http: add support for proxy

[vatsal-encs]

fixes #3232

[kira-syslogng]

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[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[MrAnno]

@kira-syslogng ok to test

[kira-syslogng]

Build SUCCESS

[furiel]

Thank you for your pull request. At first glance it looks good to me.

• Please don't forget to free the proxy string.
• Please add a news file.
• I mentioned in the issue that it would be nice to have a warning if users use different proxy scheme than the url.

Unfortunately, I have a few things to do now, but i try to come back with a proper review as soon as possible. In the meantime, could you tell your experiences? It would help the review greatly.

• How did you test the feature? What proxy implementation did you use, and how did you configure it?
• Can two different http destinations have different proxys? Is it possible to have one http destination with proxy, and another without proxy?
• The proxy setting in the grammar indeed stronger than the proxy environment variables?
• What happens if we have only one http destination, but have two urls and two workers? Do both workers use the same proxy?
• What happens to the messages if the proxy is not available? Does syslog-ng drop the messages?
• What happens to the messages if the proxy is available, but the remote isn't?

[bazsi]

I think a news file entry with an example configuration would be pretty useful, perhaps with important details included from https://curl.haxx.se/libcurl/c/CURLOPT_PROXY.html.

Apart from that the code looks good to me.

[furiel]

After discussing with @bazsi , I was wrong about the scheme mismatch. I tried the code, and indeed with http url and https proxy, syslog-ng still tries to connect to the https proxy.

So the only thing left to do is to fix the memory leak, then this PR can go in.

[kira-syslogng]

Build SUCCESS

[furiel]

Thank you. Finally, please add a news file:
https://github.com/syslog-ng/syslog-ng/blob/master/CONTRIBUTING.md#news-file

memleak fixed

[vatsal-encs]

Thank you. Finally, please add a news file:
https://github.com/syslog-ng/syslog-ng/blob/master/CONTRIBUTING.md#news-file

@furiel : Okay, I have added it.

[kira-syslogng]

Build SUCCESS

[kira-syslogng]

Build SUCCESS

[furiel]

Thank you. The patchset looks good now.

Unfortunately pytest-linters is broken, and as I see it is a bug in virtualenv, that the maintainers are currently trying to address in pypa/virtualenv#1811. We can wait a little until it is resolved. Or we can just merge this as it is: python code has not been changed, so the linters should not find any problem.

[alltilla]

Merging, as travis fails for an environment error.