io.c: make IO#reopen("pathname") atomic

* io.c (rb_io_reopen): create a new, temporary FD via rb_sysopen and
  call rb_cloexec_dup2 on it to atomically replace the file fptr->fd
  points to.  This leaves no possible window where fptr->fd is invalid
  to userspace (even for any threads running w/o GVL).  based on the
  patch by Eric Wong <normalperson@yhbt.net> at [ruby-core:57943].
  [Bug #9036]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43373 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,12 @@
+Sun Oct 20 15:41:22 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_reopen): create a new, temporary FD via rb_sysopen and
+	  call rb_cloexec_dup2 on it to atomically replace the file fptr->fd
+	  points to.  This leaves no possible window where fptr->fd is invalid
+	  to userspace (even for any threads running w/o GVL).  based on the
+	  patch by Eric Wong <normalperson@yhbt.net> at [ruby-core:57943].
+	  [Bug #9036]
+
 Sun Oct 20 15:29:05 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* error.c (rb_syserr_fail_path_in): new function split from

io.c

@@ -6665,10 +6665,14 @@ rb_io_reopen(int argc, VALUE *argv, VALUE file)
         }
     }
     else {
-        if (close(fptr->fd) < 0)
-            rb_sys_fail_path(fptr->pathv);
-        fptr->fd = -1;
-        fptr->fd = rb_sysopen(fptr->pathv, oflags, 0666);
+	int tmpfd = rb_sysopen(fptr->pathv, oflags, 0666);
+	int err = 0;
+	if (rb_cloexec_dup2(tmpfd, fptr->fd) < 0)
+	    err = errno;
+	(void)close(tmpfd);
+	if (err) {
+	    rb_syserr_fail_path(err, fptr->pathv);
+	}
     }
 
     return file;