Fix generated examples with small maximum lengths - #4009
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Goa can panic while generating examples for a valid array with
MaxLength(1). With no minimum,NewLengthsubtracts a random value from zero through two; subtracting two produces a negative allocation size. A maximum of zero has the same problem.Bound the possible subtraction by the declared maximum before generating the value. Zero and one now produce only valid lengths. Maxima of two or more preserve the existing calculation, example-size cap, and number of random draws. The shared calculation fixes string, byte, array, and map examples without changing the DSL or validation contracts.
The new deterministic regression reproduces 12 failures on the old implementation and passes all 51 cases with the fix. The complete
./exprsuite passes on Go 1.27.1: 227 top-level tests and 486 subtests. Tests exercise both the length calculation and actual example construction, including unchanged larger bounds.No runtime wire format, caller API, or persisted data changes; no migration is needed. Consumers encountering this generation failure should update Goa and regenerate. Only generated example values for these small maximum lengths may change.