Filter out duplicate relays & monitors

[jtraglia]

📝 Summary

Noticed that there were no duplication checks. You could specify the same relay or monitor multiple times.

⛱ Motivation and Context

May prevent unnecessary work by the MEV client and relays.

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #348 (00f9d46) into main (bca8407) will decrease coverage by 10.29%.
The diff coverage is 95.00%.

@@             Coverage Diff             @@
##             main     #348       +/-   ##
===========================================
- Coverage   82.50%   72.20%   -10.30%     
===========================================
  Files           5        6        +1     
  Lines         680      806      +126     
===========================================
+ Hits          561      582       +21     
- Misses         91      192      +101     
- Partials       28       32        +4     

Flag	Coverage Δ
unittests	72.20% <95.00%> (-10.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cli/main.go	20.47% <95.00%> (ø)
server/service.go	80.00% <0.00%> (-1.04%)	⬇️

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[jtraglia]

Also, without this PR, mev-boost will return a not-so-useful error if the -relays flag isn't set:

$ make build && ./mev-boost -mainnet  
go build -trimpath -ldflags "-s -X 'github.com/flashbots/mev-boost/config.Version=v1.3.2-5-gece7a08'" -v -o mev-boost .
github.com/flashbots/mev-boost
INFO[2022-09-23T13:59:38-05:00] mev-boost v1.3.2-5-gece7a08                  
INFO[2022-09-23T13:59:38-05:00] using genesis fork version: 0x00000000       
FATA[2022-09-23T13:59:38-05:00] Invalid relay URL                             error="missing relay public key" relayURL=

This is because relayURLs is an empty string ("") when not specified. That's its default value.

With this PR, it will print the "no relays specified" error like we want.

[metachris]

thanks! a test or two would be very nice for this

[jtraglia]

Yeah tests are a good idea. Added some. I would have done some relay monitor tests but it very repetitive since it's essentially the same thing. Also, it's not really possible to test invalid relay cases because parseRelayURLs will immediately exit without returning an error if something is wrong, like a missing pubkey.

[metachris]

since the final size is not known, don't initialize the slice with a size?

Suggested change

	filtered := make([]string, 0, len(stringSlice))
	filtered := []string{}

[jtraglia]

So that third argument is the capacity, not size. This is an optimization.

func make([]T, len, cap) []T

Since we know the maximum number of items there can be (the length of the input slice), I think this is reasonable. Pre-allocations can speed things up quite a bit, not that this is a time sensitive operation.

Here's some random article that explains it pretty well:

• https://medium.com/swlh/bad-go-not-sizing-slices-aed1b01cff83

[metachris]

Suggested change

	filtered := make([]string, 0, len(stringSlice))
	filtered := []string{}

the final size is not known, don't initialize it

[jtraglia]

See my other comment.

[jtraglia]

If #351 gets merged, this PR is no longer necessary.