__sflush(): on write error, if nothing was written, reset FILE state back

[kib]

Summary

otherwise the data is just dropped.  Check for current position equal to
the buffer base at the entry of the function; if not equal, setvbuf()
was done from the write method and it is not our business to override
the decision.

PR:     76398

[markj]

lib/libc/stdio/fflush.c

133

Shouldn't this also be fp->_p == old_p && p > fp->_p?

[kib]

lib/libc/stdio/fflush.c

133

May be. But this is definitely a separate change.

In fact I am not sure what should be a contract for custom streams which do setvbuf() and return error from the write method.