Description of problem:

While debugging bug 1664159, a few other fixes related to client connection handling were added, in addition to the new debugging capabilties that were added to find the problem.  This bug covers those other fixes and improvements,
and since these are the only sanlock changes upstream we can take them all with a rebase.


commit 4c3924eb226d345cfc2da3baa81c68566f2c8bbb
Author: David Teigland <teigland>
Date:   Mon Mar 9 14:52:03 2020 -0500

    sanlock: clear revents in client_free
    
    By clearing poll revents sanlock won't receive
    or process anything more from the client.
    After calling client_free() from the main loop,
    we will not have to process a poll error for
    that client and call client_free() again.

commit b9cd4651a738bab65102fdc50c7067080dd5ce45
Author: David Teigland <teigland>
Date:   Tue Mar 10 12:20:27 2020 -0500

    sanlock: per command debug logging
    
    Allow debugging logging for specific commands to be
    enabled or disabled at the command processing level.
    
    In sanlock.conf:
    
    debug_cmd = +<name>
    enables cmd level debugging for the named command.
    
    debug_cmd = -<name>
    disables cmd level debugging for the named command.
    
    debug_cmd lines can be repeated, once for each name.
    
    By default sanlock disables some cmds that are often
    repetitive and fill the in-memory debug buffer.

commit f9b09264a4857afc5b10db8b0f157676848de926
Author: David Teigland <teigland>
Date:   Mon Mar 9 15:56:05 2020 -0500

    sanlock: client debug logging
    
    New debug logging for client connections, enabled with
    debug_clients=1 in sanlock.conf.

commit d384a9cbc278756523ae59dd12131df78859a518
Author: David Teigland <teigland>
Date:   Mon Mar 9 15:21:35 2020 -0500

    sanlock: clean up poll event checks for efd
    
    Never look at clients array when checking efd.
    (No evidence of any problems from this.)

commit fd5ec4c01b4796abe9153aa45b2c26fbacc1ab2a
Author: David Teigland <teigland>
Date:   Mon Mar 9 12:34:54 2020 -0500

    sanlock: fix connection processing error path
    
    Don't close the connection fd in process_cmd_thread_unregistered
    error path, avoiding possible problems like that fixed in
    62ba5d194100f71cbed32214e0aeb76f690f4093.
    Handle it like process_cmd_thread_registered by receiving any
    data, sending an error back to the client, and resume the client.
    
    Call client_free() in the following cases to close
    the client connection instead of doing nothing and
    waiting for a poll error on the connection to clean
    it up later:
    - when recv() returns zero
    - when client_suspend() finds an error
    - when receiving an unknown cmd
    
    These error paths are not exercised by normal usage.




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