This is an attempt to describe the VmBackup state machine using ASCII diagrams.

=== Main State Machine ===

The main state machine, implemented in stateMachine.c, works as follows:

         -------------------> IDLE <-----------|
         |                      |              |
         | b, e                 | a            |
         |                      |              |
         |        e            \/              |
   SCRIPT_ERROR <--------- SCRIPT_FREEZE       |
         ^                   | b'    |         |
         |      e           \/       |         |
    SYNC_ERROR <-- SYNC_FREEZE_WAIT  |         |
         | d                 | b     | b       |
         |        e         \/      \/         | b, e
    SYNC_ERROR <----------- SYNC_FREEZE        |
         ^                      |              |
         |                      | c            |
         |     e               \/              |
         |------------------ SYNC_THAW         |
                                |              |
                                | d            |
                               \/              |
                            SCRIPT_THAW --------

The transitions mean the following events / conditions:

a. vmbackup.start RPC
   b'. vmbackup operation is running in background (Linux specific)
b. vmbackup operation is finished
c. vmbackup.snapshotDone RPC
d. sync provider operation is finished
e. error condition: runtime error, or vmbackup.abort RPC

Sending a vmbackup.start RPC while the state machine is not IDLE causes an
error to be returned to the client, but the state machine is not changed.
Same behavior occurs for multiple vmbackup.snapshotDone RPCs. Multiple
vmbackup.abort messages are ignored.

Transitions to IDLE cause the backup operation to be finalized, and a "done"
event to be sent to the VMX. Transitions from IDLE cause a "reset" event to be
sent to the VMX.

