Fix empty and directory-only 7z archive finalization

src/SharpCompress/Writers/SevenZip/SevenZipWriter.Async.cs

@@ -43,6 +43,8 @@ public partial class SevenZipWriter
 
     private async ValueTask FinalizeArchiveAsync()
     {
+        // Reserve the signature even when no file data was written.
+        await EnsurePlaceholderWrittenAsync(CancellationToken.None).ConfigureAwait(false);
         var output = OutputStream.NotNull();
 
         // Current position = end of packed data streams

src/SharpCompress/Writers/SevenZip/SevenZipWriter.cs

@@ -193,6 +193,8 @@ public partial class SevenZipWriter : AbstractWriter
 
     private void FinalizeArchive()
     {
+        // Directory-only and empty archives never pass through Write().
+        EnsurePlaceholderWritten();
         var output = OutputStream.NotNull();
 
         // Current position = end of packed data streams

tests/SharpCompress.Test/SevenZip/SevenZipWriterStreamlessTests.cs

@@ -0,0 +1,89 @@
+using System.IO;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using SharpCompress.Archives.SevenZip;
+using SharpCompress.Writers.SevenZip;
+using Xunit;
+
+namespace SharpCompress.Test.SevenZip;
+
+public class SevenZipWriterStreamlessTests
+{
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task SevenZipWriter_Streamless_ClosesOwnedStream(bool async)
+    {
+        using var output = new MemoryStream();
+        var writer = new SevenZipWriter(
+            output,
+            new SevenZipWriterOptions { LeaveStreamOpen = false }
+        );
+        writer.WriteDirectory("dir", null);
+        if (async)
+        {
+            await writer.DisposeAsync();
+        }
+        else
+        {
+            writer.Dispose();
+        }
+        Assert.False(output.CanRead);
+        using var input = new MemoryStream(output.ToArray());
+        using var archive = SevenZipArchive.OpenArchive(input);
+        Assert.True(archive.Entries.Single().IsDirectory);
+    }
+
+    [Theory]
+    [InlineData(false, false, 0)]
+    [InlineData(false, true, 0)]
+    [InlineData(true, false, 0)]
+    [InlineData(true, true, 0)]
+    [InlineData(false, false, 2)]
+    [InlineData(false, true, 2)]
+    [InlineData(true, false, 2)]
+    [InlineData(true, true, 2)]
+    public async Task SevenZipWriter_Streamless_RoundTrip(
+        bool async,
+        bool compressHeader,
+        int directories
+    )
+    {
+        using var output = new MemoryStream();
+        var writer = new SevenZipWriter(
+            output,
+            new SevenZipWriterOptions { CompressHeader = compressHeader }
+        );
+        for (var i = 0; i < directories; i++)
+        {
+            if (async)
+            {
+                await writer.WriteDirectoryAsync($"dir{i}", null, CancellationToken.None);
+            }
+            else
+            {
+                writer.WriteDirectory($"dir{i}", null);
+            }
+        }
+        if (async)
+        {
+            await writer.DisposeAsync();
+        }
+        else
+        {
+            writer.Dispose();
+        }
+
+        Assert.True(output.CanRead);
+        var bytes = output.ToArray();
+        var offset = System.BitConverter.ToUInt64(bytes, 12);
+        var size = System.BitConverter.ToUInt64(bytes, 20);
+        Assert.Equal((ulong)bytes.Length, 32 + offset + size);
+        output.Position = 0;
+        using var archive = SevenZipArchive.OpenArchive(output);
+        var entries = archive.Entries.ToList();
+        Assert.Equal(directories, entries.Count);
+        Assert.All(entries, entry => Assert.True(entry.IsDirectory));
+    }
+}