make lock-file location in CrossProcessMutex.h configurable

[frankseide]

CrossProcessMutex.h uses a file for a global lock inside /var/lock. That location cannot be universally be assumed to be writeable in all environments for all users. It should be changed to some place that is, or changeable through a config variable.

CrossProcessMutex(const std::string& name)
    : m_fd(-1),
      m_fileName("/var/lock/" + name)
{
}

[frankseide]

We probably also want to change the lock file name to something without DBN.exe in the name, as that was meant to allow coexistence of CNTK on a single server with an old internal Windows-only tool that is no longer in use.

[nslay]

How about using an environment variable?

CrossProcessMutex(const std::string& name)
    : m_fd(-1)
{
  const char * const envLockDir = getenv("CNTK_LOCK_DIR");
  if (envLockDir != NULL)
    m_fileName = envLockDir + ('/' + name);
  else
    m_fileName = "/var/lock/" + name;
}

If you insist on a setting in the configuration file, that could simply use putenv to expose this variable to CrossProcessMutex.

EDIT:
I personally would like an environment variable since write-access local storage may depend on a job ID or similar. This would save me the trouble having to sed/awk the config file to update a setting. Instead I can export a variable. See for example:
https://hpc.nih.gov/docs/b2-userguide.html#local

[wolfma61]

This should have stayed open - this is a PR in process

[timolohrenz]

even after the latest news, this would be a mandatory feature for all users which have no access to /var/lock.

As far as I can see, the solution with the environment variable never made its way into a release. Maybe reopen?