Intermittent failure in test_multi_join()

[tarka]

The test test_multi_join() will intermittently fail, although it can take a large number of runs to see this. Running the following script:

#!/bin/bash
set -e

for i in `seq 5000`; do
    echo ==================== $i ====================
    cargo test -- multi_join;
done

will eventually produce the following error:

running 1 test
test test::test_multi_join ... FAILED

failures:

---- test::test_multi_join stdout ----
thread 'test::test_multi_join' panicked at src/lib.rs:1111:9:
assertion `left == right` failed
  left: Ok(0)
 right: Err(Empty)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test::test_multi_join

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 20 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`

[tarka]

Test output with the debugging uncommented:

running 1 test
p0: 0
p0: 1
p0: 2
p0: 3
p0: 4
p0: 5
p0: 6
p0: 7
p1: 0 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(0)
p1: 1 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(1)
p1: 2 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(2)
p1: 3 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(3)
p1: 4 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: 5 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(4)
p1: send(5)
p1: 6 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: 7 -=- ThreadPool { name: Some("multi join pool0"), queued_count: 0, active_count: 0, max_count: 4 }
p1: send(6)
p1: send(7)
test test::test_multi_join ... FAILED

failures:

---- test::test_multi_join stdout ----
thread 'test::test_multi_join' panicked at src/lib.rs:1111:9:
assertion `left == right` failed
  left: Ok(0)
 right: Err(Empty)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test::test_multi_join

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 20 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`