test_tcflow hangs on DragonFly BSD

[serhiy-storchaka]

Bug report

test_termios.test_tcflow hangs on DragonFly BSD. It is not tcflow() itself which hangs, but closing the pseudo-terminal in the cleanup:

Timeout (0:02:00)!
Thread 0x0000000800af30c0 (most recent call first):
  File "Lib/unittest/case.py", line 654 in _callCleanup
  File "Lib/unittest/case.py", line 728 in doCleanups

TCIOFF and TCION transmit STOP and START characters, and nothing reads them from the master side, so closing the slave waits for the output to drain forever. Which action is used matters:

action	DragonFly BSD	Linux
TCOOFF, TCOON	close returns	close returns
TCIOFF	close blocks	close returns
TCION	close blocks	close returns

Reading the master before closing shows the two characters and unblocks the close:

master had b'\x13\x11'

Discarding them with tcflush(fd, TCOFLUSH) after the tcflow() calls fixes the hang. test_termios then takes 44 ms instead of timing out.

Linked PRs

• gh-154416: Fix hanging test_tcflow on DragonFly BSD #154417
• [3.13] gh-154416: Fix hanging test_tcflow on DragonFly BSD (GH-154417) #154421
• [3.14] gh-154416: Fix hanging test_tcflow on DragonFly BSD (GH-154417) #154422
• [3.15] gh-154416: Fix hanging test_tcflow on DragonFly BSD (GH-154417) #154423