Use correct start index when accessing Data slices (swiftlang#408)

Author: parkera

Sources/FoundationEssentials/Data/Data+Writing.swift

@@ -75,7 +75,7 @@ private func writeToFileDescriptorWithProgress(_ fd: Int32, data: Data, reportPr
         }
     }
 
-    var nextRange = 0..<length
+    var nextRange = data.startIndex..<data.startIndex.advanced(by: length)
     var numBytesRemaining = length
     while numBytesRemaining > 0 {
         if let localProgress, localProgress.isCancelled {
@@ -117,7 +117,7 @@ private func writeToFileDescriptorWithProgress(_ fd: Int32, data: Data, reportPr
                     break
                 }
 
-                nextRange = numBytesWritten..<data.count
+                nextRange = nextRange.startIndex.advanced(by: numBytesWritten)..<data.endIndex
             }
         } while numBytesWritten < 0 && errno == EINTR
     }

Tests/FoundationEssentialsTests/DataIOTests.swift

@@ -43,15 +43,17 @@ class DataIOTests : XCTestCase {
 
     func generateTestData() -> Data {
         // 16 MB file, big enough to trigger things like chunking
-        let count = 1 << 24
+        let count = 16_777_216
 
         let memory = malloc(count)!
         let ptr = memory.bindMemory(to: UInt8.self, capacity: count)
 
         // Set a few bytes so we're sure to not be all zeros
         let buf = UnsafeMutableBufferPointer(start: ptr, count: count)
-        for i in 0..<128 {
-            buf[i] = UInt8.random(in: 1..<42)
+        for i in 0..<15 {
+            for j in 0..<128 {
+                buf[j * 1024 + i] = UInt8.random(in: 1..<42)
+            }
         }
 
         return Data(bytesNoCopy: ptr, count: count, deallocator: .free)
@@ -94,6 +96,25 @@ class DataIOTests : XCTestCase {
         try writeAndVerifyTestData(to: url)
         cleanup(at: url)
     }
+    
+    func test_slicedReadWrite() throws {
+        // Be sure to use progress reporting so we get tests of the chunking
+        let url = testURL()
+        let data = generateTestData()
+        let slice = data[data.startIndex.advanced(by: 1 * 1024 * 1024)..<data.startIndex.advanced(by: 8 * 1024 * 1024)]
+
+#if FOUNDATION_FRAMEWORK
+        let p = Progress(totalUnitCount: 1)
+        p.becomeCurrent(withPendingUnitCount: 1)
+#endif
+        try slice.write(to: url, options: [])
+#if FOUNDATION_FRAMEWORK
+        p.resignCurrent()
+#endif
+        let readData = try Data(contentsOf: url, options: [])
+        XCTAssertEqual(readData, slice)
+        cleanup(at: url)
+    }
 
     // Atomic writing is a very different code path
     func test_readWriteAtomic() throws {