[BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN > received >= buf

[Julio Guerra]

Hi,

When a tty (here a slave pty) is set in noncanonical input and blocking 
read modes, a read() randomly blocks when:
"VMIN > kernel received >= user buffer size > 0".

The standard says that read() should block until VMIN bytes are received 
[1][2]. Whether this is an implementation defined case not really 
specified by POSIX or not, it should not behave randomly (otherwise it 
really should be documented in termios manpage).

I isolated it in the following example (with VMIN = 5, received = 4, 
user buffer = 3):
https://gist.github.com/Julio-Guerra/b3fdefab281403073607d81cabcea04a

Since it is random, you will need run it several times to observe both 
cases. When correctly behaving, it should block in the read() for ever 
(C-c to kill it). When incorrectly behaving, it does not block and reads 
into the user buffer what is present in the kernel receive buffer 
(string "any").

Example where it does not block 3 times out of 4:
 > $ ./a.out
 > res=3 buf=any
 > $ ./a.out
 > res=3 buf=any
 > $ ./a.out
 > ^Câ

Linux version 4.5.1-1-ARCH (builduser@tobias) (gcc version 5.3.0 (GCC) ) 
#1 SMP PREEMPT Thu Apr 14 19:19:32 CEST 2016

[1] "Canonical and noncanonical mode", man termios
[2] 
http://www.gnu.org/software/libc/manual/html_node/Noncanonical-Input.html

--
Julio Guerra