src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs

@@ -681,14 +681,7 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
                 stream.Flush(true);
             }
 
-            if (File.Exists(path))
-            {
-                ReplaceExistingFile(tempPath, path);
-            }
-            else
-            {
-                File.Move(tempPath, path);
-            }
+            StoreReplace.Replace(tempPath, path);
         }
         finally
         {
@@ -746,26 +739,4 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
             }
         }
     }
-
-    private static void ReplaceExistingFile(string sourcePath, string destinationPath)
-    {
-        for (var attempt = 0; attempt < 3; attempt++)
-        {
-            try
-            {
-                File.Replace(sourcePath, destinationPath, null);
-                return;
-            }
-            catch (IOException) when (attempt < 2)
-            {
-                Thread.Sleep(10 * (attempt + 1));
-            }
-            catch (UnauthorizedAccessException) when (attempt < 2)
-            {
-                Thread.Sleep(10 * (attempt + 1));
-            }
-        }
-
-        File.Replace(sourcePath, destinationPath, null);
-    }
 }

src/GameCult.Caching/CultCache.cs

@@ -5204,14 +5204,7 @@ namespace GameCult.Caching
                     stream.Flush(true);
                 }
 
-                if (File.Exists(path))
-                {
-                    File.Replace(tempPath, path, null);
-                }
-                else
-                {
-                    File.Move(tempPath, path);
-                }
+                StoreReplace.Replace(tempPath, path);
             }
             finally
             {

src/GameCult.Caching/StoreReplace.cs

@@ -0,0 +1,182 @@
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Threading;
+
+namespace GameCult.Caching
+{
+    /// <summary>The kernel32 calls a Windows replace needs, each answering 0 or a Win32 error code.</summary>
+    internal interface IReplaceKernel
+    {
+        /// <summary>CreateFileW(staged, DELETE|SYNCHRONIZE), FileRenameInfoEx with REPLACE_IF_EXISTS|POSIX_SEMANTICS, CloseHandle.</summary>
+        int RenamePosix(string staged, string destination);
+
+        /// <summary>MoveFileExW(staged, destination, flags).</summary>
+        int MoveFileEx(string staged, string destination, uint flags);
+
+        void Sleep(int milliseconds);
+    }
+
+    /// <summary>
+    /// Owns how a staged file becomes the store, for the single-file and directory stores. Windows tries a POSIX-semantics
+    /// rename first (which makes a moment with the path absent rare for a lock-free reader, not impossible: a stalled rename was
+    /// measured leaving it absent for 5-47 ms), falls back to write-through MoveFileExW where that is unsupported or does not
+    /// resolve, falls back to the portable File.Replace if MoveFileExW itself does not resolve, and retries transient sharing
+    /// errors; other OSes keep rename. Mirrors cultcache-ts store-file.ts
+    /// replaceStoreFile (ruling variants:ruling:windows-replace-posix-first).
+    /// </summary>
+    internal static class StoreReplace
+    {
+        private const int ErrorAccessDenied = 5;
+        private const int ErrorSharingViolation = 32;
+        private const int ErrorLockViolation = 33;
+        private const int ErrorNotSupported = 50;
+        private const int ErrorInvalidParameter = 87;
+        private const int ReplaceAttempts = 25;
+        private const int ReplaceRetryMs = 20;
+
+        /// <summary>MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH.</summary>
+        internal const uint MoveFileReplaceWriteThrough = 0x9;
+
+        internal static readonly IReplaceKernel WindowsKernel = new Kernel32Replace();
+
+        public static void Replace(string staged, string destination)
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                ReplaceWindows(staged, destination, WindowsKernel);
+            }
+            else
+            {
+                ReplacePortable(staged, destination);
+            }
+        }
+
+        private static void ReplacePortable(string staged, string destination)
+        {
+            if (File.Exists(destination))
+            {
+                File.Replace(staged, destination, null);
+            }
+            else
+            {
+                File.Move(staged, destination);
+            }
+        }
+
+        private static bool Unresolved(Exception ex) => ex is DllNotFoundException || ex is EntryPointNotFoundException;
+
+        internal static void ReplaceWindows(string staged, string destination, IReplaceKernel kernel)
+        {
+            var posix = true;
+            var lastCode = 0;
+            for (var attempt = 1; attempt <= ReplaceAttempts; attempt++)
+            {
+                int code;
+                try
+                {
+                    code = posix ? RenamePosixOrUnsupported(kernel, staged, destination) : kernel.MoveFileEx(staged, destination, MoveFileReplaceWriteThrough);
+                    if (posix && (code == 1 || code == ErrorNotSupported || code == ErrorInvalidParameter))
+                    {
+                        // POSIX semantics are unsupported here (FAT, exFAT, before Windows 10 1607): the rest of this call is write-through.
+                        posix = false;
+                        code = kernel.MoveFileEx(staged, destination, MoveFileReplaceWriteThrough);
+                    }
+                }
+                catch (Exception ex) when (Unresolved(ex))
+                {
+                    // kernel32's MoveFileExW does not resolve here (a sandboxed player): the runtime-owned replace is all that is left.
+                    ReplacePortable(staged, destination);
+                    return;
+                }
+
+                if (code == 0)
+                {
+                    return;
+                }
+
+                if (code != ErrorAccessDenied && code != ErrorSharingViolation && code != ErrorLockViolation)
+                {
+                    throw new IOException($"CultCache could not replace the store: Win32 error {code}.");
+                }
+
+                lastCode = code;
+                kernel.Sleep(ReplaceRetryMs);
+            }
+
+            throw new IOException($"CultCache could not replace the store after {ReplaceAttempts} attempts: last Win32 error {lastCode}.");
+        }
+
+        private static int RenamePosixOrUnsupported(IReplaceKernel kernel, string staged, string destination)
+        {
+            try
+            {
+                return kernel.RenamePosix(staged, destination);
+            }
+            catch (Exception ex) when (Unresolved(ex))
+            {
+                // The POSIX call does not resolve: the same answer as "unsupported", so the replace falls back to MoveFileExW.
+                return ErrorNotSupported;
+            }
+        }
+
+        private sealed class Kernel32Replace : IReplaceKernel
+        {
+            private const uint DeleteSynchronize = 0x00110000;
+            private const uint ShareAll = 0x7;
+            private const uint OpenExisting = 3;
+            private const uint FileAttributeNormal = 0x80;
+            private const int FileRenameInfoEx = 22;
+            private const uint RenameReplacePosix = 0x3;
+
+            public int RenamePosix(string staged, string destination)
+            {
+                var handle = CreateFileW(staged, DeleteSynchronize, ShareAll, IntPtr.Zero, OpenExisting, FileAttributeNormal, IntPtr.Zero);
+                if (handle == new IntPtr(-1))
+                {
+                    return Marshal.GetLastWin32Error();
+                }
+
+                try
+                {
+                    // FILE_RENAME_INFO: Flags u32 at 0, RootDirectory handle, FileNameLength u32, FileName (UTF-16, offsets for this pointer size).
+                    var name = System.Text.Encoding.Unicode.GetBytes(Path.GetFullPath(destination));
+                    var lengthOffset = 2 * IntPtr.Size;
+                    var nameOffset = lengthOffset + 4;
+                    var size = nameOffset + name.Length + 2;
+                    var info = Marshal.AllocHGlobal(size);
+                    try
+                    {
+                        for (var i = 0; i < size; i++) Marshal.WriteByte(info, i, 0);
+                        Marshal.WriteInt32(info, 0, (int)RenameReplacePosix);
+                        Marshal.WriteInt32(info, lengthOffset, name.Length);
+                        Marshal.Copy(name, 0, info + nameOffset, name.Length);
+                        return SetFileInformationByHandle(handle, FileRenameInfoEx, info, (uint)size) ? 0 : Marshal.GetLastWin32Error();
+                    }
+                    finally { Marshal.FreeHGlobal(info); }
+                }
+                finally { CloseHandle(handle); }
+            }
+
+            public int MoveFileEx(string staged, string destination, uint flags)
+                => MoveFileExW(staged, destination, flags) ? 0 : Marshal.GetLastWin32Error();
+
+            public void Sleep(int milliseconds) => Thread.Sleep(milliseconds);
+
+            [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
+            private static extern IntPtr CreateFileW(string fileName, uint access, uint share, IntPtr security, uint disposition, uint flags, IntPtr template);
+
+            [DllImport("kernel32.dll", SetLastError = true)]
+            [return: MarshalAs(UnmanagedType.Bool)]
+            private static extern bool SetFileInformationByHandle(IntPtr handle, int informationClass, IntPtr information, uint size);
+
+            [DllImport("kernel32.dll", SetLastError = true)]
+            [return: MarshalAs(UnmanagedType.Bool)]
+            private static extern bool CloseHandle(IntPtr handle);
+
+            [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
+            [return: MarshalAs(UnmanagedType.Bool)]
+            private static extern bool MoveFileExW(string existing, string target, uint flags);
+        }
+    }
+}

tests/GameCult.Caching.Tests/StoreReplaceTests.cs

@@ -0,0 +1,268 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Threading;
+using NUnit.Framework;
+
+namespace GameCult.Caching.Tests
+{
+    public class StoreReplaceTests
+    {
+        private sealed class FakeKernel : IReplaceKernel
+        {
+            private readonly Queue<int> _posix;
+            private readonly Queue<int> _moveEx;
+            public readonly List<string> Log = new List<string>();
+            public readonly List<int> Sleeps = new List<int>();
+            public Exception? PosixThrows;
+            public Exception? MoveExThrows;
+
+            public FakeKernel(int[] posix, int[] moveEx)
+            {
+                _posix = new Queue<int>(posix);
+                _moveEx = new Queue<int>(moveEx);
+            }
+
+            public int RenamePosix(string staged, string destination)
+            {
+                Log.Add("posix");
+                if (PosixThrows != null) throw PosixThrows;
+                return _posix.Count > 0 ? _posix.Dequeue() : 0;
+            }
+
+            public int MoveFileEx(string staged, string destination, uint flags)
+            {
+                Log.Add("moveex:" + flags);
+                if (MoveExThrows != null) throw MoveExThrows;
+                return _moveEx.Count > 0 ? _moveEx.Dequeue() : 0;
+            }
+
+            public void Sleep(int milliseconds) => Sleeps.Add(milliseconds);
+        }
+
+        private static IOException Fails(FakeKernel kernel)
+            => Assert.Throws<IOException>(() => StoreReplace.ReplaceWindows("a", "b", kernel))!;
+
+        [Test]
+        public void APosixRenameThatWorksIsTheWholeReplace()
+        {
+            var kernel = new FakeKernel(new[] { 0 }, new int[0]);
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix" }));
+            Assert.That(kernel.Sleeps, Is.Empty);
+        }
+
+        [TestCase(1)]
+        [TestCase(50)]
+        [TestCase(87)]
+        public void WhereAPosixRenameIsUnsupportedTheReplaceFallsBackToWriteThroughMoveFileEx(int unsupported)
+        {
+            var kernel = new FakeKernel(new[] { unsupported }, new[] { 0 });
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+        }
+
+        [Test]
+        public void OnceThePosixRenameIsUnsupportedTheRestOfTheCallStaysOnMoveFileEx()
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { 32, 0 });
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9", "moveex:9" }));
+            Assert.That(kernel.Sleeps, Is.EqualTo(new[] { 20 }));
+        }
+
+        [TestCase(5)]
+        [TestCase(32)]
+        [TestCase(33)]
+        public void ATransientErrorRetriesTheSamePrimitiveAfterTwentyMilliseconds(int transient)
+        {
+            var kernel = new FakeKernel(new[] { transient, transient, 0 }, new int[0]);
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "posix", "posix" }));
+            Assert.That(kernel.Sleeps, Is.EqualTo(new[] { 20, 20 }));
+        }
+
+        [TestCase(1)]
+        [TestCase(50)]
+        [TestCase(87)]
+        public void AnUnsupportedCodeAfterTheFallbackIsAnErrorNotAnotherFallback(int unsupported)
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { unsupported });
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("Win32 error " + unsupported));
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+        }
+
+        private static IEnumerable<Exception> Unresolved()
+        {
+            yield return new DllNotFoundException("kernel32.dll");
+            yield return new EntryPointNotFoundException("SetFileInformationByHandle");
+        }
+
+        [TestCaseSource(nameof(Unresolved))]
+        public void APosixCallThatDoesNotResolveTakesTheUnsupportedFallback(Exception unresolved)
+        {
+            var kernel = new FakeKernel(new int[0], new[] { 0 }) { PosixThrows = unresolved };
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+        }
+
+        [TestCaseSource(nameof(Unresolved))]
+        public void AMoveFileExThatDoesNotResolveDegradesToThePortableReplace(Exception unresolved)
+        {
+            var dir = Path.Combine(TestContext.CurrentContext.WorkDirectory, "storereplace-" + Guid.NewGuid().ToString("N"));
+            Directory.CreateDirectory(dir);
+            try
+            {
+                var destination = Path.Combine(dir, "store.cc");
+                var staged = Path.Combine(dir, "staged");
+                File.WriteAllBytes(destination, new byte[] { 1 });
+                File.WriteAllBytes(staged, new byte[] { 2 });
+                var kernel = new FakeKernel(new[] { 87 }, new int[0]) { MoveExThrows = unresolved };
+                StoreReplace.ReplaceWindows(staged, destination, kernel);
+                Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 2 }));
+                Assert.That(File.Exists(staged), Is.False);
+
+                // Both calls unresolved: the same portable replace, here onto an absent destination.
+                File.Delete(destination);
+                File.WriteAllBytes(staged, new byte[] { 3 });
+                var both = new FakeKernel(new int[0], new int[0]) { PosixThrows = unresolved, MoveExThrows = unresolved };
+                StoreReplace.ReplaceWindows(staged, destination, both);
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 3 }));
+            }
+            finally { Directory.Delete(dir, true); }
+        }
+
+        [Test]
+        public void TheFallbackRunsInTheSameAttemptEvenOnTheLastOne()
+        {
+            var posix = new int[25].Fill(32);
+            posix[24] = 87;
+            var kernel = new FakeKernel(posix, new[] { 0 });
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log.Count, Is.EqualTo(26));
+            Assert.That(kernel.Log[24], Is.EqualTo("posix"));
+            Assert.That(kernel.Log[25], Is.EqualTo("moveex:9"));
+        }
+
+        [Test]
+        public void ARetriedMoveFileExThatAnswersUnsupportedFailsAfterOneCallNotTwo()
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { 32, 87 });
+            Assert.That(Fails(kernel).Message, Does.Contain("Win32 error 87"));
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9", "moveex:9" }));
+        }
+
+        [Test]
+        public void ARetryThatNeverSucceedsGivesUpAfterTwentyFiveAttemptsNamingTheCode()
+        {
+            var kernel = new FakeKernel(new int[30].Fill(32), new int[0]);
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("25 attempts").And.Contain("last Win32 error 32"));
+            Assert.That(kernel.Log.Count, Is.EqualTo(25));
+            Assert.That(kernel.Sleeps.Count, Is.EqualTo(25));
+        }
+
+        [Test]
+        public void AnyOtherCodeThrowsAtOnceNamingTheCode()
+        {
+            var kernel = new FakeKernel(new[] { 2 }, new int[0]);
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("Win32 error 2"));
+            Assert.That(kernel.Log.Count, Is.EqualTo(1));
+            Assert.That(kernel.Sleeps, Is.Empty);
+        }
+
+        [Test]
+        public void AFallbackThatFailsOtherwiseThrowsItsCode()
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { 3 });
+            Assert.That(Fails(kernel).Message, Does.Contain("Win32 error 3"));
+        }
+
+        [Test]
+        public void ReplaceOverAnExistingAndThenAnAbsentDestinationLeavesTheStagedBytes()
+        {
+            var dir = Path.Combine(TestContext.CurrentContext.WorkDirectory, "storereplace-" + Guid.NewGuid().ToString("N"));
+            Directory.CreateDirectory(dir);
+            try
+            {
+                var destination = Path.Combine(dir, "store.cc");
+                var staged = Path.Combine(dir, "staged");
+                File.WriteAllBytes(destination, new byte[] { 1 });
+                File.WriteAllBytes(staged, new byte[] { 2 });
+                StoreReplace.Replace(staged, destination);
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 2 }));
+                Assert.That(File.Exists(staged), Is.False);
+
+                File.Delete(destination);
+                File.WriteAllBytes(staged, new byte[] { 3 });
+                StoreReplace.Replace(staged, destination);
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 3 }));
+            }
+            finally { Directory.Delete(dir, true); }
+        }
+
+        [Test]
+        public void OnWindowsAReplaceOverADestinationAReaderHoldsOpenSucceedsOnTheFirstAttempt()
+        {
+            if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                Assert.Ignore("The real kernel runs on Windows only.");
+
+            var dir = Path.Combine(TestContext.CurrentContext.WorkDirectory, "storereplace-" + Guid.NewGuid().ToString("N"));
+            Directory.CreateDirectory(dir);
+            try
+            {
+                var destination = Path.Combine(dir, "store.cc");
+                var staged = Path.Combine(dir, "staged");
+                File.WriteAllBytes(destination, new byte[] { 1 });
+                File.WriteAllBytes(staged, new byte[] { 2 });
+                using (new FileStream(destination, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
+                {
+                    var kernel = new CountingKernel(StoreReplace.WindowsKernel);
+                    StoreReplace.ReplaceWindows(staged, destination, kernel);
+                    Assert.That(kernel.PosixCalls, Is.EqualTo(1));
+                    Assert.That(kernel.MoveExCalls, Is.EqualTo(0));
+                    Assert.That(kernel.Sleeps, Is.EqualTo(0));
+                    Assert.That(kernel.LastCode, Is.EqualTo(0));
+                }
+
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 2 }));
+            }
+            finally { Directory.Delete(dir, true); }
+        }
+
+        private sealed class CountingKernel : IReplaceKernel
+        {
+            private readonly IReplaceKernel _inner;
+            public int PosixCalls, MoveExCalls, Sleeps, LastCode;
+            public CountingKernel(IReplaceKernel inner) { _inner = inner; }
+
+            public int RenamePosix(string staged, string destination)
+            {
+                PosixCalls++;
+                return LastCode = _inner.RenamePosix(staged, destination);
+            }
+
+            public int MoveFileEx(string staged, string destination, uint flags)
+            {
+                MoveExCalls++;
+                return LastCode = _inner.MoveFileEx(staged, destination, flags);
+            }
+
+            public void Sleep(int milliseconds) { Sleeps++; _inner.Sleep(milliseconds); }
+        }
+    }
+
+    internal static class ArrayFill
+    {
+        public static int[] Fill(this int[] array, int value)
+        {
+            for (var i = 0; i < array.Length; i++) array[i] = value;
+            return array;
+        }
+    }
+}

tests/GameCult.Mesh.Tests/CultMeshSingleFileStoreTests.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
 using FluentAssertions;
@@ -364,11 +365,11 @@ public sealed class CultMeshSingleFileStoreTests
     [Test]
     public void ACacheOpeningDuringMeshWritesNeverSeesAnEmptyStore()
     {
-        // Windows replaces with ReplaceFile, which leaves a moment with nothing at the path for a lock-free reader: 1,722 empty
-        // stores in 17,313 opens at b1ff2dcb. That is the store's replace, not Mesh's, and it is the same for a cache flush beside a
-        // cache open; the fork is in the cut report.
-        if (OperatingSystem.IsWindows())
-            Assert.Ignore("Windows File.Replace is not atomic to a lock-free reader; see cut report fork mesh-reader-windows-replace-window.");
+        // Windows replaces with a POSIX-semantics rename first (StoreReplace), which makes a lock-free reader finding the path
+        // absent rare, not impossible: a stalled rename was measured leaving it absent for 5-47 ms on Windows. So on Windows a
+        // brief not-found (a read throwing, or a cache opening onto an absent path) is tolerated and reported; elsewhere rename
+        // is atomic and none is allowed.
+        var windows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
 
         var path = Path.Combine(_root, "race.cc");
         CultMesh.WriteSingleFileDocument(path, Key, Publication());
@@ -386,7 +387,7 @@ public sealed class CultMeshSingleFileStoreTests
             Volatile.Write(ref stop, 1);
         });
 
-        int opens = 0, empty = 0, reads = 0;
+        int opens = 0, empty = 0, reads = 0, absent = 0;
         var clock = Stopwatch.StartNew();
         while (Volatile.Read(ref stop) == 0 && clock.Elapsed < TimeSpan.FromSeconds(60))
         {
@@ -397,6 +398,10 @@ public sealed class CultMeshSingleFileStoreTests
                 CultMesh.ReadSingleFileDocumentPayload(path, Key, SchemaId());
                 reads++;
             }
+            catch (Exception ex) when (windows && (ex is FileNotFoundException || ex is DirectoryNotFoundException))
+            {
+                absent++;
+            }
             catch (Exception ex)
             {
                 errors.Enqueue("read: " + ex.GetType().Name + ": " + ex.Message);
@@ -404,9 +409,10 @@ public sealed class CultMeshSingleFileStoreTests
         }
 
         writer.Wait(TimeSpan.FromSeconds(60)).Should().BeTrue();
-        TestContext.Out.WriteLine($"mesh writes={writes} cache opens={opens} empty stores={empty} mesh reads ok={reads} errors={errors.Count}");
+        TestContext.Out.WriteLine($"mesh writes={writes} cache opens={opens} empty stores={empty} not-found reads={absent} mesh reads ok={reads} errors={errors.Count}");
         errors.Take(3).Should().BeEmpty();
-        empty.Should().Be(0, "a cache opened in the window where nothing was at the path");
+        if (!windows)
+            empty.Should().Be(0, "a cache opened in the window where nothing was at the path");
         opens.Should().BeGreaterThan(0);
     }