[PATCH v4] libio: Start to return errors when flushing fwrite's buffer [BZ #29459]

[Florian Weimer]

* Tulio Magno Quites Machado Filho:

> diff --git a/stdio-common/tst-fwrite-bz29459.c b/stdio-common/tst-fwrite-bz29459.c
> new file mode 100644
> index 0000000000..2e8ebc5a08
> --- /dev/null
> +++ b/stdio-common/tst-fwrite-bz29459.c
> @@ -0,0 +1,89 @@
> +/* Test fwrite against bug 29459.

> +/* Usually this test reproduces in a few iterations.  However, keep a high
> +   number of iterations in order to avoid return false-positives due to an
> +   overwhelmed/slow system.  */
> +#define ITERATIONS 5000

> +  /* Ensure the string we send has a new line because we're dealing
> +     with a lined-buffered stream.  */
> +  const char *s = "hello\n";
> +  const size_t len = strlen(s);

The number of iterations and the length of the written string are too
small to fill all the buffers in all cases, so we end up with:

  fprintf(stderr, "Error: fwrite did not return an error\n");

I suggest to increase both substantially (maybe 500 * 1000 and string
length 10?).  The total amount of buffering is probably dependent on
kernel and coreutils versions.

Thanks,
Florian