Commits on Jun 30, 2016

1. Fix a bug in the port selection for the nested Jupyter servers. …

   Previously, Datalab was picking the next port that it wanted Jupyter
   to run on, passing that to the launched Jupyter process using the
   '--port' flag, and then assuming that the server did in fact start
   listening on that port.
   
   However, if the port selected was already in use, then Jupyter would
   automatically try the next port in sequence. That could cause an issue
   where the one Jupyter server (for one user) was listening on a port
   that we thought was being used by the Jupyter server for a different
   user.
   
   This change fixes that bug by doing the following:
   
   1. Verifying that the next port is free before we try to start a
      Jupyter server listening on it.
   2. Telling the Jupyter server to not try any additional ports if
      the one specified was not available (so it will instead die in
      that situation).
   
   The combination of these two steps effectively moves the retry
   logic for picking a port out of the Jupyter process and in to the
   wrapping Node server.
   
   There are still two failure cases that can manifest even with this
   change.
   
   The first is that if the attempt to start a Jupyter server
   fails more than 10 times, then the retry logic will give up and
   an internal error will be reported to the user. This failure mode
   always existed, but now we will be properly tracking it.
   
   The second is a race condition where another process grabs the
   requested port between the time when we verify that it is free
   and the time that the launched Jupyter server starts up. If that
   happens, then the Jupyter server will kill itself, and it will
   be removed from the users->Jupyter servers map. However, the
   request that caused us to spin up a Jupyter server to begin
   with will be forwarded to the process that took hold of the
   assigned port.
   
   This failure case should be difficult enough to trigger that it
   will not occur in practice.

   

   ojarjur committed Jun 30, 2016
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