First crack at a fix to prevent clients from stomping on their headers

[bc-sb]

First crack at a fix to prevent clients from stomping on their own headers

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area build

/area config

/area outputs

/area tests

What this PR does / why we need it:
Fixes a race condition that multiple calls to the same http Post causes headers to become unset
Which issue(s) this PR fixes:
#375

Fixes #375

Special notes for your reviewer:

[poiana]

Welcome @bc-sb! It looks like this is your first PR to falcosecurity/falcosidekick 🎉

[Issif]

Thank you for this PR, I'm in holidays this week, I'll take a look asap

[Issif]

For all outputs that use the generic client.Post method, why are you not locking directly in the method (https://github.com/spyderbat/falcosidekick/blob/d80b603dddd6a2599f6691e48cf21d40f7f2df66/outputs/client.go#L139)? I see you set your c.m.Lock() in each output.

[bc-sb]

I actually tried that approach originally of just doing a:

Lock()
defer Unlock()

But was still seeing it...admittedly I got lazy and just crammed it into the clients that were touching the headers to confirm. But now that I looked at it again, I ran the race detector on it:

==================
WARNING: DATA RACE
Write at 0x00c000620110 by goroutine 42:
  github.com/falcosecurity/falcosidekick/outputs.(*Client).Post()
      /home/bc/spyderbat/falcosidekick/outputs/client.go:232 +0x1904
  github.com/falcosecurity/falcosidekick/outputs.(*Client).WebhookPost()
      /home/bc/spyderbat/falcosidekick/outputs/webhook.go:19 +0x1fd
  main.forwardEvent.func22()
      /home/bc/spyderbat/falcosidekick/handlers.go:246 +0xa8                                                                                                                                                                                                                                                                                                                            Previous write at 0x00c000620110 by goroutine 55:
  github.com/falcosecurity/falcosidekick/outputs.(*Client).AddHeader()                                                                                                                            /home/bc/spyderbat/falcosidekick/outputs/client.go:287 +0x7f5
  github.com/falcosecurity/falcosidekick/outputs.(*Client).WebhookPost()
      /home/bc/spyderbat/falcosidekick/outputs/webhook.go:15 +0x6ff                                                                                                                           main.forwardEvent.func22()
      /home/bc/spyderbat/falcosidekick/handlers.go:246 +0xa8  

So, I think I have a better option that I will push after seeing that.

[bc-sb]

@Issif I just pushed the more robust option.

[Issif]

Can you a more explicit name than m for the mutex please, I try to keep things explicit for beginners in Go that would like to contribute

[Issif]

Can you use a longer name for clarity please.

[poiana]

LGTM label has been added.

Git tree hash: 10d4035a83f8192a189c4931da6b547ce70519ec

[poiana]

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This pull-request has been approved by: bc-sb, Issif

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