Fix RawTenantAccessWorkload Unsigned difference expression compared to zero

[odaysec]

foundationdb/fdbserver/workloads/RawTenantAccessWorkload.actor.cpp

Line 90 in c289917

bool hasExistingTenant() const { return idx2Tid.size() - lastDeletedTenants.size() > 0; }

This rule finds relational comparisons between the result of an unsigned subtraction and the value 0. Such comparisons are likely to be wrong as the value of an unsigned subtraction can never be negative. So the relational comparison ends up checking whether the result of the subtraction is equal to 0. This is probably not what the programmer intended.

Fix the issue, we need to ensure that the subtraction operation does not underflow. The best approach is to cast the unsigned values to a signed type (e.g., int64_t) before performing the subtraction. This ensures that the result of the subtraction can represent negative values if lastDeletedTenants.size() is greater than idx2Tid.size(). The comparison will then behave as intended.

The specific change involves modifying the expression idx2Tid.size() - lastDeletedTenants.size() > 0 to (int64_t)idx2Tid.size() - (int64_t)lastDeletedTenants.size() > 0.

uint32_t limit = get_limit();
uint32_t total = 0;

while (limit - total > 0) { // BAD: if `total` is greater than `limit` this will underflow and continue executing the loop.
  total += get_data();
}

while (total < limit) { // GOOD: never underflows here because there is no arithmetic.
  total += get_data();
}

while ((int64_t)limit - total > 0) { // GOOD: never underflows here because the result always fits in an `int64_t`.
  total += get_data();
}

INT02-C. Understand integer conversion rules

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[xis19]

Appreciate your contribution! It is always not-so-nice to do substractions over unsigned variables. And I am certain this function is problematic. However, this patch is assuming that the value of idx2Tid.size() and lastDeletedTenants.size() are both smaller than std::numeric_limits::int64_t::max(). This may be true or may not be true. So I would recommend a fix looks like

idx2Tid.size() > lastDeletedTenants.size()

and the assumption will be unnecessary in this case. Would you mind update the patch? Thanks a lot!

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