fix(run-engine): decrement totalWeight in fair-queue weighted env shuffle#4019
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…ffle The weighted env shuffle drew its random pivot against the full set's total weight on every iteration but never reduced that total as items were removed. Once items had been picked, the pivot routinely overshot the remaining weight, the selection loop ran off the end, and the last remaining item was chosen, over-picking whichever env sat at the tail of the set. With fair-queue biases enabled (the default), this left the first env slot fair but skewed later positions by env iteration order instead of by the intended concurrency and capacity weighting. Decrement totalWeight before splicing, matching the sibling queue-order and top-env selection paths.
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Summary
Fixes the fair-queue weighted environment shuffle, which biased environment ordering whenever fair-queue biases are enabled (the default configuration).
Root cause
#weightedShuffleinfairQueueSelectionStrategy.tscomputed the total weight once and drew its random pivot against that full-set total on every iteration, but never decremented the total as items were removed from the working set. After the first pick, the pivot frequently overshot the sum of the remaining items, so the inner selection loop ran off the end and clamped to the last remaining element. The result systematically over-selected whichever environment sat at the tail of the set.The first slot stayed fair (the full total is correct on the first draw), but later positions were ordered by environment iteration order rather than by the intended concurrency-limit and available-capacity weighting. For four equal-weight environments, the final position landed on one env ~9% of the time and another ~42%, instead of ~25% each.
The two sibling selection paths (
#weightedRandomQueueOrderand#selectTopEnvs) already decrement the total before splicing; this brings the env shuffle in line with them.Fix
Adds a regression test that runs the weighted shuffle over equal-weight envs with biases enabled and asserts each env lands in every position roughly uniformly. It fails on the old code (tail position ~37%) and passes with the fix.
Reported in #4001.