Precompile lua functions to prevent wasted CPU cycles

[tonyhb]

miniredis.runLuaScript calls lua.*LState.DoString() for every lua invocation, even if scripts are evaluated via checksums.

In turn, lua parses and compiles the same scripts on each call, which can cause high CPU usage on script-heavy test instances.

This PR uses sync.Map to store pre-compiled lua functions for each script, using the same logic as the Lua library itself: tul/gopher-lua@7a6135d

This reduces CPU usage by about 50%.

Before:

After:

This takes runLuaScript's cumulative time from 1.19s to 0.68s, or about a 57% improvement.

[alicebob]

Hi,

thanks for the clear PR! It looks good, but some error messages changed. The errors can't have the exact same wording as Redis, but they should use the same -ERR .. prefix. Nothing major, if you could have a look at that, great, otherwise I have a better look another time.

[tonyhb]

@alicebob thanks for the quick review! believe that i've fixed that error, appreciate the heads up :)

[alicebob]

Hi,

thanks for the fix. Just the test failure left. I see the Lua code eventually does this in DoString():

return ls.Load(strings.NewReader(source), "")

https://github.com/yuin/gopher-lua/blob/v1.1.0/auxlib.go#L387

so maybe do it that way?

[tonyhb]

Aha, didn't notice that — sorry! All fixed. Had to change the name of the input script to match before as you mentioned. Thank you!

[alicebob]

Thanks! It's now in master.