Mitigation for JRUBY-6576

Because in JRuby, concurrent selects can trigger blocking mode
exceptions, I've modified IO.select logic to raise a concurrency
error when multiple threads try to select on the same sockets at
the same time (and fail to do so cleanly). At present, there's
no way to do this since we use these channels in both blocking and
nonblocking mode; we can't synchronize the blocking mode changes
with multiple threads all trying to do select operations.

src/org/jruby/util/io/SelectBlob.java

@@ -29,6 +29,7 @@
 import java.io.IOException;
 import java.nio.channels.CancelledKeyException;
 import java.nio.channels.Channel;
+import java.nio.channels.IllegalBlockingModeException;
 import java.nio.channels.SelectableChannel;
 import java.nio.channels.SelectionKey;
 import java.nio.channels.Selector;
@@ -345,14 +346,22 @@ private void tidyUp() throws IOException {
         if (readBlocking != null) {
             for (int i = 0; i < readBlocking.length; i++) {
                 if (readBlocking[i] != null) {
-                    ((SelectableChannel) readIOs[i].getChannel()).configureBlocking(readBlocking[i]);
+                    try {
+                        ((SelectableChannel) readIOs[i].getChannel()).configureBlocking(readBlocking[i]);
+                    } catch (IllegalBlockingModeException ibme) {
+                        throw runtime.newConcurrencyError("can not set IO blocking after select; concurrent select detected?");
+                    }
                 }
             }
         }
         if (writeBlocking != null) {
             for (int i = 0; i < writeBlocking.length; i++) {
                 if (writeBlocking[i] != null) {
-                    ((SelectableChannel) writeIOs[i].getChannel()).configureBlocking(writeBlocking[i]);
+                    try {
+                        ((SelectableChannel) writeIOs[i].getChannel()).configureBlocking(writeBlocking[i]);
+                    } catch (IllegalBlockingModeException ibme) {
+                        throw runtime.newConcurrencyError("can not set IO blocking after select; concurrent select detected?");
+                    }
                 }
             }
         }