Multithreaded elasticsearch get operations about swallow

[fabiolab]

As a clue.
A note from the elasticsearch python driver explains that using "fork()" may make things become that weird :

Is it at all possible that it is caused by a fork()? It might happen, if you have a connection open (which we do - we keep them open to be able to re use them) and you call fork() (or your container - uWSGI or celery - does) then both new processes share the same file descriptor and things might get weird.

Thus, Swallow multithreading system relies on the multiprocessing module, whom default mode is based on "fork()" :

     Depending on the platform, multiprocessing supports three ways to start a process. These start methods are

spawn

    The parent process starts a fresh python interpreter process. The child process will only inherit those resources necessary to run the process objects run() method. In particular, unnecessary file descriptors and handles from the parent process will not be inherited. Starting a process using this method is rather slow compared to using fork or forkserver.

    Available on Unix and Windows. The default on Windows.
fork

    The parent process uses os.fork() to fork the Python interpreter. The child process, when it begins, is effectively identical to the parent process. All resources of the parent are inherited by the child process. Note that safely forking a multithreaded process is problematic.

    Available on Unix only. The default on Unix.
forkserver

    When the program starts and selects the forkserver start method, a server process is started. From then on, whenever a new process is needed, the parent process connects to the server and requests that it fork a new process. The fork server process is single threaded so it is safe for it to use os.fork(). No unnecessary resources are inherited.

    Available on Unix platforms which support passing file descriptors over Unix pipes.

A solution may be to swap from fork to forkserver mode.

[fabiolab]

Done in 1.5.0