fix(filetransfer): register a default no-op IFileProvider

src/EntglDb.Services.FileTransfer/FileTransferExtensions.cs

@@ -1,51 +1,62 @@
-using EntglDb.Network;
-using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.DependencyInjection.Extensions;
-
-namespace EntglDb.Services.FileTransfer;
-
-/// <summary>
-/// Extension methods for registering EntglDb file transfer services.
-/// </summary>
-public static class FileTransferExtensions
-{
-    /// <summary>
-    /// Adds the EntglDb file transfer service to the DI container.
-    /// </summary>
-    /// <remarks>
-    /// <para>
-    /// Registers:
-    /// <list type="bullet">
-    ///   <item><see cref="FileQueryHandler"/> as <see cref="INetworkMessageHandler"/> (server-side, wire type 1100)</item>
-    ///   <item><see cref="FileDownloadHandler"/> as <see cref="INetworkMessageHandler"/> (server-side streaming, wire type 1102)</item>
-    ///   <item><see cref="IFileTransferService"/> / <see cref="FileTransferClient"/> (client-side download API)</item>
-    /// </list>
-    /// </para>
-    /// <para>
-    /// Call <c>AddEntglDbNetwork&lt;T&gt;()</c> before this method so that
-    /// <see cref="IPeerMessenger"/> and <see cref="IPeerConnectionPool"/> are already registered.
-    /// </para>
-    /// <para>
-    /// To make files available for remote download, also register an <see cref="IFileProvider"/>:
-    /// <code>
-    /// services.AddEntglDbFileTransfer();
-    /// services.AddSingleton&lt;IFileProvider, MyFileProvider&gt;();
-    /// </code>
-    /// Without an <see cref="IFileProvider"/> the server-side handlers are registered but will
-    /// return not-found for every request.
-    /// </para>
-    /// </remarks>
-    public static IServiceCollection AddEntglDbFileTransfer(this IServiceCollection services)
-    {
-        // Server side — file query (single response)
-        services.AddSingleton<INetworkMessageHandler, FileQueryHandler>();
-
-        // Server side — file download (streaming response)
-        services.AddSingleton<INetworkMessageHandler, FileDownloadHandler>();
-
-        // Client side
-        services.TryAddSingleton<IFileTransferService, FileTransferClient>();
-
-        return services;
-    }
-}
+using EntglDb.Network;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+
+namespace EntglDb.Services.FileTransfer;
+
+/// <summary>
+/// Extension methods for registering EntglDb file transfer services.
+/// </summary>
+public static class FileTransferExtensions
+{
+    /// <summary>
+    /// Adds the EntglDb file transfer service to the DI container.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Registers:
+    /// <list type="bullet">
+    ///   <item><see cref="FileQueryHandler"/> as <see cref="INetworkMessageHandler"/> (server-side, wire type 1100)</item>
+    ///   <item><see cref="FileDownloadHandler"/> as <see cref="INetworkMessageHandler"/> (server-side streaming, wire type 1102)</item>
+    ///   <item><see cref="IFileTransferService"/> / <see cref="FileTransferClient"/> (client-side download API)</item>
+    ///   <item><see cref="IFileProvider"/> / <see cref="NullFileProvider"/> (server-side default - see below)</item>
+    /// </list>
+    /// </para>
+    /// <para>
+    /// Call <c>AddEntglDbNetwork&lt;T&gt;()</c> before this method so that
+    /// <see cref="IPeerMessenger"/> and <see cref="IPeerConnectionPool"/> are already registered.
+    /// </para>
+    /// <para>
+    /// A node that only ever downloads files (never serves any) needs nothing further - it uses the
+    /// registered <see cref="NullFileProvider"/> default, reporting every file as not found. To make files
+    /// available for remote download instead, register a real provider <em>after</em> this call:
+    /// <code>
+    /// services.AddEntglDbFileTransfer();
+    /// services.AddSingleton&lt;IFileProvider, MyFileProvider&gt;();
+    /// </code>
+    /// <see cref="FileQueryHandler"/>/<see cref="FileDownloadHandler"/> take a single (non-enumerable)
+    /// <see cref="IFileProvider"/>, so the last registration wins - the call above's own provider overrides
+    /// the default even though both end up registered.
+    /// </para>
+    /// </remarks>
+    public static IServiceCollection AddEntglDbFileTransfer(this IServiceCollection services)
+    {
+        // Server side — file query (single response)
+        services.AddSingleton<INetworkMessageHandler, FileQueryHandler>();
+
+        // Server side — file download (streaming response)
+        services.AddSingleton<INetworkMessageHandler, FileDownloadHandler>();
+
+        // Client side
+        services.TryAddSingleton<IFileTransferService, FileTransferClient>();
+
+        // Server-side default so a download-only consumer doesn't have to implement IFileProvider itself
+        // just to satisfy FileQueryHandler/FileDownloadHandler's constructor dependency - see remarks
+        // above. Must be TryAddSingleton (not AddSingleton) so a real provider the caller registers
+        // afterward via plain AddSingleton becomes the winning "last registration" for the single-instance
+        // resolution FileQueryHandler/FileDownloadHandler actually use.
+        services.TryAddSingleton<IFileProvider, NullFileProvider>();
+
+        return services;
+    }
+}

src/EntglDb.Services.FileTransfer/NullFileProvider.cs

@@ -0,0 +1,22 @@
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace EntglDb.Services.FileTransfer;
+
+/// <summary>
+/// Default <see cref="IFileProvider"/> registered by <see cref="FileTransferExtensions.AddEntglDbFileTransfer"/> -
+/// reports every file as not found. Lets a node that only ever downloads files (never serves any) use
+/// <c>AddEntglDbFileTransfer()</c> without also having to implement and register an <see cref="IFileProvider"/>
+/// of its own just to satisfy <see cref="FileQueryHandler"/>/<see cref="FileDownloadHandler"/>'s constructor
+/// dependency. A node that does want to serve files registers its own provider after this call; DI resolves
+/// a single (non-enumerable) service to the last registration, so the real provider wins.
+/// </summary>
+public sealed class NullFileProvider : IFileProvider
+{
+    public Task<FileTransferInfo?> GetInfoAsync(string fileId, CancellationToken ct = default) =>
+        Task.FromResult<FileTransferInfo?>(null);
+
+    public Task<Stream?> OpenReadAsync(string fileId, CancellationToken ct = default) =>
+        Task.FromResult<Stream?>(null);
+}