problem: jeromq is not thread-safe

Solution: port CLIENT and SERVER sockets from libzmq, which are thread-safe sockets.

The thread-safe sockets family includes other sockets, which are not part of this commit.

Also, CLIENT and SERVER cannot be used with a poller at the moment. For poller support, zmq_poller from libzmq has to be ported as well.

src/main/java/zmq/Ctx.java

@@ -106,7 +106,7 @@ private enum Side
 
     //  Array of pointers to mailboxes for both application and I/O threads.
     private int       slotCount;
-    private Mailbox[] slots;
+    private IMailbox[] slots;
 
     //  Mailbox for zmq_term thread.
     private final Mailbox termMailbox;
@@ -435,7 +435,7 @@ private void initSlots()
             finally {
                 optSync.unlock();
             }
-            slots = new Mailbox[slotCount];
+            slots = new IMailbox[slotCount];
 
             //  Initialize the infrastructure for zmq_term thread.
             slots[TERM_TID] = termMailbox;

src/main/java/zmq/IMailbox.java

@@ -0,0 +1,8 @@
+package zmq;
+
+import java.io.Closeable;
+
+public interface IMailbox extends Closeable  {
+    void send(final Command cmd);
+    Command recv(long timeout);
+}

src/main/java/zmq/Mailbox.java

@@ -1,6 +1,5 @@
 package zmq;
 
-import java.io.Closeable;
 import java.io.IOException;
 import java.nio.channels.SelectableChannel;
 import java.util.concurrent.locks.Lock;
@@ -9,7 +8,7 @@
 import zmq.pipe.YPipe;
 import zmq.util.Errno;
 
-public final class Mailbox implements Closeable
+public final class Mailbox implements IMailbox
 {
     //  The pipe to store actual commands.
     private final YPipe<Command> cpipe;
@@ -55,7 +54,8 @@ public SelectableChannel getFd()
         return signaler.getFd();
     }
 
-    void send(final Command cmd)
+    @Override
+    public void send(final Command cmd)
     {
         boolean ok = false;
         sync.lock();
@@ -72,6 +72,7 @@ void send(final Command cmd)
         }
     }
 
+    @Override
     public Command recv(long timeout)
     {
         Command cmd;

src/main/java/zmq/MailboxSafe.java

@@ -0,0 +1,129 @@
+package zmq;
+
+import zmq.pipe.YPipe;
+import zmq.util.Errno;
+
+import java.io.IOException;
+import java.util.Vector;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.Condition;
+
+public class MailboxSafe implements IMailbox
+{
+    //  The pipe to store actual commands.
+    private final YPipe<Command> cpipe;
+
+    //  Synchronize access to the mailbox from receivers and senders
+    private final ReentrantLock sync;
+
+    //  Condition variable to pass signals from writer thread to reader thread.
+    private final Condition condition;
+
+    private final Vector<Signaler> signalers;
+
+    // mailbox name, for better debugging
+    private final String name;
+
+    private final Errno errno;
+
+    public MailboxSafe(Ctx ctx, ReentrantLock sync, String name)
+    {
+        this.errno = ctx.errno();
+        this.cpipe = new YPipe<>(Config.COMMAND_PIPE_GRANULARITY.getValue());
+        this.sync = sync;
+        this.condition = this.sync.newCondition();
+        this.signalers = new Vector<Signaler>(10);
+        this.name = name;
+
+        //  Get the pipe into passive state. That way, if the users starts by
+        //  polling on the associated file descriptor it will get woken up when
+        //  new command is posted.
+        Command cmd = cpipe.read();
+        assert (cmd == null);
+    }
+
+    public void addSignaler(Signaler signaler)
+    {
+        this.signalers.add(signaler);
+    }
+
+    public void removeSignaler(Signaler signaler)
+    {
+        this.signalers.remove(signaler);
+    }
+
+    public void clearSignalers()
+    {
+        this.signalers.clear();
+    }
+
+    @Override
+    public void send(Command cmd)
+    {
+        sync.lock();
+        try {
+            cpipe.write(cmd, false);
+            boolean ok = cpipe.flush();
+
+            if (!ok) {
+                condition.signalAll();
+                signalers.forEach(Signaler::send);
+            }
+        }
+        finally {
+            sync.unlock();
+        }
+    }
+
+    @Override
+    public Command recv(long timeout)
+    {
+        Command cmd;
+
+        //  Try to get the command straight away.
+        cmd = cpipe.read();
+        if (cmd != null) {
+            return cmd;
+        }
+
+        //  If the timeout is zero, it will be quicker to release the lock, giving other a chance to send a command
+        //  and immediately relock it.
+        if (timeout == 0) {
+            sync.unlock();
+            sync.lock();
+        } else {
+            try {
+                //  Wait for signal from the command sender.
+                condition.await(timeout, TimeUnit.MILLISECONDS);
+            } catch (InterruptedException e) {
+                errno.set(ZError.EINTR);
+                return null;
+            }
+        }
+
+        //  Another thread may already fetch the command
+        cmd = cpipe.read();
+        if (cmd == null) {
+            errno.set(ZError.EAGAIN);
+            return null;
+        }
+
+        return cmd;
+    }
+
+    @Override
+    public void close() throws IOException
+    {
+        // Work around problem that other threads might still be in our
+        // send() method, by waiting on the mutex before disappearing.
+        sync.lock();
+        sync.unlock();
+    }
+
+    @Override
+    public String toString()
+    {
+        return super.toString() + "[" + name + "]";
+    }
+}

src/main/java/zmq/Msg.java

@@ -111,6 +111,8 @@ enum Type
     // keep track of relative read position
     private int readIndex = 0;
 
+    private int routingId;
+
     public Msg()
     {
         this(0);
@@ -435,4 +437,35 @@ public Msg putShortString(String data)
         writeIndex += Wire.putShortString(dup, data);
         return this;
     }
+
+    /**
+    * Return the routing id of a message. The routing id represent the CLIENT socket that sent the message to the
+    * SERVER socket.
+    * @return the routing id
+    * */
+    public int getRoutingId()
+    {
+        return routingId;
+    }
+
+    /**
+     * Set the routing id on a message. The routing id represent the CLIENT socket which the message should be sent to.
+     * Only SERVER socket is currently using the routing id.
+     * @param routingId the routing id
+     * @return true if successfully set the routing id.
+     */
+    public boolean setRoutingId(int routingId)
+    {
+        if (routingId != 0) {
+            this.routingId = routingId;
+            return true;
+        }
+
+        return false;
+    }
+
+    public void resetRoutingId()
+    {
+        routingId = 0;
+    }
 }