Bad thread-interleaving occurs, as a result test failure happens

[Alisha-0321]

Commit version- aad6654

Describe the bug
It is possible to find some bad interleaving where one thread is writing from a synchronized block and another thread is reading the same variable from another method which is not synchronized. Here, flushandclosestate is a shared variable which is updated from a synchronized block at Line 596, but another thread uses flushandclosestate at Line 615 which is not synchronized. I can observe such an execution results in the assertion fail from running an existing test org.java_websocket.issues.Issue256Test with some added delay just before Line 596 of WebSocketImpl, resulting in the following stack trace.

Debug log
Tests run: 20, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 1.823 sec <<< FAILURE!
runReconnectSocketClose6 Time elapsed: 1.013 sec <<< FAILURE!
java.lang.AssertionError: Found 1 zombie thread(s)
at org.junit.Assert.fail(Assert.java:88)
at org.java_websocket.util.ThreadCheck.checkZombies(ThreadCheck.java:74)
at org.java_websocket.util.ThreadCheck.after(ThreadCheck.java:53)
at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:50)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:252)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:141)
at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:112)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:189)
at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:165)
at org.apache.maven.surefire.booter.ProviderFactory.invokeProvider(ProviderFactory.java:85)
at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:115)
at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:75)

Results :

Tests run: 20, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 1.823 sec <<< FAILURE!
runReconnectSocketClose6 Time elapsed: 1.013 sec <<< FAILURE!
java.lang.AssertionError: Found 1 zombie thread(s)

Tests in error:
Failed tests: runReconnectSocketClose6: Found 1 zombie thread(s)
Tests run: 20, Failures: 1, Errors: 0, Skipped: 0

To Reproduce
Steps to reproduce the behavior:

Add Thread.sleep(100) before Line 596
Run mvn test -Dtest=org.java_websocket.issues.Issue256Test -Dcheckstyle.skip

Expected behavior
Test failure should not occur

Additional context
When I investigated the WebSocketImpl class, I find flushandclosestate is a shared variable that is used on Line 615 and Line 596. By injecting a delay before Line 596 of WebSocketImpl.java, I could get a different thread interleaving where the variable flushandclosestate was not set to true when other threads were trying to close connection at Line 615. As a result, the assertion fails.

Environment(please complete the following information):
I ran the test on an Ubuntu 20.04 LTS machine using OpenJDK 1.8.0_312.