Optimize memory usage on the TLS manager/store

[ddtmachado]

What does this PR do?

Optimize the memory footprint of TLS certificate handling on the TLS Manager and Store, mainly:

• Avoid parsing the certificate multiple times
• Avoid copying certificates back and forth multiple times
• Do not wipe the store on every refresh (dynamic config update)

I included a small bench test for the manager in the PR, it's goal is to switch TLS certificates in batches of 10 from a pool of 100. With it I observed a large decrease from +10k allocs/op average to 1.9k allocs/op average as follows:

#Before
122390966 ns/op	 3410744 B/op	   10843 allocs/op

#After
704874 ns/op	  187272 B/op	    1981 allocs/op

Motivation

Since v2 Traefik received many reports with concerns and problems caused by the increased memory usage, specially while handling TLS certs.

I had some free time to investigate and found the code on the manager and store overly complex and probably trading memory for compatibility without strong justification, other than "I won't risk touching this code" maybe? :D

More

• Added/updated tests
• Added/updated documentation

Additional Notes

I did not had time to properly test it in a real environment with real data. Needs way more testing.

[nmengin]

Hello @ddtmachado,

Thank you for this PR, we are very interested in it but won't be able to test/review/merge it before releasing Traefik v3.0.
That's why I moved it to the next milestone.

On your side, could you rebase it on the master branch?

[rtribotte]

Hey @ddtmachado!

Nice to see you around and thanks for this contribution!
I took a look at the changes and I have a few comments.
Regarding the next steps of the review process, do you want to work and iterate on the changes, or you don't mind if we push some review commits?

[ddtmachado]

Hey @ddtmachado!

Nice to see you around and thanks for this contribution! I took a look at the changes and I have a few comments. Regarding the next steps of the review process, do you want to work and iterate on the changes, or you don't mind if we push some review commits?

Hey Romain, great to see you're on this!!
And sorry for the delay but somehow I'm not getting notifications from anything on the traefik repo and I just checked it is enabled.

Anyway, I'm running out of free time lately due to other demands and would not mind if you just push some commits on top, feel free as I completely agree with your comments and did not validade for those use cases.

But in case you run out of time as well or get prioritized in something else let me know and I'll give it a shot in the coming weeks.

[rtribotte]

Hello @ddtmachado,

I pushed review commits and also rebased the changes.
I have marked the PR as need-review, I think I could LGTM it right away, but I prefer to sleep on it to give it maybe more thought :-)

[rtribotte]

Thanks!