Bug in yaws_revproxy

[Colm Dougan]

Hi,

I was running ab (apache bench) on the yaws reverse proxy module and
got a crash which leaves the whole server wedged.  The crash report is
below.

My theory is that the problem is related to this section in yaws_revproxy :

            P1 = proc_lib:spawn_link(?MODULE, ploop, [Cli2, Srv, GC,
SC, self()]),
            ?Debug("Client=~p, Srv=~p", [P1, self()]),
            ploop(Srv, Cli2, GC, SC, P1);

I can see that it is the process spawned with proc_lib which is the
one hitting the crash.  My assumption is that this process has been
spawned to handle further data coming from the client (e.g. for
keep-alive).  Is that correct?  Anyway,  my theory is that it is a
race condition of sorts and that the server has already sent a
response which is handled by the inline ploop and the connection is
closed *before* the spawned ploop gets a chance to be scheduled in and
so when it tries to operate on the socket it gets errors.

I have attached a patch which catches the problem.  I'm not claiming
that it is the ideal solution but I can confirm that it stops the
server getting wedged by the error case.

Colm

=CRASH REPORT==== 30-Oct-2009::15:14:56 ===
  crasher:
    initial call: yaws_revproxy:ploop/5
    pid: <0.74.0>
    registered_name: []
    exception error: no match of right hand side value {error,einval}
      in function  yaws:setopts/3
      in call from yaws:do_http_get_headers/2
      in call from yaws:http_get_headers/2
      in call from yaws_revproxy:ploop/3
    ancestors: [<0.57.0>,<0.56.0>,yaws_server,yaws_sup,<0.45.0>]
    messages: []
    links: [<0.57.0>]
    dictionary: [{sc,{sconf,8000,3,[],undefined,undefined,
                             "/home/tobbe/docroot",[],
                             {0,0,0,0},
                             "sej",16401,undefined,[],nolimit,[],yaws_outmod,
                             yaws_outmod,yaws_outmod,yaws,[],undefined,
                             [yaws,php,cgi,fcgi],
                             [],
                             [{"/",
                               {url,http,"172.16.62.84",undefined,"/",[]}}],
                             [],[],undefined,undefined,undefined,
                             [{"ys",yaws_ext_handler_yaws}]}},
                  {ssl,nossl},
                  {gc,{gconf,"/root/devel/yaws-1.85",false,204,".",
                             ["/root/devel/yaws-1.85/examples/ebin"],
                             [],15000,400,1000000,8000,nolimit,10240,[],
                             10000000,30,
                             ["/root/devel/yaws-1.85/examples/include"],
                             "/usr/bin/php-cgi","Yaws 1.85","default",false}}]
    trap_exit: false
    status: running
    heap_size: 1597
    stack_size: 24
    reductions: 286
  neighbours:
    neighbour: [{pid,<0.57.0>},
                  {registered_name,[]},
                  {initial_call,
                      {yaws_server,acceptor0,['Argument__1','Argument__2']}},
                  {current_function,{yaws_server,acceptor0,2}},
                  {ancestors,[<0.56.0>,yaws_server,yaws_sup,<0.45.0>]},
                  {messages,[]},
                  {links,[<0.56.0>,<0.74.0>,#Port<0.749>]},
                  {dictionary,
                      [{gc,{gconf,"/root/devel/yaws-1.85",false,204,".",
                               ["/root/devel/yaws-1.85/examples/ebin"],
                               [],15000,400,1000000,8000,nolimit,10240,[],
                               10000000,30,
                               ["/root/devel/yaws-1.85/examples/include"],
                               "/usr/bin/php-cgi","Yaws 1.85","default",
                               false}}]},
                  {trap_exit,false},
                  {status,waiting},
                  {heap_size,1597},
                  {stack_size,8},
                  {reductions,2353}]


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