Avoid shutdown() on shared sockets (fixes brianmario#213)

Close the mysql socket before calling mysql_close(). mysql_close() calls
shutdown() on the attached socket. In a situation involving fork() and copies
of an object spanning multiple interpreter instances this will result in the
first instance to GC breaking all other instances sharing the socket.

dylanahsmith notes that we can avoid this by simply closing the socket before
entering mysql_close(), preventing the shutdown() from having any impact on the
shared socket state.

Author: eam

ext/mysql2/client.c

@@ -164,26 +164,19 @@ static void *nogvl_connect(void *ptr) {
 
 static void *nogvl_close(void *ptr) {
   mysql_client_wrapper *wrapper;
-#ifndef _WIN32
-  int flags;
-#endif
   wrapper = ptr;
   if (wrapper->connected) {
     wrapper->active_thread = Qnil;
     wrapper->connected = 0;
-    /*
-     * we'll send a QUIT message to the server, but that message is more of a
-     * formality than a hard requirement since the socket is getting shutdown
-     * anyways, so ensure the socket write does not block our interpreter
-     *
-     *
-     * if the socket is dead we have no chance of blocking,
-     * so ignore any potential fcntl errors since they don't matter
-     */
 #ifndef _WIN32
-    flags = fcntl(wrapper->client->net.fd, F_GETFL);
-    if (flags > 0 && !(flags & O_NONBLOCK))
-      fcntl(wrapper->client->net.fd, F_SETFL, flags | O_NONBLOCK);
+    /* Call close() on the socket before calling mysql_close(). This prevents
+     * mysql_close() from sending a mysql-QUIT or from calling shutdown() on
+     * the socket. The difference is that close() will drop this process's
+     * reference to the socket only, while a QUIT or shutdown() would render
+     * the underlying connection unusable, interrupting other processes which
+     * share this object across a fork().
+     */
+    close(wrapper->client->net.fd);
 #endif
 
     mysql_close(wrapper->client);

spec/mysql2/client_spec.rb

@@ -126,6 +126,27 @@ def connect *args
     final_count.should == before_count
   end
 
+  if Process.respond_to?(:fork)
+    it "should not close connections when running in a child process" do
+      GC.start
+      sleep 1 if defined? Rubinius # Let the rbx GC thread do its work
+      client = Mysql2::Client.new(DatabaseCredentials['root'])
+
+      fork do
+        client.query('SELECT 1')
+        client = nil
+        GC.start
+        sleep 1 if defined? Rubinius # Let the rbx GC thread do its work
+      end
+
+      Process.wait
+
+      # this will throw an error if the underlying socket was shutdown by the
+      # child's GC
+      expect { client.query('SELECT 1') }.to_not raise_exception
+    end
+  end
+
   it "should be able to connect to database with numeric-only name" do
     lambda {
       creds = DatabaseCredentials['numericuser']