Implement liveness check for notebook extensions

Currently, each TensorBoard process writes its meta-information to a file in the shared .tensorboard-info temp directory, and tries to clean up the file on graceful exit. This has two problems on Windows:

• The base temporary directory %TMP% is never automatically cleaned, even after logout or reboot.
• It is not possible to gracefully shut down an arbitrary process given its PID.

The result is that most any TensorBoards started by %tensorboard will leave their info files around forever, unless manually cleaned up, and the instructions suggested to the user (“use !kill …”) are not adequate to effect this cleanup. See #2481.

We can ameliorate this by implementing the liveness check mentioned in a TODO in manager.py, cleaning up the dead info file on failure:

https://github.com/tensorflow/tensorboard/blob/c1c2771a5ed5ce1b9ad605dd1834616905c2f190/tensorboard/manager.py#L453-L458

We can also provide a notebook.kill(pid) function with implementation something vaguely like

def kill(pid):
  if os.name == "nt":
    subprocess.check_output(["taskkill", "/pid", str(int(pid)), "/f"])
    manager.remove_info_file(pid)
  else:
    os.kill(pid, signal.SIGTERM)

and then replace the “reusing TensorBoard” user-facing messaging on Windows with something like

template = (
    "Reusing TensorBoard on port {port} (pid {pid}), started {delta} ago. "
    "To kill it, run `from tensorboard import notebook; notebook.kill({pid})`."
)

though that incantation still is a bit of a mouthful.

[rmcrowley2000]

For those on Windows trying to do this from Jupyter, the following two commands will automate the process. You can just toss them in a cell and run them after you are done with the open TensorBoard instance (or before you need to check a different log file).

!taskkill /IM "tensorboard.exe" /F
!rmdir /S /Q %temp%\.tensorboard-info

[mthomp89]

Same issues: TensorBoard is not reliable within Jupyter. Based on this thread I did the following with marginal success. TensorBoard was good for only one time after the cleanup.

"taskkill /im tensorboard.exe /f" to kill all live pids in command
deleted all the pid-xxxx.info files in the "%TMP%.tensorboard-info" directory.
deleted the whole "%TMP%.tensorboard-info" directly