[PATCH 2/3] use rb_str_subseq for tail string on write

[Sokolov Yura 'funny-falcon <funny.falcon@gmail.com>]

Use rb_str_subseq for taking string's tail. rb_str_subseq do not allocate
additional memory in this case. And although it prevents from collecting
original string, it seems that tests wins both in performance and in memory
usage.

Use fallback to rb_str_substr on ruby1.8
---
 ext/kgio/extconf.rb   |    1 +
 ext/kgio/read_write.c |    5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/ext/kgio/extconf.rb b/ext/kgio/extconf.rb
index fb680f7..f6bd0cc 100644
--- a/ext/kgio/extconf.rb
+++ b/ext/kgio/extconf.rb
@@ -49,5 +49,6 @@ have_func('rb_thread_io_blocking_region')
 have_func('rb_str_set_len')
 have_func('rb_time_interval')
 have_func('rb_wait_for_single_fd')
+have_func('rb_str_subseq')
 
 create_makefile('kgio_ext')
diff --git a/ext/kgio/read_write.c b/ext/kgio/read_write.c
index 51d2d16..9924743 100644
--- a/ext/kgio/read_write.c
+++ b/ext/kgio/read_write.c
@@ -4,6 +4,9 @@
 static VALUE sym_wait_readable, sym_wait_writable;
 static VALUE eErrno_EPIPE, eErrno_ECONNRESET;
 static ID id_set_backtrace;
+#ifndef HAVE_RB_STR_SUBSEQ
+#define rb_str_subseq rb_str_substr
+#endif
 
 /*
  * we know MSG_DONTWAIT works properly on all stream sockets under Linux
@@ -338,7 +341,7 @@ done:
 				a->ptr = RSTRING_PTR(a->buf) + written;
 				return -1;
 			} else if (written > 0) {
-				a->buf = rb_str_new(a->ptr, a->len);
+				a->buf = rb_str_subseq(a->buf, written, a->len);
 			} else {
 				a->buf = sym_wait_writable;
 			}
-- 
1.7.9.5