commit ecc7c3deb9f347649c2078fcc0f94d4cedf92d60
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Jan 2 14:36:17 2024 +0100

    libio: Check remaining buffer size in _IO_wdo_write (bug 31183)
    
    The multibyte character needs to fit into the remaining buffer space,
    not the already-written buffer space.  Without the fix, we were never
    moving the write pointer from the start of the buffer, always using
    the single-character fallback buffer.
    
    Fixes commit 04b76b5aa8b2d1d19066e42dd1 ("Don't error out writing
    a multibyte character to an unbuffered stream (bug 17522)").

diff --git a/libio/wfileops.c b/libio/wfileops.c
index d3deb34ba058ca39..6a6421f8880f9356 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -57,7 +57,7 @@ _IO_wdo_write (FILE *fp, const wchar_t *data, size_t to_do)
 	  char mb_buf[MB_LEN_MAX];
 	  char *write_base, *write_ptr, *buf_end;
 
-	  if (fp->_IO_write_ptr - fp->_IO_write_base < sizeof (mb_buf))
+	  if (fp->_IO_buf_end - fp->_IO_write_ptr < sizeof (mb_buf))
 	    {
 	      /* Make sure we have room for at least one multibyte
 		 character.  */