Do not double call termContext in QueryEx

QueryEx was calling termContext and rows.fatal on err of sendPreparedQuery.
rows.fatal calls rows.Close which already calls termContext. This sequence of
calls was causing underlying io timeout errors to be returned instead of context
errors.

In addition, added fatalWriteErr helper method to allow recovery of write
timeout errors where no bytes were written.

This should solve flickering errors on Travis.

conn.go

@@ -795,9 +795,11 @@ func (c *Conn) prepareEx(name, sql string, opts *PrepareExOptions) (ps *Prepared
 	buf = append(buf, 'S')
 	buf = pgio.AppendInt32(buf, 4)
 
-	_, err = c.conn.Write(buf)
+	n, err := c.conn.Write(buf)
 	if err != nil {
+		if fatalWriteErr(n, err) {
 			c.die(err)
+		}
 		return nil, err
 	}
 	c.readyForQuery = false
@@ -1085,8 +1087,8 @@ func (c *Conn) sendPreparedQuery(ps *PreparedStatement, arguments ...interface{}
 	buf = append(buf, 'S')
 	buf = pgio.AppendInt32(buf, 4)
 
-	_, err = c.conn.Write(buf)
+	n, err := c.conn.Write(buf)
-	if err != nil {
+	if err != nil && fatalWriteErr(n, err) {
 		c.die(err)
 	}
 	c.readyForQuery = false
@@ -1094,6 +1096,17 @@ func (c *Conn) sendPreparedQuery(ps *PreparedStatement, arguments ...interface{}
 	return err
 }
 
+// fatalWriteError takes the response of a net.Conn.Write and determines if it is fatal
+func fatalWriteErr(bytesWritten int, err error) bool {
+	// Partial writes break the connection
+	if bytesWritten > 0 {
+		return true
+	}
+
+	netErr, is := err.(net.Error)
+	return !(is && netErr.Timeout())
+}
+
 // Exec executes sql. sql can be either a prepared statement name or an SQL string.
 // arguments should be referenced positionally from the sql string as $1, $2, etc.
 func (c *Conn) Exec(sql string, arguments ...interface{}) (commandTag CommandTag, err error) {

query.go

@@ -398,16 +398,15 @@ func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions,
 	err = c.initContext(ctx)
 	if err != nil {
 		rows.fatal(err)
-		return rows, err
+		return rows, rows.err
 	}
 
 	err = c.sendPreparedQuery(ps, args...)
 	if err != nil {
 		rows.fatal(err)
-		err = c.termContext(err)
 	}
 
-	return rows, err
+	return rows, rows.err
 }
 
 func (c *Conn) sanitizeAndSendSimpleQuery(sql string, args ...interface{}) (err error) {