libio: Update internal fmemopen position after write (BZ #20005)

Current GLIBC fmemopen fails with a simple testcase:

  char buffer[500] = "x";
  FILE *stream;
  stream = fmemopen(buffer, 500, "r+");
  fwrite("fish",sizeof(char),5,stream);
  printf("pos-1:%ld\n",ftell(stream));
  fflush(stream);
  printf("pos-2:%ld\n",ftell(stream));

It returns:

  pos-1:5
  pos-2:0

Where it should return:

  pos-1:5
  pos-2:5

This is due the internal write function does not correctly update the internal
object position state and then the seek operation returns a wrong value.  This
patch fixes it.

It fixes both BZ #20005 and BZ #19230 (marked as duplicated). A new test is
added to check for such case.

Tested on x86_64 and i686.

	* libio/fmemopen.c (fmemopen_write): Update internal position after
	write.
	* stdio-common/Makefile (tests): Add tst-fmemopen4.c.
	* stdio-common/tst-fmemopen4.c: New file..

1 files changed, 3 insertions, 3 deletions

diff --git a/libio/fmemopen.c b/libio/fmemopen.c
index 23b5c5fea8..9264b724ee 100644
--- a/libio/fmemopen.c
+++ b/libio/fmemopen.c
@@ -86,10 +86,10 @@ fmemopen_write (void *cookie, const char *b, size_t s)
 
   memcpy (&(c->buffer[pos]), b, s);
 
-  pos += s;
-  if ((size_t) pos > c->maxpos)
+  c->pos += s;
+  if ((size_t) c->pos > c->maxpos)
     {
-      c->maxpos = pos;
+      c->maxpos = c->pos;
       if (addnullc)
 	c->buffer[c->maxpos] = '\0';
     }