do not periodically re-read /etc/hosts by default

[mostroverkhov]

Motivation:

After #11922, by default io.netty.resolver.DefaultHostsFileEntriesResolver synchronously reads /etc/hosts every 60 seconds, while prior above change It read /etc/hosts once upon instantiation.

Modification:

By default io.netty.resolver.DefaultHostsFileEntriesResolver reads /etc/hosts once upon instantiation. This corresponds to io.netty.hostsFileRefreshInterval = 0

Negative values for io.netty.hostsFileRefreshInterval are forbidden

Result:

By default io.netty.resolver.DefaultHostsFileEntriesResolver reads /etc/hosts once upon instantiation, which corresponds to behavior before #11922

[mostroverkhov]

I am wondering why DefaultHostsFileEntriesResolver was changed In first place - there exists HostsFileEntriesResolver interface, and DnsNameResolverBuilder may be configured externally with any impl: periodic updates, filesystem watcher etc.

[mostroverkhov]

original pr had io.netty.hostsFileRefreshInterval as nanos - maybe seconds are more appropriate?

[normanmaurer]

@mostroverkhov what is wrong with re-reading by default ?

[violetagg]

@mostroverkhov what is wrong with re-reading by default ?

@normanmaurer This is my perspective: currently with this re-reading the event loop can be blocked. If you are on the server and you want to make a remote call you need always to offload to another thread in order to not block.

It was ok to exclude this one

netty/common/src/main/java/io/netty/util/internal/Hidden.java

Lines 133 to 135 in e90f9bc

	builder.allowBlockingCallsInside(
	"io.netty.resolver.HostsFileEntriesProvider$ParserImpl",
	"parse");

because you can take care for the initialisation of io.netty.resolver.DefaultHostsFileEntriesResolver#DefaultHostsFileEntriesResolver(io.netty.resolver.HostsFileEntriesProvider.Parser, long) (i.e. do this out of the event loop)

but now you cannot, because every time when you try to resolve it might try to re-read the file.

[mostroverkhov]

@normanmaurer It is blocking call on filesystem

netty/resolver/src/main/java/io/netty/resolver/HostsFileEntriesProvider.java

Line 215 in 15196ad

while ((line = buff.readLine()) != null) {

, It comes from

netty/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java

Line 862 in 15196ad

final List<InetAddress> hostsFileEntries = resolveHostsFileEntries(hostname);

, so given resolveAll(..) signature I'd expect most people call It on event loop

[normanmaurer]

@mostroverkhov @violetagg good point! Thanks a lot!