asyn-ares: resolver timeout follows the c-ares mechanism

[sunlin7]

Hi,
The curl should set c-ares's timeout only for c-ares < 1.20.0.

curl use a fixed timeout=2s for c-ares from commit:
a181b4a,
actually c-ares used default timeout to 2s since c-ares-1.20.0 release (also mentioned on prevous commit).
c-ares/c-ares#542
and the timeout=2 from curl will overwrite the option: timeout from the /etc/resolv.conf, which supported since c-ares-1.23.0:
https://github.com/c-ares/c-ares/pull/632/files

So setting c-ares timeout to 2s should only apply to old c-ares < 1.20.0, won't overwrite timeout for c-ares > 1.23.0.

[bagder]

The curl should set c-ares's timeout only for c-ares < 1.20.0.

Why? It risks/makes the timeouts to be different with different c-ares versions. Why do curl users want that?

Also, this patch checks the version of c--ares used when building curl, not the version that is actually used in run-time. That might not be the same...

[sunlin7]

@bagder

Why? It risks/makes the timeouts to be different with different c-ares versions. Why do curl users want that?

curl failed example (curl 8.4+, c-ares 1.24+):

1. user config DNS timeout in /etc/resolv.conf with options timeout:1 for DNS timeout 1 second
2. first DNS service down (or un-accessable IP address eg: 10.10.10.10, to trigger timeout case)
3. time ./curl http://www.google.com then will timeout for 2 seconds.
   Expected timeout is 1 second, for user had set timeout in resolv.conf file.
   This PR will let curl follow user timeout setting.

Also, this patch checks the version of c--ares used when building curl, not the version that is actually used in run-time. That might not be the same...

It's already there: curl-8.3 with c-ares the DNS timeout is 5 seconds; while curl-8.4 with c-ares timeout is 2 seconds.

I like to change curl to get the version from loaded libcares dynamicly, give me minutes.

[sunlin7]

Hi @bagder
I pushed the changed one that getting the version from loaded libcares, please help review again, thanks.

[bagder]

user config DNS timeout in /etc/resolv.conf with options timeout:1 for DNS timeout 1 second

I did not even know that thing existed.

This then begs the question: what if we want to set the limit to N seconds but allow /etc/resolv.conf to override if set. Is that then not possible, you know?

[sunlin7]

Hi @bagder
This change is strong enough:
if libares < 1.20.0: curl set timeout to 2s;
if libares >= 1.20.0 it already has the timeout to 2s, curl don't need to set the timeout value;
if libares >= 1.24.0, user can set the timeout via /etc/resolv.conf to overwrite the libares's default timeout.

This then begs the question: what if we want to set the limit to N seconds but allow /etc/resolv.conf to override if set. Is that then not possible, you know?

Currently no grace way to do that (unless we parse the /etc/resolv.conf, but that's not curl's scope.)

[bagder]

thanks!

[sunlin7]

Thank you !