closeLock) {
                useCount++;
            }
        }

        private void lower() throws IOException {
            synchronized (closeLock) {
                useCount--;
                if (useCount == 0 && closePending) {
                    closePending = false;
                    super.close();
                }
            }
        }

        @Override
        public int read() throws IOException {
            raise();
            try {
                return super.read();
            } finally {
                lower();
            }
        }

        @Override
        public int read(byte[] b) throws IOException {
            raise();
            try {
                return super.read(b);
            } finally {
                lower();
            }
        }

        @Override
        public int read(byte[] b, int off, int len) throws IOException {
            raise();
            try {
                return super.read(b, off, len);
            } finally {
                lower();
            }
        }

        @Override
        public long skip(long n) throws IOException {
            raise();
            try {
                return super.skip(n);
            } finally {
                lower();
            }
        }

        @Override
        public int available() throws IOException {
            raise();
            try {
                return super.available();
            } finally {
                lower();
            }
        }

        @Override
        public void close() throws IOException {
            // BufferedInputStream#close() is not synchronized unlike most other
            // methods. Synchronizing helps avoid racing with drainInputStream().
            synchronized (closeLock) {
                if (useCount == 0) {
                    super.close();
                }
                else {
                    closePending = true;
                }
            }
        }
    }
}