[PATCH] tcp_splice: Set OUT_WAIT_ flag whenever pipe isn't emptied

[Stefano Brivio <sbrivio@redhat.com>]

In tcp_splice_sock_handler(), if we get EAGAIN on the second splice(),
from pipe to receiving socket, that doesn't necessarily mean that the
pipe is empty: the receiver buffer might be full instead.

Hence, we can't use the 'never_read' flag to decide that there's
nothing to wait for: even if we didn't read anything from the sending
side in a given iteration, we might still have data to send in the
pipe. Use read/written counters, instead.

This fixes an issue where large bulk transfers would occasionally
hang. From a corresponding strace:

     0.000061 epoll_wait(4, [{events=EPOLLOUT, data={u32=29442, u64=12884931330}}], 8, 1000) = 1
     0.005003 epoll_ctl(4, EPOLL_CTL_MOD, 211, {events=EPOLLIN|EPOLLRDHUP, data={u32=54018, u64=8589988610}}) = 0
     0.000089 epoll_ctl(4, EPOLL_CTL_MOD, 115, {events=EPOLLIN|EPOLLRDHUP, data={u32=29442, u64=12884931330}}) = 0
     0.000081 splice(211, NULL, 151, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
     0.000073 splice(150, NULL, 115, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = 1048576
     0.000087 splice(211, NULL, 151, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
     0.000045 splice(150, NULL, 115, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = 520415
     0.000060 splice(211, NULL, 151, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
     0.000044 splice(150, NULL, 115, NULL, 1048576, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = -1 EAGAIN (Resource temporarily unavailable)
     0.000044 epoll_wait(4, [], 8, 1000) = 0

we're reading from socket 211 into to the pipe end numbered 151,
which connects to pipe end 150, and from there we're writing into
socket 115.

We initially drop EPOLLOUT from the set of monitored flags for socket
115, because it already signaled it's ready for output. Then we read
nothing from socket 211 (the sender had nothing to send), and we keep
emptying the pipe into socket 115 (first 1048576 bytes, then 520415
bytes).

This call of tcp_splice_sock_handler() ends with EAGAIN on the writing
side, and we just exit this function without setting the OUT_WAIT_1
flag (and, in turn, EPOLLOUT for socket 115). However, it turns out,
the pipe wasn't actually emptied, and while socket 211 had nothing
more to send, we should have waited on socket 115 to be ready for
output again.

As a further step, we could consider not clearing EPOLLOUT at all,
unless the read/written counters match, but I'm first trying to fix
this ugly issue with a minimal patch.

Link: https://github.com/containers/podman/issues/22575
Link: https://github.com/containers/podman/issues/22593
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp_splice.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tcp_splice.c b/tcp_splice.c
index 42b7be0..4c36b72 100644
--- a/tcp_splice.c
+++ b/tcp_splice.c
@@ -616,7 +616,7 @@ eintr:
 			if (errno != EAGAIN)
 				goto close;
 
-			if (never_read)
+			if (conn->read[fromside] == conn->written[fromside])
 				break;
 
 			conn_event(c, conn,
-- 
@@ -616,7 +616,7 @@ eintr:
 			if (errno != EAGAIN)
 				goto close;
 
-			if (never_read)
+			if (conn->read[fromside] == conn->written[fromside])
 				break;
 
 			conn_event(c, conn,
-- 
2.43.0