Motivation:
Doc of ChannelGroup says, that it can be used to manage server and child channels at once.
However, in DefaultChannelGroup, there is a race condition. When a server channel accepts a child, it schedules its
registration on an event loop, which takes some time. If the ChannelGroup, which is supposed
to close server and child channels at once, is closed after the child channel has been scheduled
for registration and before this registration actually happens, this child channel is not closed
and remains connected. This could lead to connection leaks.
Modifications:
To fix this, the DefaultChannelGroup is changed to has a closed flag.
This flag is set to true, just before the close() method is actually closing channels.
The add() method checks after adding a new channel, if this flag has been set to true.
If yes, the new channel is closed. If not, we have the guarantee, that this channel will be
closed by the ChannelGroup, because setting the closed flag to true happens-before closing any channels.
This behaviour can be activated by two new constructors. The old constructors are still there and behave like before.
Therefore, no existing code should be affected directly.
Result:
If activating this feature, the DefaultChannelGroup can be used, for managing server and child channels at once.
But this activating this feature means also, that a ChannelGroup cannot be reused after calling close().
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