Occasionally, during multiplanning, a $expr will fail nondeterministically during execution of the candidate plans. In the classic engine, we used ExprMatchExpressionMatchesReturnsFalseOnException as a failpoint to deal with these types of errors. However, since SBE cannot easily simulate a try/catch, we decided in SERVER-56152 to instead relax the fuzzers to handle these types of errors.

In agg_compare_results.js, we relax the fuzzers by checking if both sides used SBE, and if there was a failure in a $expr, we ignore it. However, the way we check if both sides used SBE is to examine the explain output. In some cases, such as BF-23057, the failure happens before the explain output has a chance to be generated. Therefore, it is not caught that both sides are using SBE, and the failure is not ignored by the fuzzer.

SERVER-60848, currently in-progress, will add logging and easier ways to check if SBE was used in a particular query. I propose that, rather than examining the explain output to check if both sides used SBE, we simply modify the check to access this information more directly, presumably checking a value that will be set/created in SERVER-60848.

However, there has been a lot of discussion surrounding this BF, and I am open to discussion and alternative approaches to this ticket.