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Re: [libmicrohttpd] shutdown of listen socket does not work on solaris 1

From: Christian Grothoff
Subject: Re: [libmicrohttpd] shutdown of listen socket does not work on solaris 10
Date: Sun, 18 Sep 2011 00:15:47 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110818 Icedove/3.0.11

On 09/16/2011 01:53 PM, Frank Meier wrote:

I tried to use libmicrohttpd on solaris 10, and my simple programm only
starting and stopping the daemon (see below) did hang in.
after some investigation (see debug code) I found, that the shutdown
call in daemon.c fails, since the daemon socket is not a connected
fullduplex but a listening socket.

Ah, I see. Well, on *BSD and GNU/Linux (and AFAIK even W32), 'shutdown' of a listen socket causes a corresponding select/poll call to unblock. I didn't know Solaris disagreed here. I see where this comes from, but shutdown can be used to close a socket 'for reading' and on a listen socket, you read for incoming connections (see usage of listen sockets in select), so if I close a listen socket for reading, then that means that I won't be accepting on it anymore. At least, that's what most POSIX-implementations do...

this means the daemon thread keeps
hanging in select() and main hangs in pthread_join() (see pstack
output). problem is since the socket doesn't get closed, select does not
interrupt. After adding CLOSE (fd) after the point of SHUTDOWN () the
programm did work. It looks like solaris and linux behave differently
when shutdown is called on a listening socket. But on the other hand I'm
not sure if it is correct to call shutdown in the first place, since the
man page states "The shutdown() call causes all or part of a full-duplex
connection on the socket associated with sockfd to be shut down" and the
listening socket does not have a connection.

I think the socket should be closed with CLOSE before calling
pthread_join() and use SHUTDOWN only on connection sockets.

Well, that would seem to solve the problem, but would introduce a race. With 'close', another thread might open a file or socket and MHD's might then select/poll/accept on that unrelated socket. So 'close' is actually incorrect (admittedly, it is a "solution" that will work in at least 99.999% of all actual runs, but still not correct -- EBADF is not an errno value I like to see, ever).

Also, other solutions in this context, some involving pipes, signals for threads (pthread_kill) etc. were rejected because they are not supported on some platforms and/or are inefficient. 'shutdown' was specifically used because it seemed to be a clean way to avoid the problems with the various alternatives. Except, it seems, on Solaris. Yuck.

So I'm not sure I have a good answer for you at this point. Suggestions welcome.

Happy hacking!


regards, Frank

/*test programm*/
struct MHD_Daemon* mhdDaemon = 0;
mhdDaemon = MHD_start_daemon (MHD_USE_THREAD_PER_CONNECTION, mhdPort,
NULL, NULL, &answer_to_connection, NULL, MHD_OPTION_END);
if (mhdDaemon)
MHD_stop_daemon (mhdDaemon); // <-here it hangs
mhdDaemon = 0;

/*debug code in daemon.c*/
--- daemon/daemon.c (revision 16873)
+++ daemon/daemon.c (working copy)
@@ -2477,7 +2477,11 @@
daemon->worker_pool[i].shutdown = MHD_YES;
daemon->worker_pool[i].socket_fd = -1;
+ int res = SHUTDOWN (fd, SHUT_RDWR);
+ int tmpErrno = errno;
+ printf("shutdown res:%d %s\n", res, strerror(tmpErrno));
+ CLOSE (fd);

->output with debug code:
shutdown res:-1 Transport endpoint is not connected

/*pstack output -> damon thread is in select, main hangs in pthread join*/
$ pstack 16802
16802: ./test.bin
----------------- lwp# 1 / thread# 1 --------------------
feee9365 lwp_wait (2, 80478d8)
feee2c1f _thrp_join (2, 0, 8047918, 1) + 5a
feee2d9e pthread_join (2, 8047918, feeec7e0, fef9665d) + 2b
fef96777 MHD_stop_daemon (8060d40, 0, 0, fee63301) + 12b
0805080a main (1, 8047988, 8047990) + 6f
080506ac _start (1, 8047a88, 0, 8047a93, 8047abe, 8047b78) + 60
----------------- lwp# 2 / thread# 2 --------------------
feee8da5 pollsys (fed5ed50, 1, 0, 0)
fee93afa pselect (4, fed5ef30, fed5eeb0, fed5ee30, 0, 0) + 18e
fee93df0 select (4, fed5ef30, fed5eeb0, fed5ee30, 0, 0) + 82
fef95ba4 MHD_select (fec50200, fec50200, fed5efec, feee59b9, 8060d40, 0)
+ 130
fef96248 MHD_select_thread (8060d40) + 18
feee59b9 _thr_setup (fec50200) + 4e
feee5ca0 _lwp_start (fec50200, 0, 0, fed5eff8, feee5ca0, fec50200)

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