Tcl Library Source Code

::websocket server socket is set to *blocking* mode.  As a
consequence, misbehaving clients can completely block the server if they
refuse to read or write a complete message.

The following recipe (*nix only) reproduces the issue.  Two shells are
needed, one to run a ::websocket server, the other to create a
misbehaving client.

In the first shell run the Tcl script below.  The script creates a
WebSocket server (handshake ommited) and prints a message at regular
intervals; this message will stop being printed when the client blocks
the server.

---------------->8----------------
package require websocket;
namespace eval ::s {};
proc ::s::accept {sock args} {
    puts "server: new connection $args";
    ::websocket::takeover $sock [list ::s::handler] 1 {};
}
proc ::s::handler {sock type args} {
    puts "$sock: $type $args";
}
proc ::s::canary {interval} {
    puts "canary: Still alive...";
    ::after $interval [list ::s::canary $interval];
}

::s::canary 1000;
::socket -server [list ::s::accept] 54321;
::vwait forever;
----------------8<----------------

In the second shell run the following command:

---------------->8----------------
    { echo -n 1; sleep 5s; } | nc localhost 54321
----------------8<----------------

The server is blocked for 5 seconds.