Infinite hang when channel incompletely read

[jyasskin]

The following program hangs forever in trying to destroy the to_child chan, because it's waiting for the queue to be emptied by the child, which has completed and will never do so.

io fn incomplete_read(chan[chan[str]] for_incoming_chan) {
  let port[str] incoming = port();
  for_incoming_chan <| chan(incoming);
  auto read_one <- incoming;
  // Never read the second string.
}

io fn main() {
  let port[chan[str]] get_to_child = port();
  auto child = spawn incomplete_read(chan(get_to_child));
  let chan[str] to_child <- get_to_child;
  to_child <| "str 1";
  to_child <| "never read";
}

[froystig]

Somewhat relevant: a similar race is commented about (and worked-around) in test/run-pass/obj-dtor.rs.

[graydon]

Fixed long ago. Now completes with:

rt: warning: '_unread == 0', at: rt/circular_buffer.cpp:29 freeing circular_buffer with 4 unread bytes