Frequently, tests must do something like:

• enable a failpoint,
• wait for the code to reach the point where it checks if the failpoint is enabled,
• the test continues.

Today we do this by manually adding a log line to the C++ code like:

        if (MONGO_unlikely(rsSyncApplyStop.shouldFail())) {
            log() << "Oplog Applier - rsSyncApplyStop fail point enabled. Blocking until fail "
                     "point is disabled.";
            /* ... snip ... */
        }

Then in Javascript we use the checkLog mechanism to wait for that log message to appear. This is prone to various errors:

• We don't apply this pattern everywhere we need it, which causes race conditions like SERVER-43703
• We don't apply this pattern in Python test fixtures, because we haven't implemented checkLog in Python yet
• We forget to clear the log before enabling the failpoint, so we see an old log message and continue immediately instead of waiting for the new log message
• We change a log message because we didn't know a test depended on it

It would be awesome if we had a better mechanism for synchronizing tests with failpoints. I propose something approximately like:

        if (MONGO_unlikely(rsSyncApplyStop.shouldFail())) {
            rsSyncApplyStop.signal(); /* new method */
            /* ... snip ... */
        }

Then in jsTests:

        db.adminCommand({
            configureFailPoint: 'rsSyncApplyStop',
            mode: 'alwaysOn',
            wait: true /* new parameter */
        });

If "wait" is true, the command first changes the failpoint's mode, then waits for the next call to signal() before returning. If the mechanism were this convenient I think we would use it more consistently and with fewer mistakes.