fix: cache concatList.Size() to prevent O(N^2) evaluation time

[Flo354]

concatList.Size() recomputes the total size by recursively calling Size() on prevList and nextList. After N concatenation operations, the resulting tree has depth N, and each Size() call traverses the full tree. List operations like exists(), all(), and filter() call Size() on each iteration, producing O(N^2) total time.

This means the CEL cost tracker (which uses Size() for cost estimation) reports O(N) cost while the actual evaluation time is O(N^2), bypassing CostLimit.

With N=1000 and CostLimit set: reported cost is 17011 (within budget), but actual evaluation takes 12s (706x the expected time).

The fix caches the computed size in a sync.Once field so repeated calls return in O(1).

Files changed: common/types/list.go

[jnthntatum]

/gcbrun

[Flo354]

According to your last comment, I have added a newConcatList() constructor that precomputes the size, both Add() methods go through it now and removed the sync.

[TristonianJones]

Thanks, @Flo354!

[TristonianJones]

/gcbrun

[Flo354]

I did the changes, it's better indeed.
And from your comment on my other PR, I added a test case. It's on two angles. The first one verifies the size is correct, and the second one to check about the time required.

[jnthntatum]

/gcbrun