Fixes subtle bug in the case of multiple Dispose calls

adams85 commented 1 year ago

Describe the purpose of your pull request

Consider the following pattern:

// 1. Creates a new instance of ConfigCatClient under the hood
var client1 = ConfigCatClient.Get("<KEY>");

// 2. Removes the previously created instance from the client cache
client1.Dispose();

// 3. Creates another instance of ConfigCatClient with the *same* key
var client2 = ConfigCatClient.Get("<KEY>");

// 4. Dispose can be called multiple times (it's not prohibited) but it should have no effect.
// However, if the Dispose logic just blindly removes the current cached instance by key,
// it will remove the other instance (client2), which would mess up the logic of shared instances.
// That is, we need to safeguard against this by making Dispose truly idempotent!
client1.Dispose();

A possible way of fixing this is presented in this PR. Another solution could be a disposed flag.