Agreed, and this is why some languages have annotations that ask the compiler to check a function is indeed tail recursive.
However I don't think that is the case in Expat. If the algorithm is tail recursive, but accidentally not expressed in that way, its a simple (but perhaps tedious to manually apply) program transform to express it in a way that does not consume unbounded memory (heap or stack). From the scant details on the fix in the article it appears the parsing algorithm itself was completely changed. ("The third variant "Parameter entities" reuses ... the same mechanism of delayed interpretation.") If this is the case the issue is not recursion, as the original parsing algorithm would consume unbounded resources no matter how it was expressed in C.
However I don't think that is the case in Expat. If the algorithm is tail recursive, but accidentally not expressed in that way, its a simple (but perhaps tedious to manually apply) program transform to express it in a way that does not consume unbounded memory (heap or stack). From the scant details on the fix in the article it appears the parsing algorithm itself was completely changed. ("The third variant "Parameter entities" reuses ... the same mechanism of delayed interpretation.") If this is the case the issue is not recursion, as the original parsing algorithm would consume unbounded resources no matter how it was expressed in C.