Module interface and use of static robj

[JimB123]

The module API uses robj for passing keys. By using robj (rather than an sds or char*) this provides the ability for the module to save the string via ref-counts. See: RM_RetainString.

In Redis 6.2 RC2, I'm seeing the use of stack allocated robj being passed to a module interface. Of course this won't work if the module attempts to retain the value.

Code reference:
https://github.com/redis/redis/blame/303465af35c13691f989b3400b028a94df1235d4/src/rdb.c#L2554-L2582

There are a couple of other uses of a stack-based robj in this file, but I can't immediately see that they are wrong. Any use of a stack-based robj is risky as it's hard to know how the robj is used by called routines. It's also hard to predict future modifications to the code.

I suggest that a real heap-based robj should be used in almost all cases. It should be readily apparent that a stack-based robj is only used locally. Note: this implies that the referenced string must also be heap-based and that the string will not be deleted independently from the robj.

[oranagra]

We identified some of these problems before redis 6.0 went out. The main one was the key names in RM_GetKeyNameFromIO that suffered from an optimization in rdb.c, but considering this optimization resulted in huge speed improvement for the rdb parsing, and the fact that API wasn't highly used, we decided to create RM_HoldString that can create a new heap allocation, and depreciate RM_RetainString.