n_tty: Fix hanfling of buffer full corner cases
Fix the handling of input characters when the tty buffer is full or nearly full. This includes tests that are done in n_tty_receive_char() and handling of PARMRK. Problems with the buffer-full tests done in receive_char() caused characters to be lost at times when the buffer(s) filled. Also, these full conditions would often only be detected with echo on, and PARMRK was not accounted for properly in all cases. One symptom of these problems, in addition to lost characters, was early termination from unix commands like tr and cat when ^Q was used to break from a stopped tty with full buffers (note that breaking out was often previously not possible, due to the pty getting in "gridlock", which will be addressed in another patch). Note space is always reserved at the end of the buffer for a newline (or EOF/EOL) in canonical mode. Signed-off-by: Joe Peterson <joe@skyrush.com> Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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