Intermittent crash in /websockets/Create-blocked-port.htm (too many open files)

[jdm]

  ▶ CRASH [expected OK] /websockets/Create-blocked-port.htm
  │ 
  │ VMware, Inc.
  │ softpipe
  │ 3.3 (Core Profile) Mesa 17.3.0-devel
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:389/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:465/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:514/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:526/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ 2018-02-07 21:17:37.186 servo[20645:505960784] Metadata.framework [Error]: couldn't get the client port
  │ SCAN: opendir [/Users/servo/buildbot/slave/mac-nightly/build/target/release] failed with errno 24 (Too many open files)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:556/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:563/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:587/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:601/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:636/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:993/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:995/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:2049/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:3659/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:4045/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6000/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6665/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6666/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6667/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6668/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  │ could not initialize thread_rng: Too many open files (os error 24) (thread WebSocket connection to ws://web-platform.test:6669/echo, at /Users/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.3.15/src/lib.rs:894)
  └ ERROR:layout_thread: Failed to load UA stylesheet user-agent.css!