Allow setting ZipArchiveEntry general-purpose flag bits (#98278)

* Use CompressionLevel to set general-purpose bit flags

Also changed mapping of ZipPackage's compression options, such that CompressionOption.Maximum now sets the compression level to SmallestSize, and SuperFast now sets the compression level to NoCompression.
Both of these changes restore compatibility with the .NET Framework.

* Made function verbs consistent

* Added test to verify read file contents

* Corrected failing Packaging test

This test was intended to ensure that bit 11 isn't set. It was actually performing a blind comparison of the entire bit field. Other tests in System.IO.Packaging function properly.

* Changes following code review

* Updated the conditional compilation directives for the .NET Framework/Core package CompressionLevel mappings.
* Specifying a CompressionMethod other than Deflate or Deflate64 will now set the compression level to NoCompression, and will write zeros to the relevant general purpose bits.
* The CompressionLevel always defaulted to Optimal for new archives, but this is now explicitly set (rather than relying upon setting it to null and null-coalescing it to Optimal.) This removes a condition to test for.
* Updated the test data for the CreateArchiveEntriesWithBitFlags test. If the compression level is set to NoCompression, we should expect the CompressionMethod to become Stored (which unsets the general purpose bits, returning an expected result of zero.)

* Code review changes

* Updated mapping between general purpose bit fields and CompressionLevel.
* Updated mapping from CompressionOption to CompressionLevel.
* Added test to verify round-trip of CompressionOption and its setting of the general purpose bit fields.

---------

Co-authored-by: Carlos Sánchez López <1175054+carlossanlop@users.noreply.github.com>

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveEntry.cs

@@ -44,7 +44,7 @@ public partial class ZipArchiveEntry
         private List<ZipGenericExtraField>? _cdUnknownExtraFields;
         private List<ZipGenericExtraField>? _lhUnknownExtraFields;
         private byte[] _fileComment;
-        private readonly CompressionLevel? _compressionLevel;
+        private readonly CompressionLevel _compressionLevel;
 
         // Initializes a ZipArchiveEntry instance for an existing archive entry.
         internal ZipArchiveEntry(ZipArchive archive, ZipCentralDirectoryFileHeader cd)
@@ -86,7 +86,7 @@ internal ZipArchiveEntry(ZipArchive archive, ZipCentralDirectoryFileHeader cd)
 
             _fileComment = cd.FileComment;
 
-            _compressionLevel = null;
+            _compressionLevel = MapCompressionLevel(_generalPurposeBitFlag, CompressionMethod);
         }
 
         // Initializes a ZipArchiveEntry instance for a new archive entry with a specified compression level.
@@ -98,6 +98,7 @@ internal ZipArchiveEntry(ZipArchive archive, string entryName, CompressionLevel
             {
                 CompressionMethod = CompressionMethodValues.Stored;
             }
+            _generalPurposeBitFlag = MapDeflateCompressionOption(_generalPurposeBitFlag, _compressionLevel, CompressionMethod);
         }
 
         // Initializes a ZipArchiveEntry instance for a new archive entry.
@@ -111,8 +112,9 @@ internal ZipArchiveEntry(ZipArchive archive, string entryName)
             _versionMadeByPlatform = CurrentZipPlatform;
             _versionMadeBySpecification = ZipVersionNeededValues.Default;
             _versionToExtract = ZipVersionNeededValues.Default; // this must happen before following two assignment
-            _generalPurposeBitFlag = 0;
+            _compressionLevel = CompressionLevel.Optimal;
             CompressionMethod = CompressionMethodValues.Deflate;
+            _generalPurposeBitFlag = MapDeflateCompressionOption(0, _compressionLevel, CompressionMethod);
             _lastModified = DateTimeOffset.Now;
 
             _compressedSize = 0; // we don't know these yet
@@ -138,8 +140,6 @@ internal ZipArchiveEntry(ZipArchive archive, string entryName)
 
             _fileComment = Array.Empty<byte>();
 
-            _compressionLevel = null;
-
             if (_storedEntryNameBytes.Length > ushort.MaxValue)
                 throw new ArgumentException(SR.EntryNamesTooLong);
 
@@ -632,7 +632,7 @@ private CheckSumAndSizeWriteStream GetDataCompressor(Stream backingStream, bool
                 case CompressionMethodValues.Deflate:
                 case CompressionMethodValues.Deflate64:
                 default:
-                    compressorStream = new DeflateStream(backingStream, _compressionLevel ?? CompressionLevel.Optimal, leaveBackingStreamOpen);
+                    compressorStream = new DeflateStream(backingStream, _compressionLevel, leaveBackingStreamOpen);
                     break;
 
             }
@@ -799,6 +799,46 @@ private bool IsOpenable(bool needToUncompress, bool needToLoadIntoMemory, out st
 
         private bool SizesTooLarge() => _compressedSize > uint.MaxValue || _uncompressedSize > uint.MaxValue;
 
+        private static CompressionLevel MapCompressionLevel(BitFlagValues generalPurposeBitFlag, CompressionMethodValues compressionMethod)
+        {
+            // Information about the Deflate compression option is stored in bits 1 and 2 of the general purpose bit flags.
+            // If the compression method is not Deflate, the Deflate compression option is invalid - default to NoCompression.
+            if (compressionMethod == CompressionMethodValues.Deflate || compressionMethod == CompressionMethodValues.Deflate64)
+            {
+                return ((int)generalPurposeBitFlag & 0x6) switch
+                {
+                    0 => CompressionLevel.Optimal,
+                    2 => CompressionLevel.SmallestSize,
+                    4 => CompressionLevel.Fastest,
+                    6 => CompressionLevel.Fastest,
+                    _ => CompressionLevel.Optimal
+                };
+            }
+            else
+            {
+                return CompressionLevel.NoCompression;
+            }
+        }
+
+        private static BitFlagValues MapDeflateCompressionOption(BitFlagValues generalPurposeBitFlag, CompressionLevel compressionLevel, CompressionMethodValues compressionMethod)
+        {
+            ushort deflateCompressionOptions = (ushort)(
+                // The Deflate compression level is only valid if the compression method is actually Deflate (or Deflate64). If it's not, the
+                // value of the two bits is undefined and they should be zeroed out.
+                compressionMethod == CompressionMethodValues.Deflate || compressionMethod == CompressionMethodValues.Deflate64
+                    ? compressionLevel switch
+                    {
+                        CompressionLevel.Optimal => 0,
+                        CompressionLevel.SmallestSize => 2,
+                        CompressionLevel.Fastest => 6,
+                        CompressionLevel.NoCompression => 6,
+                        _ => 0
+                    }
+                    : 0);
+
+            return (BitFlagValues)(((int)generalPurposeBitFlag & ~0x6) | deflateCompressionOptions);
+        }
+
         // return value is true if we allocated an extra field for 64 bit headers, un/compressed size
         private bool WriteLocalFileHeader(bool isEmptyFile)
         {

src/libraries/System.IO.Compression/tests/ZipArchive/zip_CreateTests.cs

@@ -142,6 +142,78 @@ public static void CreateUncompressedArchive()
             }
         }
 
+        // This test checks to ensure that setting the compression level of an archive entry sets the general-purpose
+        // bit flags correctly. It verifies that these have been set by reading from the MemoryStream manually, and by
+        // reopening the generated file to confirm that the compression levels match.
+        [Theory]
+        // Special-case NoCompression: in this case, the CompressionMethod becomes Stored and the bits are unset.
+        [InlineData(CompressionLevel.NoCompression, 0)]
+        [InlineData(CompressionLevel.Optimal, 0)]
+        [InlineData(CompressionLevel.SmallestSize, 2)]
+        [InlineData(CompressionLevel.Fastest, 6)]
+        public static void CreateArchiveEntriesWithBitFlags(CompressionLevel compressionLevel, ushort expectedGeneralBitFlags)
+        {
+            var testfilename = "testfile";
+            var testFileContent = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
+            var utf8WithoutBom = new Text.UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
+
+            byte[] zipFileContent;
+
+            using (var testStream = new MemoryStream())
+            {
+
+                using (var zip = new ZipArchive(testStream, ZipArchiveMode.Create))
+                {
+                    ZipArchiveEntry newEntry = zip.CreateEntry(testfilename, compressionLevel);
+                    using (var writer = new StreamWriter(newEntry.Open(), utf8WithoutBom))
+                    {
+                        writer.Write(testFileContent);
+                        writer.Flush();
+                    }
+
+                    ZipArchiveEntry secondNewEntry = zip.CreateEntry(testFileContent + "_post", CompressionLevel.NoCompression);
+                }
+
+                zipFileContent = testStream.ToArray();
+            }
+
+            // expected bit flags are at position 6 in the file header
+            var generalBitFlags = System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(zipFileContent.AsSpan(6));
+
+            Assert.Equal(expectedGeneralBitFlags, generalBitFlags);
+
+            using (var reReadStream = new MemoryStream(zipFileContent))
+            {
+                using (var reReadZip = new ZipArchive(reReadStream, ZipArchiveMode.Read))
+                {
+                    var firstArchive = reReadZip.Entries[0];
+                    var secondArchive = reReadZip.Entries[1];
+                    var compressionLevelFieldInfo = typeof(ZipArchiveEntry).GetField("_compressionLevel", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
+                    var generalBitFlagsFieldInfo = typeof(ZipArchiveEntry).GetField("_generalPurposeBitFlag", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
+
+                    var reReadCompressionLevel = (CompressionLevel)compressionLevelFieldInfo.GetValue(firstArchive);
+                    var reReadGeneralBitFlags = (ushort)generalBitFlagsFieldInfo.GetValue(firstArchive);
+
+                    Assert.Equal(compressionLevel, reReadCompressionLevel);
+                    Assert.Equal(expectedGeneralBitFlags, reReadGeneralBitFlags);
+
+                    reReadCompressionLevel = (CompressionLevel)compressionLevelFieldInfo.GetValue(secondArchive);
+                    Assert.Equal(CompressionLevel.NoCompression, reReadCompressionLevel);
+
+                    using (var strm = firstArchive.Open())
+                    {
+                        var readBuffer = new byte[firstArchive.Length];
+
+                        strm.Read(readBuffer);
+
+                        var readText = Text.Encoding.UTF8.GetString(readBuffer);
+
+                        Assert.Equal(readText, testFileContent);
+                    }
+                }
+            }
+        }
+
         [Fact]
         public static void CreateNormal_VerifyDataDescriptor()
         {

src/libraries/System.IO.Packaging/src/System/IO/Packaging/ZipPackage.cs

@@ -383,7 +383,11 @@ internal static string GetOpcNameFromZipItemName(string zipItemName)
                     break;
                 case CompressionOption.Maximum:
                     {
+#if NET
+                        compressionLevel = CompressionLevel.SmallestSize;
+#else
                         compressionLevel = CompressionLevel.Optimal;
+#endif
                     }
                     break;
                 case CompressionOption.Fast:

src/libraries/System.IO.Packaging/tests/ReflectionTests.cs

@@ -34,7 +34,7 @@ public void Verify_GeneralPurposeBitFlag_NotSetTo_Unicode()
                 FieldInfo fieldInfo = typeof(ZipArchiveEntry).GetField("_generalPurposeBitFlag", BindingFlags.Instance | BindingFlags.NonPublic);
                 object fieldObject = fieldInfo.GetValue(entry);
                 ushort shortField = (ushort)fieldObject;
-                Assert.Equal(0, shortField); // If it was UTF8, we would set the general purpose bit flag to 0x800 (UnicodeFileNameAndComment)
+                Assert.Equal(0, shortField & 0x800); // If it was UTF8, we would set the general purpose bit flag to 0x800 (UnicodeFileNameAndComment)
                 CheckCharacters(entry.Name);
                 CheckCharacters(entry.Comment); // Unavailable in .NET Framework
             }

src/libraries/System.IO.Packaging/tests/Tests.cs

@@ -3988,6 +3988,47 @@ public void CreatePackUriWithFragment()
 
         }
 
+        [Theory]
+#if NET
+        [InlineData(CompressionOption.NotCompressed, CompressionOption.Normal, 0)]
+        [InlineData(CompressionOption.Normal, CompressionOption.Normal, 0)]
+        [InlineData(CompressionOption.Maximum, CompressionOption.Normal, 2)]
+        [InlineData(CompressionOption.Fast, CompressionOption.Normal, 6)]
+        [InlineData(CompressionOption.SuperFast, CompressionOption.Normal, 6)]
+#else
+        [InlineData(CompressionOption.NotCompressed, CompressionOption.NotCompressed, 0)]
+        [InlineData(CompressionOption.Normal, CompressionOption.Normal, 0)]
+        [InlineData(CompressionOption.Maximum, CompressionOption.Maximum, 2)]
+        [InlineData(CompressionOption.Fast, CompressionOption.Fast, 4)]
+        [InlineData(CompressionOption.SuperFast, CompressionOption.SuperFast, 6)]
+#endif
+        public void Roundtrip_Compression_Option(CompressionOption createdCompressionOption, CompressionOption expectedCompressionOption, ushort expectedZipFileBitFlags)
+        {
+            var documentPath = "untitled.txt";
+            Uri partUriDocument = PackUriHelper.CreatePartUri(new Uri(documentPath, UriKind.Relative));
+
+            using (MemoryStream ms = new MemoryStream())
+            {
+                Package package = Package.Open(ms, FileMode.Create, FileAccess.ReadWrite);
+                PackagePart part = package.CreatePart(partUriDocument, "application/text", createdCompressionOption);
+
+                package.Flush();
+                package.Close();
+                (package as IDisposable).Dispose();
+
+                ms.Seek(0, SeekOrigin.Begin);
+
+                var zipBytes = ms.ToArray();
+                var generalBitFlags = System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(zipBytes.AsSpan(6));
+
+                package = Package.Open(ms, FileMode.Open, FileAccess.Read);
+                part = package.GetPart(partUriDocument);
+
+                Assert.Equal(expectedZipFileBitFlags, generalBitFlags);
+                Assert.Equal(expectedCompressionOption, part.CompressionOption);
+            }
+        }
+
         private const string DocumentRelationshipType = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument";
     }