add alert function to pacrunners

[multiSnow]

This PR add the (widely used?) alert function to each pacrunner, which directly print to stderr, prefixed with PAC-alert:.

Update:
Unless the _PX_DEBUG_PACALERT environment is set, this alert function will do nothing (since there is no 'browser console'), which just create the function name in pacrunner context to avoid ReferenceError while interpreting pac script.

[Vogtinator]

I wonder whether it's a good idea to unconditionally allow PAC scripts to pollute stderr of arbitrary programs. That can also cause fun with escape sequences.

[mcatanzaro]

I wonder whether it's a good idea to unconditionally allow PAC scripts to pollute stderr of arbitrary programs. That can also cause fun with escape sequences.

I don't see why not. The PAC script is inherently trusted, and it seems useful. So this looks good to me.

Note this codebase will be obsoleted by https://github.com/janbrummer/libproxy2, so since this is new functionality, you'll want to submit this there too.

[Vogtinator]

I don't see why not. The PAC script is inherently trusted, and it seems useful. So this looks good to me.

It's trusted to perform redirections (at most), but not to possibly cause remote code execution...

[mcatanzaro]

Programs print untrusted stuff to stderr all the time. Is this really unsafe?

[Vogtinator]

Programs print untrusted stuff to stderr all the time. Is this really unsafe?

Programs might, but not libraries.

[mcatanzaro]

I think it's pretty normal for libraries to print error messages that may contain untrusted data to stderr.

[janbrummer]

Please rebase.

[janbrummer]

Thanks for the update, but i'm currently confused why there are some minor code style issues. I expected the commit hook to run our style checker before commiting. Have you build the current code?

[janbrummer]

LGTM, but we should ensure that this parameter is also documented. Could you add a small section to the doc?

[codecov-commenter]

Codecov Report

Patch coverage: 18.18% and project coverage change: -0.74 ⚠️

Comparison is base (de3bc39) 72.49% compared to head (87af768) 71.76%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   72.49%   71.76%   -0.74%     
==========================================
  Files          17       17              
  Lines         807      818      +11     
  Branches      215      217       +2     
==========================================
+ Hits          585      587       +2     
- Misses        130      139       +9     
  Partials       92       92              

Impacted Files	Coverage Δ
...kend/plugins/pacrunner-duktape/pacrunner-duktape.c	58.82% <18.18%> (-6.05%)	⬇️

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