Ensure that hashing data with zero bytes avoids empty allocations and fix bridged empty data hashes from de-referencing null values

Author: phausler

stdlib/public/SDK/Foundation/Data.swift

@@ -1626,9 +1626,11 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
     public var hashValue: Int {
         var hashValue = 0
         let hashRange: Range<Int> = _sliceRange.lowerBound..<Swift.min(_sliceRange.lowerBound + 80, _sliceRange.upperBound)
-        _withStackOrHeapBuffer(hashRange.count) { buffer in
-            _backing.withUnsafeBytes(in: hashRange) {
-                memcpy(buffer.pointee.memory, $0.baseAddress!, hashRange.count)
+        _withStackOrHeapBuffer(hashRange.count + 1) { buffer in
+            if hashRange.count > 0 {
+                _backing.withUnsafeBytes(in: hashRange) {
+                    memcpy(buffer.pointee.memory, $0.baseAddress!, hashRange.count)
+                }
             }
             hashValue = Int(bitPattern: CFHashBytes(buffer.pointee.memory.assumingMemoryBound(to: UInt8.self), hashRange.count))
         }

test/stdlib/TestData.swift

@@ -3663,6 +3663,20 @@ class TestData : TestDataSuper {
         expectEqual(Data(bytes: [0, 1, 2, 3, 4]), regionData[1]) //passes
         expectEqual(Data(bytes: [0]), regionData[2]) //fails
     }
+
+    func test_hashEmptyData() {
+        let d1 = Data()
+        let h1 = d1.hashValue
+
+        let d2 = NSData() as Data
+        let h2 = d2.hashValue
+        expectEqual(h1, h2)
+
+        let data = Data(bytes: [0, 1, 2, 3, 4, 5, 6])
+        let d3 = data[4..<4]
+        let h3 = d3.hashValue
+        expectEqual(h1, h3)
+    }
 }
 
 #if !FOUNDATION_XCTEST
@@ -3973,6 +3987,7 @@ DataTests.test("test_validateMutation_slice_cow_customMutableBacking_replaceSubr
 DataTests.test("test_sliceHash") { TestData().test_sliceHash() }
 DataTests.test("test_slice_resize_growth") { TestData().test_slice_resize_growth() }
 DataTests.test("test_sliceEnumeration") { TestData().test_sliceEnumeration() }
+DataTests.test("test_hashEmptyData") { TestData().test_hashEmptyData() }
 
 // XCTest does not have a crash detection, whereas lit does
 DataTests.test("bounding failure subdata") {