Re: Accessing an invalid pointer in BufferManagerRelation structure

Hello,

Dmitriy Bondar and I decided to review this patch as part of the CommitFest Review.  

The first thing we both noticed is that the macro calls a function that won't be available without an additional header. This seems a bit inconvenient.  

I also have a question: is the logic correct that if the relation is valid, we should fetch it rather than the other way around? Additionally, is checking only the `rd_isvalid` flag sufficient, or should we also consider the flag below?  

```
bool rd_isvalid; /* relcache entry is valid */
```  

Other than that, the tests pass, and there are no issues with code style. Thanks for the patch - it could indeed help prevent future issues.
Best regards,
Stepan Neretin