Fix misleading zeroTime documentation in TokenBucket.h#2658
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The constructors and reset() of TokenBucketStorage, BasicDynamicTokenBucket, and BasicTokenBucket document the default zeroTime = 0 as producing an empty bucket. The actual balance is min((time - zeroTime) * rate, burstSize), so zeroTime = 0 (the clock epoch) yields a *full* bucket at all but very low rates, and a partially-filled one at very low rates. This mismatch caused the confusion in facebook#2430, where a default-constructed DynamicTokenBucket at rate 0.0001 was expected to hold one token but held only uptime * rate tokens. Document the real semantics and how to start with an actually-empty bucket. Fixes facebook#2430 Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Summary
The constructors and
reset()ofTokenBucketStorage,BasicDynamicTokenBucket, andBasicTokenBucketdocument the defaultzeroTime = 0as producing an empty bucket. The actual balance ismin((time - zeroTime) * rate, burstSize), so the default of 0 (the clock epoch -- machine boot forsteady_clock) yields a full bucket at all but very low rates, and a partially-filled one at very low rates.This mismatch is the root cause of the confusion in #2430: a default-constructed
DynamicTokenBucketatrate = 0.0001was expected to hold one token (empty buckets that immediately served single-token consumes at normal rates reinforced that expectation), but it actually helduptime * ratetokens (~0.376 on the reporter's machine), soconsumeWithBorrowNonBlockingcorrectly borrowed the remaining ~0.624 tokens and returned a wait of0.624 / 0.0001 = 6241.23s-- exactly the value in the reported failure. There is no precision bug; the documentation was describing semantics the code never had.This updates the five affected doc comments to state the real balance formula and how to start with an actually-empty bucket (pass the current time /
defaultClockNow()aszeroTime).Fixes #2430
Test plan
Documentation-only change; no code modified.
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