Prevent pinned CarrierThreads on JDK-21 while using Virtual Threads

[rjbaucells]

This PR replaces some of the synchronized() blocks with ReentrantLock equivalents to prevent pinned CarrierThreads when using JDK-21 Virtual Threads.

When a service instance is requested from a Virtual Thread and the service initialization runs a native method or foreign function, the JVM Carrier Thread is pinned preventing the execution of other Virtual Threads. See the following trace logged by using the -Djdk.tracePinnedThreads=full command line argument to the java command:

Thread[#18,ForkJoinPool-1-worker-2,5,CarrierThreads]
    java.base/java.lang.VirtualThread$VThreadContinuation.onPinned(VirtualThread.java:185)
    java.base/jdk.internal.vm.Continuation.onPinned0(Continuation.java:393)
    java.base/java.lang.VirtualThread.park(VirtualThread.java:592)
    java.base/java.lang.System$2.parkVirtualThread(System.java:2639)
    java.base/jdk.internal.misc.VirtualThreads.park(VirtualThreads.java:54)
    java.base/java.util.concurrent.locks.LockSupport.park(LockSupport.java:219)
    java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:754)
    java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:990)
    java.base/java.util.concurrent.locks.ReentrantLock$Sync.lock(ReentrantLock.java:153)
    java.base/java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:322)

    .. service initialization ...

    [email protected]/dagger.internal.DoubleCheck.get(DoubleCheck.java:47) <== monitors:1

After replacing the synchronized(monitor) {} by the ReentrantLock the CarrierThread is no longer pinned and the JVM no longer logs the above Error message.

See JDK related documentation