Motivation:
SslHandler must generate control data as part of the TLS protocol, for example
to do handshakes. SslHandler doesn't capture the status of the future
corresponding to the writes when writing this control (aka non-application
data). If there is another handler before the SslHandler that wants to fail
these writes the SslHandler will not detect the failure and we must wait until
the handshake timeout to detect a failure.
Modifications:
- SslHandler should detect if non application writes fail, tear down the
channel, and clean up any pending state.
Result:
SslHandler detects non application write failures and cleans up immediately.
Motivation:
How we tried to detect if KeyManagerFactory is supported was not good enough for OpenSSL 1.1.0+ as it partly provided the API but not all of what is required.
This then lead to failures like:
[ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 1.102 s <<< FAILURE! - in io.netty.channel.epoll.EpollDomainSocketStartTlsTest
[ERROR] initializationError(io.netty.channel.epoll.EpollDomainSocketStartTlsTest) Time elapsed: 0.016 s <<< ERROR!
javax.net.ssl.SSLException: failed to set certificate and key
at io.netty.handler.ssl.ReferenceCountedOpenSslServerContext.newSessionContext(ReferenceCountedOpenSslServerContext.java:130)
at io.netty.handler.ssl.OpenSslServerContext.<init>(OpenSslServerContext.java:353)
at io.netty.handler.ssl.OpenSslServerContext.<init>(OpenSslServerContext.java:334)
at io.netty.handler.ssl.SslContext.newServerContextInternal(SslContext.java:468)
at io.netty.handler.ssl.SslContextBuilder.build(SslContextBuilder.java:457)
at io.netty.testsuite.transport.socket.SocketStartTlsTest.data(SocketStartTlsTest.java:93)
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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.runners.Parameterized.allParameters(Parameterized.java:280)
at org.junit.runners.Parameterized.<init>(Parameterized.java:248)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at org.junit.internal.builders.AnnotatedBuilder.buildRunner(AnnotatedBuilder.java:104)
at org.junit.internal.builders.AnnotatedBuilder.runnerForClass(AnnotatedBuilder.java:86)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:26)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at org.junit.internal.requests.ClassRequest.getRunner(ClassRequest.java:33)
at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:362)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:273)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:238)
at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:159)
at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:384)
at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:345)
at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:126)
at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:418)
Caused by: java.lang.Exception: Requires OpenSSL 1.0.2+
at io.netty.internal.tcnative.SSLContext.setCertificateCallback(Native Method)
at io.netty.handler.ssl.ReferenceCountedOpenSslServerContext.newSessionContext(ReferenceCountedOpenSslServerContext.java:126)
... 32 more
Modifications:
Also try to set the certification callback and only if this works as well mark KeyManagerFactory support as enabled.
Result:
Also correctly work when OpenSSL 1.1.0 is used.
Motivation:
Usafe of io.netty.handler.ssl.openssl.useKeyManagerFactory system property was deprecated in 4.1 so let us remove it.
Modifications:
Remove io.netty.handler.ssl.openssl.useKeyManagerFactory usage.
Result:
Remove support of deprecated system property
Motivation:
OOME is occurred by increasing suppressedExceptions because other libraries call Throwable#addSuppressed. As we have no control over what other libraries do we need to ensure this can not lead to OOME.
Modifications:
Only use static instances of the Exceptions if we can either dissable addSuppressed or we run on java6.
Result:
Not possible to OOME because of addSuppressed. Fixes https://github.com/netty/netty/issues/9151.
Motivation:
Previously, any 'relative' pipeline operations, such as
ctx.pipeline().replace(), .addBefore(), addAfter(), etc
would fail as the handler was not present in the pipeline.
Modification:
Used the pattern from ChannelInitializer when invoking configurePipeline().
Result:
Fixes#9131
Motivation:
As brought up in https://github.com/netty/netty/issues/8998, JKS can be substantially faster than pkcs12, JDK's new default. Without an option to set the KeyStore type you must change the configuration of the entire JVM which is impractical.
Modification:
- Allow to specify KeyStore type
- Add test case
Result:
Fixes https://github.com/netty/netty/issues/8998.
Motivation:
Bootstrap allows you to set a localAddress for outbound TCP connections, either via the Bootstrap.localAddress(localAddress) or Bootstrap.connect(remoteAddress, localAddress) methods. This works well if you want to bind to just one IP address on an interface. Sometimes you want to bind to a specific address based on the resolved remote address which should be possible.
Modifications:
Add DynamicAddressConnectHandler and tests
Result:
Fixes https://github.com/netty/netty/issues/8940.
Motivation:
Subclasses of `OpenSslKeyMaterial` implement `ReferenceCounted`. This means that a new object should have an initial refcount of 1. An `OpenSslPrivateKey.OpenSslPrivateKeyMaterial` object shares its refcount with the enclosing `OpenSslPrivateKey` object. This means the enclosing object's refcount must be incremented by 1 when an instance of `OpenSslPrivateKey.OpenSslPrivateKeyMaterial` is created. Otherwise, when the key material object is `release()`-ed, the refcount on the enclosing object will drop to 0 while it is still in use.
Modification:
- Increment the refcount in the constructor of `OpenSslPrivateKey.OpenSslPrivateKeyMaterial`
- Ensure we also always release the native certificates as well.
Result:
Refcount is now correct.
Motivation:
While OpenSslPrivateKeyMethod.* should never return null we should still guard against it to prevent any possible segfault.
Modifications:
- Throw SignatureException if null is returned
- Add unit test
Result:
No segfault when user returns null.
Motivation:
Seems like some analyzer / validation tools scan code to detect if it may produce some security risk because of just blindly accept certificates. Such a tool did tag our code because we have such an implementation (which then is actually never be used). We should just change the impl to not do this as it does not matter for us and it makes such tools happier.
Modifications:
Throw CertificateException
Result:
Fixes https://github.com/netty/netty/issues/9032
Motivation:
IdleStateHandler may trigger unexpected idle events when flushing large entries to slow clients.
Modification:
In netty design, we check the identity hash code and total pending write bytes of the current flush entry to determine whether there is a change in output. But if a large entry has been flushing slowly (for some reason, the network speed is slow, or the client processing speed is too slow to cause the TCP sliding window to be zero), the total pending write bytes size and identity hash code would remain unchanged.
Avoid this issue by adding checks for the current entry flush progress.
Result:
Fixes#8912 .
Motivation:
BoringSSL allows to customize the way how key signing is done an even offload it from the IO thread. We should provide a way to plugin an own implementation when BoringSSL is used.
Modifications:
- Introduce OpenSslPrivateKeyMethod that can be used by the user to implement custom signing by using ReferenceCountedOpenSslContext.setPrivateKeyMethod(...)
- Introduce static methods to OpenSslKeyManagerFactory which allows to create a KeyManagerFactory which supports to do keyless operations by let the use handle everything in OpenSslPrivateKeyMethod.
- Add testcase which verifies that everything works as expected
Result:
A user is able to customize the way how keys are signed.
Motivation:
During OpenSsl.java initialization, a SelfSignedCertificate is created
during the static initialization block to determine if OpenSsl
can be used.
The default key strength for SelfSignedCertificate was too low if FIPS
mode is used and BouncyCastle-FIPS is the only available provider
(necessary for compliance). A simple fix is to just augment the key
strength to the minimum required about by FIPS.
Modification:
Set default key bit length to 2048 but also allow it to be dynamically set via a system property for future proofing to more stricter security compliance.
Result:
Fixes#9018
Signed-off-by: Farid Zakaria <farid.m.zakaria@gmail.com>
Motivation:
Some SslProvider do support different types of keys and chains. We should fail fast if we can not support the type.
Related to https://github.com/netty/netty-tcnative/issues/455.
Modifications:
- Try to parse key / chain first and if if this fails throw and SslException
- Add tests.
Result:
Fail fast.
Motivation:
A callback may already have stored a initial handshake exception in ReferenceCountedOpenSslEngine so we should include it when throwing a SslHandshakeException to ensure the user has all the infos when debugging.
Modifications:
Include initial handshake exception
Result:
Include all erros when throwing the SslHandshakeException.
Motivation:
DefaultPromise requires an EventExecutor which provides the thread to notify listeners on and this EventExecutor can never change. We can remove the code that supported the possibility of a changing the executor as this is not possible anymore.
Modifications:
- Remove constructor which allowed to construct a *Promise without an EventExecutor
- Remove extra state
- Adjusted SslHandler and ProxyHandler for new code
Result:
Fixes https://github.com/netty/netty/issues/8517.
Motivation:
We do not need to call SSL.setHostNameValidation(...) as it should be done as part of the TrustManager implementation. This is consistent with the JDK implementation of SSLEngine.
Modifications:
Remove call to SSL.setHostNameValidation(...)
Result:
More consistent behaviour between our SSLEngine implementation and the one that comes with the JDK.
Motivation:
We use SSL.setKeyMaterial(...) in our implementation when using the KeyManagerFactory so we should also use it to detect if we can support KeyManagerFactory.
Modifications:
Use SSL.setKeyMaterial(...) as replacement for SSL.setCertificateBio(...)
Result:
Use the same method call to detect if KeyManagerFactory can be supported as we use in the real implementation.
Motivation:
In 42742e233f we already added default methods to Channel*Handler and deprecated the Adapter classes to simplify the class hierarchy. With this change we go even further and merge everything into just ChannelHandler. This simplifies things even more in terms of class-hierarchy.
Modifications:
- Merge ChannelInboundHandler | ChannelOutboundHandler into ChannelHandler
- Adjust code to just use ChannelHandler
- Deprecate old interfaces.
Result:
Cleaner and simpler code in terms of class-hierarchy.
Motivation:
We have multiple places where we store the exception that was produced by a callback in ReferenceCountedOpenSslEngine, and so have a lot of code-duplication.
Modifications:
- Consolidate code into a package-private method that is called from the callbacks if needed
Result:
Less code-duplication and cleaner code.
Motivation:
BoringSSL supports offloading certificate validation to a different thread. This is useful as it may need to do blocking operations and so may block the EventLoop.
Modification:
- Adjust ReferenceCountedOpenSslEngine to correctly handle offloaded certificate validation (just as we already have code for certificate selection).
Result:
Be able to offload certificate validation when using BoringSSL.
Motivation:
When the verification of the server cert fails because of the used TrustManager on the client-side we need to ensure we produce the correct alert and send it to the remote peer before closing the connection.
Modifications:
- Use the correct verification mode on the client-side by default.
- Update tests
Result:
Fixes https://github.com/netty/netty/issues/8942.
Motivation:
As we now us java8 as minimum java version we can deprecate ChannelInboundHandlerAdapter / ChannelOutboundHandlerAdapter and just move the default implementations into the interfaces. This makes things a bit more flexible for the end-user and also simplifies the class-hierarchy.
Modifications:
- Mark ChannelInboundHandlerAdapter and ChannelOutboundHandlerAdapter as deprecated
- Add default implementations to ChannelInboundHandler / ChannelOutboundHandler
- Refactor our code to not use ChannelInboundHandlerAdapter / ChannelOutboundHandlerAdapter anymore
Result:
Cleanup class-hierarchy and make things a bit more flexible.
Motivation:
The SSLSession that is returned by SSLEngine.getHandshakeSession() must be able to provide the local certificates when the TrustManager is invoked on the server-side.
Modifications:
- Correctly return the local certificates
- Add unit test
Result:
Be able to obtain local certificates from handshake SSLSession during verification on the server side.
Motivation:
SSLEngine API has a notion of tasks that may be expensive and offload these to another thread. We did not support this when using our native implementation but can now for various operations during the handshake.
Modifications:
- Support offloading tasks during the handshake when using our native SSLEngine implementation
- Correctly handle the case when NEED_TASK is returned and nothing was consumed / produced yet
Result:
Be able to offload long running tasks from the EventLoop when using SslHandler with our native SSLEngine.
Motivation:
We must only remove ReferenceCountedOpenSslEngine from OpenSslEngineMap when engine is destroyed as the verifier / certificate callback may be called multiple times when the remote peer did initiate a renegotiation.
If we fail to do so we will cause an NPE like this:
```
13:16:36.750 [testsuite-oio-worker-5-18] DEBUG i.n.h.s.ReferenceCountedOpenSslServerContext - Failed to set the server-side key material
java.lang.NullPointerException: null
at io.netty.handler.ssl.OpenSslKeyMaterialManager.setKeyMaterialServerSide(OpenSslKeyMaterialManager.java:69)
at io.netty.handler.ssl.ReferenceCountedOpenSslServerContext$OpenSslServerCertificateCallback.handle(ReferenceCountedOpenSslServerContext.java:212)
at io.netty.internal.tcnative.SSL.readFromSSL(Native Method)
at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.readPlaintextData(ReferenceCountedOpenSslEngine.java:575)
at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.unwrap(ReferenceCountedOpenSslEngine.java:1124)
at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.unwrap(ReferenceCountedOpenSslEngine.java:1236)
at io.netty.handler.ssl.ReferenceCountedOpenSslEngine.unwrap(ReferenceCountedOpenSslEngine.java:1279)
at io.netty.handler.ssl.SslHandler$SslEngineType$1.unwrap(SslHandler.java:217)
at io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1330)
at io.netty.handler.ssl.SslHandler.decodeNonJdkCompatible(SslHandler.java:1237)
at io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1274)
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:502)
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:441)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:278)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:359)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:345)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:337)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1408)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:359)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:345)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:930)
at io.netty.channel.oio.AbstractOioByteChannel.doRead(AbstractOioByteChannel.java:170)
at io.netty.channel.oio.AbstractOioChannel$1.run(AbstractOioChannel.java:40)
at io.netty.channel.ThreadPerChannelEventLoop.run(ThreadPerChannelEventLoop.java:69)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:905)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:834)
```
While the exception is kind of harmless (as we will reject the renegotiation at the end anyway) it produces some noise in the logs.
Modifications:
Don't remove engine from map after handshake is complete but wait for it to be removed until the engine is destroyed.
Result:
No more NPE and less noise in the logs.
Motivation:
ChunkedWriteHandler needs to close both successful and failed
ChunkInputs. It used to never close successful ones.
Modifications:
* ChunkedWriteHandler always closes ChunkInput before completing
the write promise.
* Ensure only ChunkInput#close() is invoked
on a failed input.
* Ensure no methods are invoked on a closed input.
Result:
Fixes https://github.com/netty/netty/issues/8875.
Motivation:
fa6a8cb09c introduced correct dispatching of delegated tasks for SSLEngine but did not correctly handle some cases for resuming wrap / unwrap after the task was executed. This could lead to stales, which showed up during tests when running with Java11 and BoringSSL.
Modifications:
- Correctly resume wrap / unwrap in all cases.
- Fix timeout value which was changed in previous commit by mistake.
Result:
No more stales after task execution.
Motivation:
The SSLEngine does provide a way to signal to the caller that it may need to execute a blocking / long-running task which then can be offloaded to an Executor to ensure the I/O thread is not blocked. Currently how we handle this in SslHandler is not really optimal as while we offload to the Executor we still block the I/O Thread.
Modifications:
- Correctly support offloading the task to the Executor while suspending processing of SSL in the I/O Thread
- Add new methods to SslContext to specify the Executor when creating a SslHandler
- Remove @deprecated annotations from SslHandler constructor that takes an Executor
- Adjust tests to also run with the Executor to ensure all works as expected.
Result:
Be able to offload long running tasks to an Executor when using SslHandler. Partly fixes https://github.com/netty/netty/issues/7862 and https://github.com/netty/netty/issues/7020.
Motivation:
SPDY has been superseded by HTTP/2. Chrome has dropped support in 2016 and GFE no longer negociate it.
Modifications:
* drop codec
* drop examples
* drop constants from `ApplicationProtocolNames`
Result:
SPDY support dropped from Netty 5
Motivation:
We can just use Objects.requireNonNull(...) as a replacement for ObjectUtil.checkNotNull(....)
Modifications:
- Use Objects.requireNonNull(...)
Result:
Less code to maintain.
Motivation:
We can use lambdas now as we use Java8.
Modification:
use lambda function for all package, #8751 only migrate transport package.
Result:
Code cleanup.
Motivation:
SslContext implementations have tons of contructors, most of them deprecated as we want to enforce builder usage in Netty 5.
Cleaning them up is a requirement prior to introducing new parameters such as hostname verification.
Modifications:
* Make SslContext implementations classes and constructors package private, users are supposed to use the SslContextBuilder.
* Drop all but one constructor. The exception for now is with Jdk(Client|Server)Context that still has an additional constructor that takes an ApplicationProtocolNegotiator parameter. ApplicationProtocolNegotiator usage is supposed to be dropped in favor of ApplicationProtocolConfig and this constructor is only used in tests, so I guess it will be dropped to in a follow up.
Result:
Deprecated code dropped. Path cleaned up for introducing new features with having to introduce yet another constructor.
Motivation:
As netty 4.x supported Java 6 we had various if statements to check for java versions < 8. We can remove these now.
Modification:
Remove unnecessary if statements that check for java versions < 8.
Result:
Cleanup code.
Motivation:
We need to update to a new checkstyle plugin to allow the usage of lambdas.
Modifications:
- Update to new plugin version.
- Fix checkstyle problems.
Result:
Be able to use checkstyle plugin which supports new Java syntax.
Motivation:
Custom Netty ThreadLocalRandom and ThreadLocalRandomProvider classes are no longer needed and can be removed.
Modification:
Remove own ThreadLocalRandom
Result:
Less code to maintain
Motivation:
PlatformDependent.newConcurrentHashMap() is no longer needed so it could be easily removed and new ConcurrentHashMap<>() inlined instead of invoking PlatformDependent.newConcurrentHashMap().
Modification:
Use ConcurrentHashMap provided by the JDK directly.
Result:
Less code to maintain.
Motivation:
We can use the diamond operator these days.
Modification:
Use diamond operator whenever possible.
Result:
More modern code and less boiler-plate.
Motivation:
Replace "if else" conditions with string switch. It is easier to read the code, for large "if else" constructions switch also could be faster.
Modification:
Replaced "if else" with a string switch.
Result:
Use new language features
Motivation:
The concurrent set is present in Java 8 and above so we can use it instead of own implementation.
Modification:
io.netty.utik.internal.ConcurrentSet replaced with ConcurrentHashMap.newKeySet().
Result:
Less code to maintain.
Motivation:
ChunkedWriteHandler should report write operation as failed
in case *any* chunked was not written. Right now this is not
true for the last chunk.
Modifications:
* Check if the appropriate write operation was succesfull when
reporting the last chunk
* Skip writing chunks if the write operation was already marked
as "done"
* Test cases to cover write failures when dealing with chunked input
Result:
Fix https://github.com/netty/netty/issues/8700
Motivation:
At the moment it’s possible to have a Channel in Netty that is not registered / assigned to an EventLoop until register(...) is called. This is suboptimal as if the Channel is not registered it is also not possible to do anything useful with a ChannelFuture that belongs to the Channel. We should think about if we should have the EventLoop as a constructor argument of a Channel and have the register / deregister method only have the effect of add a Channel to KQueue/Epoll/... It is also currently possible to deregister a Channel from one EventLoop and register it with another EventLoop. This operation defeats the threading model assumptions that are wide spread in Netty, and requires careful user level coordination to pull off without any concurrency issues. It is not a commonly used feature in practice, may be better handled by other means (e.g. client side load balancing), and therefore we propose removing this feature.
Modifications:
- Change all Channel implementations to require an EventLoop for construction ( + an EventLoopGroup for all ServerChannel implementations)
- Remove all register(...) methods from EventLoopGroup
- Add ChannelOutboundInvoker.register(...) which now basically means we want to register on the EventLoop for IO.
- Change ChannelUnsafe.register(...) to not take an EventLoop as parameter (as the EventLoop is supplied on custruction).
- Change ChannelFactory to take an EventLoop to create new Channels and introduce ServerChannelFactory which takes an EventLoop and one EventLoopGroup to create new ServerChannel instances.
- Add ServerChannel.childEventLoopGroup()
- Ensure all operations on the accepted Channel is done in the EventLoop of the Channel in ServerBootstrap
- Change unit tests for new behaviour
Result:
A Channel always has an EventLoop assigned which will never change during its life-time. This ensures we are always be able to call any operation on the Channel once constructed (unit the EventLoop is shutdown). This also simplifies the logic in DefaultChannelPipeline a lot as we can always call handlerAdded / handlerRemoved directly without the need to wait for register() to happen.
Also note that its still possible to deregister a Channel and register it again. It's just not possible anymore to move from one EventLoop to another (which was not really safe anyway).
Fixes https://github.com/netty/netty/issues/8513.
Motivation:
Users who want to construct a `FlushConsolidationHandler` with a default `explicitFlushAfterFlushes` but non-default `consolidateWhenNoReadInProgress` may benefit from having an easy way to get the default "flush after flushes" count.
Modifications:
- Moved default `explicitFlushAfterFlushes` value to a public constant.
- Adjusted Javadoc accordingly.
Result:
Default `explicitFlushAfterFlushes` is accessible to callers.
Motivation:
SSLSession.putValue / getValue / removeValue / getValueNames must be thread-safe as it may be called from multiple threads. This is also the case in the OpenJDK implementation.
Modifications:
Guard with synchronized (this) blocks to keep the memory overhead low as we do not expect to have these called frequently.
Result:
SSLSession implementation is thread-safe.
