Data slice mutation support (#11939) #12011
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* Mutations of slices of data should preserve relative indexing as well as cow semantics of slices * Ensure hashes of ranges are uniform to the expected hash for Data * Correct a few mistakes in the slice mutation tests * Update sequence initializations to avoid directly calling mutableCopy which prevents slice offset mismatches * Avoid invalid index slices in creating mirrors * Restore the original Data description * Resetting a slice region should expand the slice to the maximum of the region (not a out of bounds index of the backing buffer) * Remove stray comment and use a stack buffer for sequence appending * Return false when allocations fail in _resizeConditionalAllocationBuffer (not yet in use) * Enumeration of regions of a slice should be limited to the slice range in the case of custom backing (e.g. dispatch_data_t) * adjust assertion warnings for data indexes that are negative
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Explanation:
Mutation of Data slices had issues when it comes to re-indexing to the base slice region. Previously when a slice of Data was mutated the re-calculation of the slice region was incorrectly assigned to a relative region to the previous backing. Now the slice region will copy the specific region out creating a new data upon CoW semantics and offset the index region upon access. Mutating a non CoW slice will retain the previous region backing but adjust the remaining allocation buffer accordingly. To validate this I have added tests for combinations that should approximate all possible combinations of mutations, backing storages and CoW scenarios.
Scope:
This change applies to Data in all cases for the fix. It modifies all codes paths that handle mutation and re-works the slice region calculations.
Radar (and possibly SR Issue):
rdar://problem/34206043
SR-5887
SR-5873
SR-5810
Risk:
This has medium to high risk (for Data itself) since it is a critical path for the manipulation and mutation of Data. However mitigating funnel points were used to reduce overall impact as well as a large swath of tests covering the changes.
Reviewed by:
Itai Ferber, Michael LeHew Jr, and Tony Parker
Testing:
240+ new tests were added to cover all possible permutations of mutations and backing storage of Data while sliced.