Commits on Sep 19, 2023

1. Avoid accessing a null ClientConnection instance …

   ### Motivation
   
   We observed server null `ClientConnection` accesses in test environment.
   See the `this=0x0` outputs in the following two typical stacks.
   
   ```
   #8  bytesWritten (this=0xb8, size=371) at lib/SharedBuffer.h:166
   #9  pulsar::ClientConnection::handleRead (this=0x0, err=..., bytesTransferred=371, minReadSize=4) at lib/ClientConnection.cc:609
   ```
   
   ```
   #12 0x00007f33202933d2 in unique_lock (__m=..., this=0x7f3311c82800) at /opt/rh/devtoolset-7/root/usr/include/c++/7/bits/std_mutex.h:197
   #13 pulsar::ClientConnection::sendPendingCommands (this=0x0) at lib/ClientConnection.cc:1071
   #14 0x00007f3320293d2d in pulsar::ClientConnection::handleSendPair (this=0x0, err=...) at lib/ClientConnection.cc:1066
   ```
   
   Though `shared_from_this()` is always passed to the `std::bind`
   function, when the method of `ClientConnection` is called, the pointer
   is still `null`.
   
   ### Modifications
   
   First, replace all `std::bind` calls with the lambda expression that
   catches `std::weak_ptr<ClientConnection>` and perform null checks
   explicitly on the value returned by the `lock()` method.
   
   Since now all asio callbacks don't hold a `shared_ptr`, the owner of
   the `ClientConnection` object should be `ConnectionPool`, i.e. the pool
   maintains some connections, while all asio callbacks use `weak_ptr` to
   test if the connection is present.
   
   Second, make `ClientConnection::getConnection` return `shared_ptr`
   rather than `weak_ptr` so that the caller side does not need to check if
   `lock()` returns null in the callback of this future.
   
   We cannot make `ConnectionPool::getConnectionAsync` return `shared_ptr`
   because it could return the future of `connectPromise_`, which is hold
   by `ClientConnection` itself. We should avoid holding a `shared_ptr` of
   `ClientConnection` because its owner is `ConnectionPool`.

   

   BewareMyPower committed Sep 19, 2023
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