### The Proposed Change
The proposed change updates JNDI's `DnsClient` internal implementation to use
`DatagramChannel` (DC) as a replacement for `DatagramSocket` (DS).
The main motivation behind this change is to make JNDI/DNS lookups friendly to
virtual-thread environments and update its underlying implementation to use
efficient `DatagramChannel` APIs.
The list of proposed changes:
- Replace DS usage with DC. That includes the `DNSDatagramSocketFactory` class
updates to return DC instead of DS. The factory class was renamed to
`DNSDatagramChannelFactory` to reflect that.
- Change DNS query timeouts implementation - the current version introduces a
nio channels selector to implement timeouts. One selector is created for each
instance of `DnsClient`.
- Adjust query retries logic to the new implementation of timeouts.
- Modify the `Timeout` test to create a bound UDP socket to simulate an
unresponsive DNS server. Before this change, the test was using the '10.0.0.0'
network address that doesn't belong to any host. The proposed change with a
bound unresponsive UDP socket is better for test stability on different
platforms.
### Testing
`jdk-tier1` to `jdk-tier3 `tests are showing no failures related to the changes.
JNDI regression and JCK tests also didn't highlight any problems with the
changes.
Also, an app performing a DNS lookup from a virtual thread context executed
with the following options `--enable-preview -Djdk.tracePinnedThreads=full`
showed no pinned carrier threads. Before the proposed change the following
pinned stack trace was observed:
```
java.base/sun.nio.ch.DatagramChannelImpl.park(DatagramChannelImpl.java:486)
java.base/sun.nio.ch.DatagramChannelImpl.trustedBlockingReceive(DatagramChannelImpl.java:734)
java.base/sun.nio.ch.DatagramChannelImpl.blockingReceive(DatagramChannelImpl.java:661)
java.base/sun.nio.ch.DatagramSocketAdaptor.receive(DatagramSocketAdaptor.java:241)
<== monitors:1
java.base/java.net.DatagramSocket.receive(DatagramSocket.java:714)
jdk.naming.dns/com.sun.jndi.dns.DnsClient.doUdpQuery(DnsClient.java:430)
<== monitors:1
jdk.naming.dns/com.sun.jndi.dns.DnsClient.query(DnsClient.java:216)
jdk.naming.dns/com.sun.jndi.dns.Resolver.query(Resolver.java:81)
jdk.naming.dns/com.sun.jndi.dns.DnsContext.c_lookup(DnsContext.java:290)
java.naming/com.sun.jndi.toolkit.ctx.ComponentContext.p_lookup(ComponentContext.java:542)
java.naming/com.sun.jndi.toolkit.ctx.PartialCompositeContext.lookup(PartialCompositeContext.java:177)
java.naming/com.sun.jndi.toolkit.ctx.PartialCompositeContext.lookup(PartialCompositeContext.java:166)
java.naming/javax.naming.InitialContext.lookup(InitialContext.java:409)
After proposed changes - pinned threads are not detectable.
-------------
Commit messages:
- 8290714: Make com.sun.jndi.dns.DnsClient virtual threads friendly
Changes: https://git.openjdk.org/jdk/pull/11007/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11007&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8290714
Stats: 691 lines in 4 files changed: 362 ins; 305 del; 24 mod
Patch: https://git.openjdk.org/jdk/pull/11007.diff
Fetch: git fetch https://git.openjdk.org/jdk pull/11007/head:pull/11007
PR: https://git.openjdk.org/jdk/pull/11007
