Underflow error when size of pinned queries is larger than CACHE_MAX_SIZE

[hannahbast]

Since #1067, memory sizes are no longer specified as integers, but with units, and they have a proper type in QLever.

However, that change gave rise to an error that did not occur before (or went unnoticed). Namely, when the size of the pinned queries is larger than CACHE_MAX_SIZE, then any further query results in the following error message:

Underflow error: Subtraction of the two given 'MemorySize's is not possible. It would result in a size_t underflow.

There are two possible fixes (and maybe it makes sense to do both):

1. Don't allow pinning of a query when the total size of the pinned queries would then exceed CACHE_MAX_SIZE. This would also be consistent with the fact that it is already not allowed now that a query is pinned, the size of which exceeds CACHE_MAX_SIZE_SINGLE_ENTRY.

2. Catch the underflow error above and return a proper error message (something along the lines of: no memory left; we already have such a message in other places).

@schlegan @joka921 Can you have a look and what do you think?

[schlegan]

Alright, I took a look and found the only line, which could be responsible for this error in src/util/Cache.h:343:

if (_maxSize - _totalSizePinned < sizeToMakeRoomFor) {
      return false;
}

I'm definitely for the first solution (Don't allow pinning of a query when the total size of the pinned queries would then exceed CACHE_MAX_SIZE.) and I was under the impression, that this was already the case.

After all, the documentation of the FlexibleCache says:

@brief Associative array for almost arbitrary keys and values that acts as a cache with fixed memory capacity.

What would be the point of a cache with a maximum capacity, if the maximum capacity is ignored?