Never block for read(0), return blank string. Fixes #1637.

Author: headius

core/src/main/java/org/jruby/RubyIO.java

@@ -3136,6 +3136,8 @@ private IRubyObject readNotAll(ThreadContext context, OpenFile myOpenFile, int l
         Ruby runtime = context.runtime;
         str.empty();
 
+        if (length == 0) return str;
+
         try {
             ByteList newBuffer = readNotAllCommon(context, myOpenFile, length);
 
@@ -3161,6 +3163,8 @@ private IRubyObject readNotAll(ThreadContext context, OpenFile myOpenFile, int l
     private IRubyObject readNotAll(ThreadContext context, OpenFile myOpenFile, int length) {
         Ruby runtime = context.runtime;
 
+        if (length == 0) return RubyString.newEmptyString(runtime);
+
         try {
             ByteList newBuffer = readNotAllCommon(context, myOpenFile, length);
 

test/test_socket.rb

@@ -3,6 +3,7 @@
 require 'thread'
 require 'test/test_helper'
 require 'ipaddr'
+require 'timeout'
 
 WINDOWS = RbConfig::CONFIG['host_os'] =~ /Windows|mswin/
 
@@ -512,5 +513,23 @@ def test_syswrite_raises_epipe
       assert Errno::EPIPE === ex
     end
   end
+
+  # jruby/jruby#1637
+  def test_read_zero_never_blocks
+    assert_nothing_raised do
+      server = TCPServer.new(nil, 12345)
+      t = Thread.new do
+        s = server.accept
+      end
+      client = TCPSocket.new(nil, 12345)
+      Timeout.timeout(1) do
+        assert_equal "", client.read(0)
+      end
+      t.join
+    end
+  ensure
+    server.close rescue nil
+    client.close rescue nil
+  end
 end