tree 6d7b5fa9ae4678589dc4f663fc3e69de7dede8e3
parent 530249c593d945d635e3727d53d34c6710ca70c8
author Willy Tarreau <w@1wt.eu> 1563166074 +0200
committer Willy Tarreau <w@1wt.eu> 1563182059 +0200
encoding latin1

BUG/MINOR: mux-pt: do not pretend there's more data after a read0

Commit 8706c8131 ("BUG/MEDIUM: mux_pt: Always set CS_FL_RCV_MORE.")
was a bit excessive in setting this flag, it refrained from removing
it after read0 unless it was on an empty call. The problem it causes
is that read0 is thus ignored on the first call :

  $ strace -tts200 -e trace=recvfrom,epoll_wait,sendto  ./haproxy -db -f tcp.cfg
  06:34:23.956897 recvfrom(9, "blah\n", 15360, 0, NULL, NULL) = 5
  06:34:23.956938 recvfrom(9, "", 15355, 0, NULL, NULL) = 0
  06:34:23.956958 recvfrom(9, "", 15355, 0, NULL, NULL) = 0
  06:34:23.957033 sendto(8, "blah\n", 5, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 5
  06:34:23.957229 epoll_wait(3, [{EPOLLIN|EPOLLHUP|EPOLLRDHUP, {u32=8, u64=8}}], 200, 0) = 1
  06:34:23.957297 recvfrom(8, "", 15360, 0, NULL, NULL) = 0

If CO_FL_SOCK_RD_SH is reported by the transport layer, it indicates the
read0 was already seen thus we must not try again and we must immedaitely
report it. The simple fix consists in removing the test on ret==0 :

  $ strace -tts200 -e trace=recvfrom,epoll_wait,sendto  ./haproxy -db -f tcp.cfg
  06:44:21.634835 recvfrom(9, "blah\n", 15360, 0, NULL, NULL) = 5
  06:44:21.635020 recvfrom(9, "", 15355, 0, NULL, NULL) = 0
  06:44:21.635056 sendto(8, "blah\n", 5, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 5
  06:44:21.635269 epoll_wait(3, [{EPOLLIN|EPOLLHUP|EPOLLRDHUP, {u32=8, u64=8}}], 200, 0) = 1
  06:44:21.635330 recvfrom(8, "", 15360, 0, NULL, NULL) = 0

The issue is minor, it only results in extra syscalls and CPU usage.
This fix should be backported to 2.0 and 1.9.

(cherry picked from commit 9cca8dfc0b96a2cbe35e789e2ff0b65aec78dbe6)
Signed-off-by: Willy Tarreau <w@1wt.eu>