Use Netty's asynchronous DNS resolver

[yueki1993]

Bug Report

According to the motivation of #498, netty's non-blocking DNS resolver should be used.
However netty's blocking DNS resolver seems to be used now.

netty's release note suggests if we want to use async DNS resolver we need to pass resolver to Bootstrap; they say By default it will use the JDK name resolution, just as before, which is blocking.
The modifications of commit 3c8d021 and cf71131 is to simply delegate netty's DNS resolution, which seems insufficient to use async DNS resolver.

Maybe due to this behaviour I observe lettuce' epoll event loop is blocked during topology refresh (with dynamicRefreshSources=false) when DNS lookup is heavily loaded.

thread dump:

"lettuce-epollEventLoop-5-1" #25 daemon prio=5 os_prio=0 tid=xxxxxx nid=0x2d64 runnable [xxxxxxx]
   java.lang.Thread.State: RUNNABLE
	at java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method)
	at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:929)
	at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1324)
	at java.net.InetAddress.getAllByName0(InetAddress.java:1277)
	at java.net.InetAddress.getAllByName(InetAddress.java:1193)
	at java.net.InetAddress.getAllByName(InetAddress.java:1127)
	at java.net.InetAddress.getByName(InetAddress.java:1077)
	at io.netty.util.internal.SocketUtils$8.run(SocketUtils.java:156)
	at io.netty.util.internal.SocketUtils$8.run(SocketUtils.java:153)
	at java.security.AccessController.doPrivileged(Native Method)
	at io.netty.util.internal.SocketUtils.addressByName(SocketUtils.java:153)
	at io.netty.resolver.DefaultNameResolver.doResolve(DefaultNameResolver.java:41)
	at io.netty.resolver.SimpleNameResolver.resolve(SimpleNameResolver.java:61)
	at io.netty.resolver.SimpleNameResolver.resolve(SimpleNameResolver.java:53)
	at io.netty.resolver.InetSocketAddressResolver.doResolve(InetSocketAddressResolver.java:55)
	at io.netty.resolver.InetSocketAddressResolver.doResolve(InetSocketAddressResolver.java:31)
	at io.netty.resolver.AbstractAddressResolver.resolve(AbstractAddressResolver.java:106)
	at io.netty.bootstrap.Bootstrap.doResolveAndConnect0(Bootstrap.java:206)
	at io.netty.bootstrap.Bootstrap.access$000(Bootstrap.java:46)
	at io.netty.bootstrap.Bootstrap$1.operationComplete(Bootstrap.java:180)
	at io.netty.bootstrap.Bootstrap$1.operationComplete(Bootstrap.java:166)
	at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:577)
	at io.netty.util.concurrent.DefaultPromise.notifyListenersNow(DefaultPromise.java:551)
	at io.netty.util.concurrent.DefaultPromise.notifyListeners(DefaultPromise.java:490)
	at io.netty.util.concurrent.DefaultPromise.setValue0(DefaultPromise.java:615)
	at io.netty.util.concurrent.DefaultPromise.setSuccess0(DefaultPromise.java:604)
	at io.netty.util.concurrent.DefaultPromise.trySuccess(DefaultPromise.java:104)
	at io.netty.channel.DefaultChannelPromise.trySuccess(DefaultChannelPromise.java:84)
	at io.netty.channel.AbstractChannel$AbstractUnsafe.safeSetSuccess(AbstractChannel.java:984)
	at io.netty.channel.AbstractChannel$AbstractUnsafe.register0(AbstractChannel.java:504)
	at io.netty.channel.AbstractChannel$AbstractUnsafe.access$200(AbstractChannel.java:417)
	at io.netty.channel.AbstractChannel$AbstractUnsafe$1.run(AbstractChannel.java:474)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:472)
	at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:384)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

Current Behavior

netty's blocking DNS resolver(DefaultNameResolver) seems to be used.

Input Code

Expected behavior/code

netty's non-blocking DNS resolver(DnsNameResolver) should be used.

Environment

• Lettuce version(s): 6.1.0.BUILD-SNAPSHOT (commit: fe70d44)
• Redis version: 6.0.5

Possible Solution

yueki1993@be7a99a

Additional context

To check this behaviour debugging a test with intellij would be the simplest.
Debug run AtLeastOnceTest#connectionIsConnectedAfterConnect with breakpoints at:

• https://github.com/lettuce-io/lettuce-core/blob/8011d5ca2698b8f57b7b8c73920f4e7af575daf2/src/main/java/io/lettuce/core/AbstractRedisClient.java#L390
• https://github.com/netty/netty/blob/netty-4.1.53.Final/transport/src/main/java/io/netty/bootstrap/Bootstrap.java#L206
• https://github.com/netty/netty/blob/d4a0050ef33cab2542a80e11489a4977a63859f8/resolver/src/main/java/io/netty/resolver/InetSocketAddressResolver.java#L55
  and you should see DefaultNameResolver is used.
  

[yueki1993]

Hi @mp911de,
If my understanding is correct and you agree with replacing blocking dns resolver with the non-blocking one, can I submit a PR?
Or is it better to wait for evaluating this report for a while?
I am really sorry if this report is nonsense. If so I will close this issue.
(I understand you are really busy and I am willing to do anything I can if you need.)

[mp911de]

Thanks for bringing this issue up. We're happy to receive a pull request to use netty's asynchronous DNS resolver.