Don't read from socket when data is pending in conn.block

conn.block still reads data from the socket, but only after checking the result queue and waiting for readability first.
Reading data from the socket (PQconsumeInput) takes much more time than just checking available results (PQisBusy).
So doing it as a precaution to conn.block is a performance issue, now that we call block before each get_result in pg-1.3.x.

The test was introduced in commit 09bdc16 .
It is not particular specific to conn.block, so that we can check the error class on consume_input equaly.

Fixes #442

ext/pg_connection.c

@@ -2339,21 +2339,12 @@ pg_rb_io_wait(VALUE io, VALUE events, VALUE timeout) {
 static void *
 wait_socket_readable( VALUE self, struct timeval *ptimeout, void *(*is_readable)(PGconn *))
 {
-	VALUE socket_io;
 	VALUE ret;
 	void *retval;
 	struct timeval aborttime={0,0}, currtime, waittime;
 	VALUE wait_timeout = Qnil;
 	PGconn *conn = pg_get_pgconn(self);
 
-	socket_io = pgconn_socket_io(self);
-
-	/* Check for connection errors (PQisBusy is true on connection errors) */
-	if ( PQconsumeInput(conn) == 0 ) {
-		pgconn_close_socket_io(self);
-		rb_raise( rb_eConnectionBad, "PQconsumeInput() %s", PQerrorMessage(conn) );
-	}
-
 	if ( ptimeout ) {
 		gettimeofday(&currtime, NULL);
 		timeradd(&currtime, ptimeout, &aborttime);
@@ -2368,6 +2359,7 @@ wait_socket_readable( VALUE self, struct timeval *ptimeout, void *(*is_readable)
 
 		/* Is the given timeout valid? */
 		if( !ptimeout || (waittime.tv_sec >= 0 && waittime.tv_usec >= 0) ){
+			VALUE socket_io = pgconn_socket_io(self);
 			/* Wait for the socket to become readable before checking again */
 			ret = pg_rb_io_wait(socket_io, RB_INT2NUM(PG_RUBY_IO_READABLE), wait_timeout);
 		} else {

spec/pg/connection_spec.rb

@@ -1449,15 +1449,15 @@ def interrupt_thread(exc=nil)
 		conn.finish
 	end
 
-	it "block should raise ConnectionBad for a closed connection" do
+	it "consume_input should raise ConnectionBad for a closed connection" do
 		serv = TCPServer.new( '127.0.0.1', 54320 )
 		conn = described_class.connect_start( '127.0.0.1', 54320, "", "", "me", "xxxx", "somedb" )
 		while [PG::CONNECTION_STARTED, PG::CONNECTION_MADE].include?(conn.connect_poll)
 			sleep 0.1
 		end
 		serv.close
-		expect{ conn.block }.to raise_error(PG::ConnectionBad, /server closed the connection unexpectedly/)
-		expect{ conn.block }.to raise_error(PG::ConnectionBad, /can't get socket descriptor|connection not open/)
+		expect{ conn.consume_input }.to raise_error(PG::ConnectionBad, /server closed the connection unexpectedly/)
+		expect{ conn.consume_input }.to raise_error(PG::ConnectionBad, /can't get socket descriptor|connection not open/)
 	end
 
 	it "calls the block supplied to wait_for_notify with the notify payload if it accepts " +