BUG/MEDIUM: stream-int: completely detach connection on connect error

Tim Butler reported a troubling issue affecting all versions since 1.5.
When a connection error occurs and a retry is performed on the same server,
the server connection first goes into the turn-around state (SI_ST_TAR) for
one second. During this time, the client may speak and try to push some
data. The tests in place confirm that the stream interface is in a state
<= SI_ST_EST and that a connection exists, so all ingredients are present
to try to perform a send() to forward data. The send() cannot be performed
since the connection's control layer is marked as not ready, but the
polling flags are changed, and due to the remaining error flag present
on the connection, the polling on the FD is disabled in both directions.
But if this FD was reassigned to another connection in the mean time, it
is this FD which is disabled, and it causes a timeout on another connection.

A configuration allowing to reproduce the issue looks like this :

   listen test
        bind :8003
        server s1 127.0.0.1:8001    # this one should be closed

   listen victim
        bind :8002
        server s1 127.0.0.1:8000    # this one should respond slowly (~50ms)

Two parallel injections should be run with short time-outs (100ms). After
some time, some dead connections will appear in listener "victim" due to
their I/Os being disabled by some of the failed transfers on "test"
instance. These ones will only be flushed on time out. A dead connection
looks like this :

  > show sess 0x7dcb70
  0x7dcb70: [07/Aug/2016:08:58:40.120151] id=3771 proto=tcpv4 source=127.0.0.1:34682
    flags=0xce, conn_retries=3, srv_conn=0x7da020, pend_pos=(nil)
    frontend=victim (id=3 mode=tcp), listener=? (id=1) addr=127.0.0.1:8002
    backend=victim (id=3 mode=tcp) addr=127.0.0.1:37736
    server=s1 (id=1) addr=127.0.0.1:8000
    task=0x7dcaf8 (state=0x08 nice=0 calls=2 exp=<NEVER> age=30s)
    si[0]=0x7dcd68 (state=EST flags=0x08 endp0=CONN:0x7e2410 exp=<NEVER>, et=0x000)
    si[1]=0x7dcd88 (state=EST flags=0x18 endp1=CONN:0x7e0cd0 exp=<NEVER>, et=0x000)
    co0=0x7e2410 ctrl=tcpv4 xprt=RAW data=STRM target=LISTENER:0x7d9ea8
        flags=0x0020b306 fd=122 fd.state=25 fd.cache=0 updt=0
    co1=0x7e0cd0 ctrl=tcpv4 xprt=RAW data=STRM target=SERVER:0x7da020
        flags=0x0020b306 fd=93 fd.state=20 fd.cache=0 updt=0
    req=0x7dcb80 (f=0x848000 an=0x0 pipe=0 tofwd=-1 total=129)
        an_exp=<NEVER> rex=<NEVER> wex=<NEVER>
        buf=0x7893c0 data=0x7893d4 o=0 p=0 req.next=0 i=0 size=0
    res=0x7dcbc0 (f=0x80008000 an=0x0 pipe=0 tofwd=-1 total=0)
        an_exp=<NEVER> rex=<NEVER> wex=<NEVER>
        buf=0x7893c0 data=0x7893d4 o=0 p=0 rsp.next=0 i=0 size=0

The solution against this issue is to completely detach the connection upon
error instead of only performing a forced close.

This fix should be backported to 1.6 and 1.5.

Special thanks to Tim who did all the troubleshooting work and provided a
lot of traces allowing to find the root cause of this problem.

src/stream.c

@@ -555,7 +555,6 @@ static int sess_update_st_con_tcp(struct stream *s)
 	struct stream_interface *si = &s->si[1];
 	struct channel *req = &s->req;
 	struct channel *rep = &s->res;
-	struct connection *srv_conn = __objt_conn(si->end);
 
 	/* If we got an error, or if nothing happened and the connection timed
 	 * out, we must give up. The CER state handler will take care of retry
@@ -575,7 +574,7 @@ static int sess_update_st_con_tcp(struct stream *s)
 		si->exp   = TICK_ETERNITY;
 		si->state = SI_ST_CER;
 
-		conn_force_close(srv_conn);
+		si_release_endpoint(si);
 
 		if (si->err_type)
 			return 0;