reproducible builds working as expected

explicitly setting tar archive entry modified dates & gzip modified dates. 

https://buildpacks.io/docs/features/reproducibility/
buildpacks/lifecycle#823
buildpacks/lifecycle#108

src/Dotknet/Commands/PublishCommand.cs

@@ -41,8 +41,8 @@ public void Execute()
 
     using var tarArchive = TarArchive.Create();
     tarArchive.AddAllFromDirectory(_options.Output!, "dotknet-app");
-
     using var layer = new TarArchiveLayer(tarArchive);
-    _logger.LogInformation("Layer Created: {Layer}", layer);
+
+    _logger.LogInformation("Output will create layer {Layer}", layer);
   }
 }

src/Dotknet/Extensions/IWritableArchiveExtensions.cs

@@ -2,15 +2,19 @@
 
 public static class IWritableArchiveExtensions
 {
-  public static void AddAllFromDirectory(this IWritableArchive writableArchive, string filePath, string rootDir, string searchPattern = "*.*", SearchOption searchOption = SearchOption.AllDirectories)
+  // timestamp necessary for reproducability
+  public static void AddAllFromDirectory(this IWritableArchive writableArchive, string filePath, string rootDir, string searchPattern = "*.*", SearchOption searchOption = SearchOption.AllDirectories, DateTime? timestamp = null)
   {
+    if (!timestamp.HasValue) {
+      timestamp = new DateTime(1980, 01, 01, 00, 00, 01);
+    }
     using (writableArchive.PauseEntryRebuilding())
     {
       foreach (var path in Directory.EnumerateFiles(filePath, searchPattern, searchOption))
       {
         var fileInfo = new FileInfo(path);
-        var key = rootDir + "/" + path.Substring(filePath.Length);
-        writableArchive.AddEntry(key, fileInfo.OpenRead(), true, fileInfo.Length, fileInfo.LastWriteTime);
+        var key = rootDir + path.Substring(filePath.Length);
+        writableArchive.AddEntry(key, fileInfo.OpenRead(), true, fileInfo.Length, timestamp);
       }
     }
   }

src/Dotknet/Models/Layer.cs

@@ -1,10 +1,11 @@
+using System.IO.Compression;
 using Dotknet.Extensions;
 using SharpCompress.Archives.Tar;
 
 namespace Dotknet.Models;
 
 /// ref: https://github.com/google/go-containerregistry/blob/main/pkg/v1/layer.go
-public interface ILayer: IDisposable
+public interface ILayer : IDisposable
 {
   // Digest returns the Hash of the compressed layer.
   public Hash Digest();
@@ -28,19 +29,34 @@ public interface ILayer: IDisposable
 //todo: consider perf/caching
 public class TarArchiveLayer : ILayer
 {
-  public TarArchiveLayer(TarArchive tarArchive)
+  // timestamp necessary for reproducibility
+  public TarArchiveLayer(TarArchive tarArchive, DateTime? modified = null)
   {
     _tarArchive = tarArchive;
+    if (!modified.HasValue)
+    {
+      modified = new DateTime(1980, 01, 01, 00, 00, 01, DateTimeKind.Utc);
+    }
+    _modified = modified;
   }
 
+  private DateTime EPOCH = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+
   private bool disposedValue;
   private readonly TarArchive _tarArchive;
+  private readonly DateTime? _modified;
 
   public Stream Compressed()
   {
-    var memoryStream = new MemoryStream();
-    _tarArchive.SaveTo(memoryStream, new SharpCompress.Writers.WriterOptions(SharpCompress.Common.CompressionType.GZip));
-    return memoryStream;
+    using var originalStream = new MemoryStream();
+    _tarArchive.SaveTo(originalStream, new SharpCompress.Writers.WriterOptions(SharpCompress.Common.CompressionType.GZip));
+    var buffer = originalStream.ToArray();
+    var seconds = (int)(_modified! - EPOCH).Value.TotalSeconds;
+    buffer[4] = (byte)seconds;
+    buffer[5] = (byte)(seconds >> 8);
+    buffer[6] = (byte)(seconds >> 16);
+    buffer[7] = (byte)(seconds >> 24);
+    return new MemoryStream(buffer);
   }
 
   public Hash DiffId()
@@ -89,7 +105,8 @@ public void Dispose()
     GC.SuppressFinalize(this);
   }
 
-  public override string ToString(){
+  public override string ToString()
+  {
     return $"Layer DiffId: {DiffId().Hex.Substring(0, 6)} Digest: {Digest().Hex.Substring(0, 6)}";
   }
 }

test/Dotknet.Test/Models/LayerTests.cs

@@ -0,0 +1,44 @@
+using System;
+using System.IO;
+using System.Linq;
+using Dotknet.Models;
+using FluentAssertions;
+using SharpCompress.Archives.Tar;
+using Xunit;
+
+namespace Dotknet.Test.Models;
+
+public class LayerTests
+{
+  [Fact]
+  public void LayerModifiedTests()
+  {
+    var archive = TarArchive.Create();
+    var timestamp = new DateTime(1990, 05, 13, 0, 0, 0);
+    using var text = GenerateStreamFromString("foo foo foo foo foo foo foo");
+    archive.AddEntry("bar", text, text.Length, timestamp);
+    archive.AddEntry("baz", text, text.Length, timestamp);
+    using var layer = new TarArchiveLayer(archive);
+    using var compressed = layer.Compressed();
+    compressed.Seek(0, SeekOrigin.Begin);
+    using var ms2 = new MemoryStream();
+    compressed.CopyTo(ms2);
+    ms2.Seek(0, SeekOrigin.Begin);
+    var bytes = ms2.ToArray();
+    var timestampBytes = bytes.Skip(4).Take(4).ToArray();
+    var seconds = BitConverter.ToUInt32(timestampBytes, 0);
+    var EPOCH = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+    var datetime = EPOCH.AddSeconds(seconds);
+    datetime.Should().Be(new DateTime(1980, 01, 01, 00, 00, 01, DateTimeKind.Utc));
+  }
+
+  public static Stream GenerateStreamFromString(string s)
+  {
+    var stream = new MemoryStream();
+    var writer = new StreamWriter(stream);
+    writer.Write(s);
+    writer.Flush();
+    stream.Position = 0;
+    return stream;
+  }
+}