[Windows] Fix Interpretation of Volume Name Array

Taking &wszPathNAmes[...] doesn't properly get the string and leads to
garbled data. Instead, compute the pointer address and pass that.

Author: gmittert

Foundation/FileManager+Win32.swift

@@ -39,19 +39,19 @@ extension FileManager {
 
             var wszPathNames: [WCHAR] = Array<WCHAR>(repeating: 0, count: Int(dwCChReturnLength + 1))
             if !GetVolumePathNamesForVolumeNameW(&wszVolumeName, &wszPathNames, DWORD(wszPathNames.count), &dwCChReturnLength) {
-                // TODO(compnerd) handle error
-                continue
+                return nil
             }
 
-            var pPath: DWORD = 0
-            repeat {
-                let path: String = String(decodingCString: &wszPathNames[Int(pPath)], as: UTF16.self)
-                if path.length == 0 {
-                    break
-                }
-                urls.append(URL(fileURLWithPath: path, isDirectory: true))
-                pPath += DWORD(path.length + 1)
-            } while pPath < dwCChReturnLength
+            var pPath: Int = 0
+            while wszPathNames[pPath] != 0 {
+                guard let (url, pathLength): (URL, Int) = wszPathNames.withUnsafeBufferPointer({
+                    guard let pathStart = $0.baseAddress else { return nil }
+                    let path = String(decodingCString: pathStart + pPath, as: UTF16.self)
+                    return (URL(fileURLWithPath: path, isDirectory: true), path.length + 1)
+                }) else { return nil }
+                urls.append(url)
+                pPath += pathLength
+            }
         } while FindNextVolumeW(hVolumes, &wszVolumeName, DWORD(wszVolumeName.count))
 
         return urls

TestFoundation/TestFileManager.swift

@@ -975,7 +975,12 @@ class TestFileManager : XCTestCase {
             return
         }
         XCTAssertNotEqual(0, volumes.count)
+#if os(Windows)
+        let url = URL(fileURLWithPath: String(NSTemporaryDirectory().prefix(3)))
+        XCTAssertTrue(volumes.contains(url))
+#else
         XCTAssertTrue(volumes.contains(URL(fileURLWithPath: "/")))
+#endif
 #if os(macOS)
         // On macOS, .skipHiddenVolumes should hide 'nobrowse' volumes of which there should be at least one
         guard let visibleVolumes = FileManager.default.mountedVolumeURLs(includingResourceValuesForKeys: [], options: [.skipHiddenVolumes]) else {