(#102) FileCache: support older versions of Windows

Emulsion.ContentProxy/FileCache.fs

@@ -83,6 +83,30 @@ type FileCache(logger: ILogger,
         )
     }
 
+    let deleteFileSafe (fileInfo: FileInfo) = async {
+        if OperatingSystem.IsWindows() then
+            // NOTE: On older versions of Windows (known to reproduce on windows-2019 GitHub Actions image), the
+            // following scenario may be defunct:
+            // - open a file with FileShare.Delete (i.e. for download)
+            // - delete a file (i.e. during the cache cleanup)
+            // - try to create a file with the same name again
+            //
+            // According to this article
+            // (https://boostgsoc13.github.io/boost.afio/doc/html/afio/FAQ/deleting_open_files.html), it is impossible
+            // to do since file will occupy its disk name until the last handle is closed.
+            //
+            // In practice, this is allowed (checked at least on Windows 10 20H2 and windows-2022 GitHub Actions image),
+            // but is known to be broken on older versions of Windows (windows-2019).
+            //
+            // As a workaround, let's rename the file to a random name before deleting it.
+            //
+            // This workaround may be removed after these older versions of Windows goes out of support.
+            fileInfo.MoveTo(Path.Combine(fileInfo.DirectoryName, $"{Guid.NewGuid().ToString()}.deleted"))
+            fileInfo.Delete()
+        else
+            fileInfo.Delete()
+    }
+
     let ensureFreeCache size = async {
         if size > settings.FileSizeLimitBytes || size > settings.TotalCacheSizeLimitBytes then
             return false
@@ -106,7 +130,7 @@ type FileCache(logger: ILogger,
                 currentSize <- currentSize + Checked.uint64 info.Length
                 if currentSize + size > settings.TotalCacheSizeLimitBytes then
                     logger.Information("Deleting a cache item \"{FileName}\" ({Size} bytes)", info.Name, info.Length)
-                    info.Delete()
+                    do! deleteFileSafe info
 
             return true
     }