libio: Multiple fixes for open_{w}memstram (BZ#18241 and BZ#20181)

This patches fixes multiples issues on open_{w}memstream reported on both
BZ#18241 and BZ#20181:

  - failed fseek does not set errno.
  - negative offset in fseek fails even when resulting position is
    a valid one.
  - a flush after write if the current write position is not at the
    end of the stream currupt data.

The main fix is on seek operation for memstream (_IO_{w}str_seekoff), where
both _IO_read_ptr and _IO_read_end pointer are updated if a write operation
has occured (similar to default file operations).  Also, to calculate the
offset on both read and write pointers, a temporary value is instead of
updating the argument supplied value.  Negative offset are valid if resulting
internal pointer is within the range of _IO_{read,write}_base and
_IO_{read,write}_end.

Also POSIX states that a null or wide null shall be appended to the current
buffer iff a write moves the position to a value larger than the current
lenght.  Current implementation appends a null or wide null regardless
of this condition.  This patch fixes it by removing the 'else' condition
on _IO_{w}mem_sync.

Checked on x86_64.

	[BZ #18241]
	[BZ #20181]
	* libio/Makefile (test): Add tst-memstream3 and tst-wmemstream3.
	* libio/memstream.c (_IO_mem_sync): Only append a null byte if
	write position is at the end the buffer.
	* libio/wmemstream.c (_IO_wmem_sync): Likewise.
	* libio/strops.c (_IO_str_switch_to_get_mode): New function.
	(_IO_str_seekoff): Set correct offset from negative displacement and
	set EINVAL for invalid ones.
	* libio/wstrops.c (enlarge_userbuf): Use correct function to calculate
	buffer length.
	(_IO_wstr_switch_to_get_mode): New function.
	(_IO_wstr_seekoff): Set correct offset from negative displacement and
	set EINVAL for invalid ones.
	* libio/tst-memstream3.c: New file.
	* libio/tst-wmemstream3.c: Likewise.
	* manual/examples/memstrm.c: Remove warning when priting size_t.

1 files changed, 2 insertions, 2 deletions

diff --git a/manual/examples/memstrm.c b/manual/examples/memstrm.c
index 0d443b1371..5701ba1f39 100644
--- a/manual/examples/memstrm.c
+++ b/manual/examples/memstrm.c
@@ -27,10 +27,10 @@ main (void)
   stream = open_memstream (&bp, &size);
   fprintf (stream, "hello");
   fflush (stream);
-  printf ("buf = `%s', size = %d\n", bp, size);
+  printf ("buf = `%s', size = %zu\n", bp, size);
   fprintf (stream, ", world");
   fclose (stream);
-  printf ("buf = `%s', size = %d\n", bp, size);
+  printf ("buf = `%s', size = %zu\n", bp, size);
 
   return 0;
 }