RFR: 8298381: Improve handling of session tickets for multiple SSLContexts [v6]

[Volker Simonis]

> Currently, TLS session tickets introduced by [JDK-8211018](https://bugs.openjdk.org/browse/JDK-8211018) in JDK 13 (i.e. `SessionTicketExtension$StatelessKey`) are generated in the class `SessionTicketExtension` and they use a single, global key ID (`currentKeyID`) for all `SSLContext`s.
> 
> This is problematic if more than one `SSLContext` is used, because every context which requests a session ticket will increment the global id `currentKeyID` when it creates a ticket. This means that in turn all the other contexts won't be able to find a ticket under the new id in their `SSLContextImpl` and create a new one (again incrementing `currentKeyID`). In fact, every time a ticket is requested from a different context, this will transitively trigger a new ticket creation in all the other contexts. We've observed millions of session ticket accumulating for some workloads.
> 
> Another issue with the curent implementation is that cleanup is racy because the underlying data structure (i.e. `keyHashMap` in `SSLContextImpl`) as well as the cleanup code itself are not threadsafe.
> 
> I therefor propose to move `currentKeyID` into the `SSLContextImpl` to solve these issues.
> 
> The following test program (contributed by Steven Collison (https://raycoll.com/)) can be used to demonstrate the current behaviour. It outputs the number of `StatelessKey` instances at the end of the program. Opening 1000 connections with a single `SSLContext` results in a single `StatelessKey` instance being created:
> 
> $ java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 1 1000
> 605:             1             32  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)
> 
> The same example with the 1000 connections being opened alternatively on thwo different contexts will instead create 1000 `StatelessKey` instances:
> 
> $ java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 2 1000
>   11:          1000          32000  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)
> 
> With my proposed patch, the numbers goes back to two instances again:
> 
> $ java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 2 1000
> 611:             2             64  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)
> 
> 
> I've attached the test program to the [JBS issue](https://bugs.openjdk.org/browse/JDK-8298381). If you think it makes sense, I can probably convert it into a JTreg test.

Volker Simonis has updated the pull request incrementally with one additional commit since the last revision:

  Updated copyright year to 2023

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11590/files
  - new: https://git.openjdk.org/jdk/pull/11590/files/68ed71dd..eb2d8e25

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11590&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11590&range=04-05

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11590.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11590/head:pull/11590

PR: https://git.openjdk.org/jdk/pull/11590