Motivation:
A recent change to DefaultThreadFactory modified it so that it is sticky
with respect to thread groups. In particular, this change made it so
that DefaultThreadFactory would hold on to the thread group that created
it, and then use that thread group to create threads.
This can have problematic semantics since it can lead to violations of a
tenet of thread groups that a thread can only modify threads in its own
thread group and descendant thread groups. With a sticky thread group, a
thread triggering the creation of a new thread via
DefaultThreadFactory#newThread will be modifying a thread from the
sticky thread group which will not necessarily be its own nor a
descendant thread group. When a security manager is in place that
enforces this requirement, these modifications are now impossible. This
is especially problematic in the context of Netty because certain global
singletons like GlobalEventExecutor will create a
DefaultThreadFactory. If all DefaultThreadFactory instances are sticky
about their thread groups, it means that submitting tasks to the
GlobalEventExecutor singleton can cause a thread to be created from the
DefaultThreadFactory sticky thread group, exactly the problem with
DefaultThreadFactory being sticky about thread groups. A similar problem
arises from the ThreadDeathWatcher.
Modifications:
This commit modifies DefaultThreadFactory so that a null thread group
can be set with the behavior that all threads created by such an
instance will inherit the default thread group (the thread group
provided by the security manager if there is one, otherwise the thread
group of the creating thread). The construction of the instances of
DefaultThreadFactory used by the GlobalEventExecutor singleton and
ThreadDeathWatcher are modified to use this behavior. Additionally, we
also modify the chained constructor invocations of the
DefaultThreadFactory that do not have a parameter to specify a thread
group to use the thread group from the security manager is available,
otherwise the creating thread's thread group. We also add unit tests
ensuring that all of this behavior is maintained.
Result:
It will be possible to have DefaultThreadFactory instances that are not
sticky about the thread group that led to their creation. Instead,
threads created by such a DefaultThreadFactory will inherit the default
thread group which will either be the thread group from the security
manager or the current thread's thread group.
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