Lua LDB debugging: Float precision is lost in stack values and [p]rint of Lua vars

[tw-bert]

Redis 4.0.8

Follow up on: pkulchenko/ZeroBranePackage#66
Redis docs: https://redis.io/topics/ldb

In short: 20180212.1045 is serialized as 2.01802e+07 which is unusable (because: it's a timestamp: YYYYMMDD.HHMM). The non mantissa precision of 53 bit (15 digits in base-10) should not be lost in the serialization. In this example the precision goes down from 12 digits to 6 digits.

Possible solution: Use format "%015.15g" and/or let the user set a custom session-persistent float serialization string.

[tw-bert]

@itamarhaber : Some more people on our team are starting to use redis' ldb (with or without ZeroBraneStudio). We'd greatly appreciate some feedback on this issue.

[itamarhaber]

I think this is a good suggestion, and it shouldn't break anything major as this is ldb after all. Pong @antirez ?

[antirez]

Hello @tw-bert, @itamarhaber, what conversion function is that in LDB? I've no idea about what ZeroBrane does so with the informations in this issue I'm not sure what the problem is, a set of commands directly send to the Redis LDB showing the issue would be appreciated. Thanks.

[tw-bert]

@antirez @itamarhaber Here's a repro:

tw_bert@srv-flux-dev-01:~$ printf 'local f=20180713.143205\nreturn tostring(f)' > test.lua
tw_bert@srv-flux-dev-01:~$ redis-cli -p 14213 --ldb --eval test.lua
Lua debugging session started, please use:
quit    -- End the session.
restart -- Restart the script in debug mode again.
help    -- Show Lua script debugging commands.

* Stopped at 1, stop reason = step over
-> 1   local f=20180713.143205
lua debugger> s
* Stopped at 2, stop reason = step over
-> 2   return tostring(f)
lua debugger> p
<value> f = 2.01807e+07
lua debugger> s

"20180713.143205"

(Lua debugging session ended -- dataset changes rolled back)

The issue is, that 2.01807e+07 should be serialized/cast to string as 20180713.143205 . Applying the Lua format "%015.15g" was just a suggestion, you can disregard that if you wish.