Configurable DB hash seed for SCAN family commands consistency

[sarthakaggarwal97]

Introduce a new config hash-seed which can be set only at startup and controls the hash seed for the server. This includes all hash tables. This change makes it so that both primaries and replicas will return the same results for SCAN/HSCAN/ZSCAN/SSCAN cursors. This is useful in order to make sure SCAN behaves correctly after a failover.

Resolves #4

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.43%. Comparing base (a49d469) to head (f077e5e).
⚠️ Report is 2 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/config.c	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2608      +/-   ##
============================================
+ Coverage     72.39%   72.43%   +0.03%     
============================================
  Files           128      128              
  Lines         70245    70261      +16     
============================================
+ Hits          50853    50891      +38     
+ Misses        19392    19370      -22     

Files with missing lines	Coverage Δ
src/server.c	88.44% <100.00%> (-0.02%)	⬇️
src/server.h	100.00% <ø> (ø)
src/util.c	66.53% <100.00%> (+0.36%)	⬆️
src/config.c	78.44% <50.00%> (-0.08%)	⬇️

... and 11 files with indirect coverage changes

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[sarthakaggarwal97]

@madolson I think it's ready for review 🙂

[sarthakaggarwal97]

@madolson thanks for the review, I think I have addressed all the comments!

[madolson]

Looks closer, just some more minor comments.

[zuiderkwast]

Sorry for the delay. Looks good in general, but I have a comment about the string-to-seed algorithm, which will becomes part of the interface, in the sense that if we change it in the future, it will break the scan cursor compatibility between nodes, so we may want think through this choice of algorithm.

[zuiderkwast]

LGTM!

[zuiderkwast]

Should we mention this config on the documentation page for SCAN?

[sarthakaggarwal97]

@zuiderkwast I also had this question. But when do we actually add the documentation, since the change is only in unstable and not released.

[zuiderkwast]

But when do we actually add the documentation, since the change is only in unstable and not released.

We typically formulate things like this as "Starting with Valkey 9.1, it's possible to bla-bla-bla", because we want the docs to be useful regardless of which version a user is running, so we can add it any time once it's in unstable.

[sarthakaggarwal97]

sure, makes sense. If people agree that we should add it, I will raise a PR asap.

[zuiderkwast]

Yeah I think it'd be good to have. Maybe under this section? https://valkey.io/commands/scan/#scan-guarantees

We only need to add it for SCAN, since the other ones [HSZ]SCAN link to SCAN. Oh.. they don't, but they should(!). See https://valkey.io/commands/sscan/ . It was auto-linked in the redis times, but we should use an explicit link in the markdown.