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Data.mutableSpan and Data.mutableBytes do not perform uniqueness check #1530

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Merged
merged 4 commits into from
Oct 1, 2025
Merged

Data.mutableSpan and Data.mutableBytes do not perform uniqueness check #1530

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b7683a2
(161666595) Data.mutableSpan and Data.mutableBytes do not perform uni…
jmschonfeld Sep 30, 2025
b545ffd
Address feedback
jmschonfeld Oct 1, 2025
c100157
Fix Linux build failure
jmschonfeld Oct 1, 2025
1202f28
Fix test failure
jmschonfeld Oct 1, 2025
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24 changes: 20 additions & 4 deletions Sources/FoundationEssentials/Data/Data.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -2255,11 +2255,19 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
count: _representation.count
)
case .large(let slice):
case .large(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
_representation = .empty
slice.ensureUniqueReference()
_representation = .large(slice)
buffer = unsafe UnsafeMutableRawBufferPointer(
start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
)
case .slice(let slice):
case .slice(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
_representation = .empty
slice.ensureUniqueReference()
_representation = .slice(slice)
buffer = unsafe UnsafeMutableRawBufferPointer(
start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
)
Expand Down Expand Up @@ -2288,11 +2296,19 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
count: _representation.count
)
case .large(let slice):
case .large(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
_representation = .empty
slice.ensureUniqueReference()
_representation = .large(slice)
buffer = unsafe UnsafeMutableRawBufferPointer(
start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
)
case .slice(let slice):
case .slice(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
_representation = .empty
slice.ensureUniqueReference()
_representation = .slice(slice)
buffer = unsafe UnsafeMutableRawBufferPointer(
start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
)
Expand Down
44 changes: 44 additions & 0 deletions Tests/FoundationEssentialsTests/DataTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -1439,6 +1439,50 @@ private final class DataTests {
#expect(data.count == 0)
}
}

@Test func validateMutation_cow_mutableBytes() {
var data = Data(Array(repeating: 0, count: 32))
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Don't we have Data(count: 32) to get a zero-initialized instance?

holdReference(data) {
var bytes = data.mutableBytes
bytes.storeBytes(of: 1, toByteOffset: 0, as: UInt8.self)

#expect(data[0] == 1)
#expect(heldData?[0] == 0)
}

var data2 = Data(Array(repeating: 0, count: 32))
// Escape the pointer to compare after a mutation without dereferencing the pointer
let originalPointer = data2.withUnsafeBytes { $0.baseAddress }

var bytes = data2.mutableBytes
bytes.storeBytes(of: 1, toByteOffset: 0, as: UInt8.self)
#expect(data2[0] == 1)
data2.withUnsafeBytes {
#expect($0.baseAddress == originalPointer)
}
}

@Test func validateMutation_cow_mutableSpan() {
var data = Data(Array(repeating: 0, count: 32))
holdReference(data) {
var bytes = data.mutableSpan
bytes[0] = 1

#expect(data[0] == 1)
#expect(heldData?[0] == 0)
}

var data2 = Data(Array(repeating: 0, count: 32))
// Escape the pointer to compare after a mutation without dereferencing the pointer
let originalPointer = data2.withUnsafeBytes { $0.baseAddress }

var bytes = data2.mutableSpan
bytes[0] = 1
#expect(data2[0] == 1)
data2.withUnsafeBytes {
#expect($0.baseAddress == originalPointer)
}
}

@Test func sliceHash() {
let base1 = Data([0, 0xFF, 0xFF, 0])
Expand Down