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nicktorwald 
Race condition in TarantoolClientImpl

A state of a client is a set of the following flags: {READING, WRITING,
RECONNECT, CLOSED}. Let's name a state when no flags are set
UNINITIALIZED.

A reader thread sets READING, performs reading until an error or an
interruption, drops READING and tries to trigger reconnection (if a
state allows, see below). A writer do quite same things, but with the
WRITING flag. The key point here is that a reconnection is triggered
from a reader/writer thread and only when certain conditions are met.

The prerequisite to set RECONNECT and signal (unpark) a connector thread
is that a client has UNINITIALIZED state.

There are several problems here:

- Say, a reader stalls a bit after dropping READING, then a writer drops
WRITING and trigger reconnection. Then reader wokes up and set RECONNECT
again.

- Calling unpark() N times for a connector thread when it is alive
doesn't lead to skipping next N park() calls, so the problem above is
not just about extra reconnection, but lead the connector thread to be
stuck.

- Say, a reader stalls just before setting READING. A writer is hit by
an IO error and triggers reconnection (set RECONNECT, unpark connector).
Then the reader wakes up and set READING+RECONNECT state that disallows
a connector thread to proceed further (it expects pure RECONNECT). Even
when the reader drops READING it will not wake up (unpark) the connector
thread, because RECONNECT was already set (state is not UNINITIALIZED).

This commit introduces several changes that eliminate the problems
above:

- Use ReentrantLock + Condition instead of park() / unpark() to never
miss signals to reconnect, does not matter whether a connector is
parked.

- Ensure a reader and a writer threads from one generation (that are
created on the same reconnection iteration) triggers reconnection once.

- Hold RECONNECT state most of time a connector works (while acquiring
a new socket, connecting and reading Tarantool greeting) and pre... (continued)

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