Reduce memory usage when updating ZipArchives

src/libraries/Common/tests/System/IO/Compression/ZipTestHelper.cs

@@ -435,6 +435,31 @@ internal static void AddEntry(ZipArchive archive, string name, string contents,
             }
         }
 
+        public static byte[] CreateZipFile(int entryCount, byte[] entryContents)
+        {
+            using (MemoryStream ms = new())
+            {
+                using (ZipArchive createdArchive = new(ms, ZipArchiveMode.Create, true))
+                {
+                    for (int i = 0; i < entryCount; i++)
+                    {
+                        string fileName = $"dummydata/{i}.bin";
+                        ZipArchiveEntry newEntry = createdArchive.CreateEntry(fileName);
+
+                        newEntry.LastWriteTime = DateTimeOffset.Now.AddHours(-1.0);
+                        using (Stream entryWriteStream = newEntry.Open())
+                        {
+                            entryWriteStream.Write(entryContents);
+                            entryWriteStream.WriteByte((byte)(i % byte.MaxValue));
+                        }
+                    }
+                }
+                ms.Flush();
+
+                return ms.ToArray();
+            }
+        }
+
         protected const string Utf8SmileyEmoji = "\ud83d\ude04";
         protected const string Utf8LowerCaseOUmlautChar = "\u00F6";
         protected const string Utf8CopyrightChar = "\u00A9";

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

@@ -31,6 +31,8 @@ public class ZipArchive : IDisposable
         private byte[] _archiveComment;
         private Encoding? _entryNameAndCommentEncoding;
 
+        private long _firstDeletedEntryOffset;
+
 #if DEBUG_FORCE_ZIP64
         public bool _forceZip64;
 #endif
@@ -168,12 +170,14 @@ public ZipArchive(Stream stream, ZipArchiveMode mode, bool leaveOpen, Encoding?
                 _entries = new List<ZipArchiveEntry>();
                 _entriesCollection = new ReadOnlyCollection<ZipArchiveEntry>(_entries);
                 _entriesDictionary = new Dictionary<string, ZipArchiveEntry>();
+                Changed = ChangeState.Unchanged;
                 _readEntries = false;
                 _leaveOpen = leaveOpen;
                 _centralDirectoryStart = 0; // invalid until ReadCentralDirectory
                 _isDisposed = false;
                 _numberOfThisDisk = 0; // invalid until ReadCentralDirectory
                 _archiveComment = Array.Empty<byte>();
+                _firstDeletedEntryOffset = long.MaxValue;
 
                 switch (mode)
                 {
@@ -221,7 +225,11 @@ public ZipArchive(Stream stream, ZipArchiveMode mode, bool leaveOpen, Encoding?
         public string Comment
         {
             get => (EntryNameAndCommentEncoding ?? Encoding.UTF8).GetString(_archiveComment);
-            set => _archiveComment = ZipHelper.GetEncodedTruncatedBytesFromString(value, EntryNameAndCommentEncoding, ZipEndOfCentralDirectoryBlock.ZipFileCommentMaxLength, out _);
+            set
+            {
+                _archiveComment = ZipHelper.GetEncodedTruncatedBytesFromString(value, EntryNameAndCommentEncoding, ZipEndOfCentralDirectoryBlock.ZipFileCommentMaxLength, out _);
+                Changed |= ChangeState.DynamicLengthMetadata;
+            }
         }
 
         /// <summary>
@@ -389,6 +397,10 @@ private set
             }
         }
 
+        // This property's value only relates to the top-level fields of the archive (such as the archive comment.)
+        // New entries in the archive won't change its state.
+        internal ChangeState Changed { get; private set; }
+
         private ZipArchiveEntry DoCreateEntry(string entryName, CompressionLevel? compressionLevel)
         {
             ArgumentException.ThrowIfNullOrEmpty(entryName);
@@ -415,7 +427,7 @@ internal void AcquireArchiveStream(ZipArchiveEntry entry)
             {
                 if (!_archiveStreamOwner.EverOpenedForWrite)
                 {
-                    _archiveStreamOwner.WriteAndFinishLocalEntry();
+                    _archiveStreamOwner.WriteAndFinishLocalEntry(true);
                 }
                 else
                 {
@@ -447,6 +459,11 @@ internal void RemoveEntry(ZipArchiveEntry entry)
             _entries.Remove(entry);
 
             _entriesDictionary.Remove(entry.FullName);
+            // Keep track of the offset of the earliest deleted entry in the archive
+            if (entry.OriginallyInArchive && entry.OffsetOfLocalHeader < _firstDeletedEntryOffset)
+            {
+                _firstDeletedEntryOffset = entry.OffsetOfLocalHeader;
+            }
         }
 
         internal void ThrowIfDisposed()
@@ -502,6 +519,9 @@ private void ReadCentralDirectory()
                     numberOfEntries++;
                 }
 
+                // Sort _entries by each archive entry's position
+                _entries.Sort(ZipArchiveEntry.LocalHeaderOffsetComparer.Instance);
+
                 if (numberOfEntries != _expectedNumberOfEntries)
                     throw new InvalidDataException(SR.NumEntriesWrong);
             }
@@ -633,41 +653,108 @@ private void WriteFile()
             // if we are in update mode, we call EnsureCentralDirectoryRead, which sets readEntries to true
             Debug.Assert(_readEntries);
 
+            // Entries starting after this offset have had a dynamically-sized change. Everything on or after this point must be rewritten.
+            long dynamicDirtyStartingOffset = 0;
+            List<ZipArchiveEntry> entriesToWrite = _entries;
+
             if (_mode == ZipArchiveMode.Update)
             {
-                List<ZipArchiveEntry> markedForDelete = new List<ZipArchiveEntry>();
+                // Entries starting after this offset have some kind of change made to them. It might just be a fixed-length field though, in which case
+                // that single entry's metadata can be rewritten without impacting anything else.
+                long startingOffset = _firstDeletedEntryOffset;
+                long nextFileOffset = 0;
+                dynamicDirtyStartingOffset = startingOffset;
+
+                entriesToWrite = new(_entries.Count);
                 foreach (ZipArchiveEntry entry in _entries)
                 {
-                    if (!entry.LoadLocalHeaderExtraFieldAndCompressedBytesIfNeeded())
-                        markedForDelete.Add(entry);
+                    if (entry.OriginallyInArchive)
+                    {
+                        if (entry.Changed == ChangeState.Unchanged)
+                        {
+                            // Keep track of the expected position of the file entry after the final untouched file entry so that when the loop completes,
+                            // we'll know which position to start writing new entries from.
+                            nextFileOffset = Math.Max(nextFileOffset, entry.OffsetOfCompressedData + entry.CompressedLength);
+                        }
+                        // When calculating the starting offset to load the files from, only look at dirty entries which are already in the archive.
+                        else
+                        {
+                            startingOffset = Math.Min(startingOffset, entry.OffsetOfLocalHeader);
+                        }
+
+                        // We want to re-write entries which are after the starting offset of the first entry which has pending data to write.
+                        // NB: the existing ZipArchiveEntries are sorted in _entries by their position ascending.
+                        if (entry.OffsetOfLocalHeader >= startingOffset)
+                        {
+                            // If the pending data to write is fixed-length metadata in the header, there's no need to load the full file for
+                            // inflation and deflation.
+                            if ((entry.Changed & (ChangeState.DynamicLengthMetadata | ChangeState.StoredData)) != 0)
+                            {
+                                dynamicDirtyStartingOffset = Math.Min(dynamicDirtyStartingOffset, entry.OffsetOfLocalHeader);
+                            }
+                            if (entry.OffsetOfLocalHeader >= dynamicDirtyStartingOffset)
+                            {
+                                entry.LoadLocalHeaderExtraFieldAndCompressedBytesIfNeeded();
+                            }
+
+                            entriesToWrite.Add(entry);
+                        }
+                    }
+                    else
+                    {
+                        entriesToWrite.Add(entry);
+                    }
+                }
+
+                // If the offset of entries to write from is still at long.MaxValue, then we know that nothing has been deleted,
+                // nothing has been modified - so we just want to move to the end of all remaining files in the archive.
+                if (startingOffset == long.MaxValue)
+                {
+                    startingOffset = nextFileOffset;
                 }
-                foreach (ZipArchiveEntry entry in markedForDelete)
-                    entry.Delete();
 
-                _archiveStream.Seek(0, SeekOrigin.Begin);
-                _archiveStream.SetLength(0);
+                _archiveStream.Seek(startingOffset, SeekOrigin.Begin);
             }
 
-            foreach (ZipArchiveEntry entry in _entries)
+            foreach (ZipArchiveEntry entry in entriesToWrite)
             {
-                entry.WriteAndFinishLocalEntry();
+                // We don't always need to write the local header entry, ZipArchiveEntry is usually able to work out when it doesn't need to.
+                // We want to force this header entry to be written (even for completely untouched entries) if the entry comes after one
+                // which had a pending dynamically-sized write.
+                bool forceWriteLocalEntry = !entry.OriginallyInArchive || (entry.OriginallyInArchive && entry.OffsetOfLocalHeader >= dynamicDirtyStartingOffset);
+
+                entry.WriteAndFinishLocalEntry(forceWriteLocalEntry);
             }
 
             long startOfCentralDirectory = _archiveStream.Position;
+            // If there are no entries in the archive, we still want to create the archive epilogue.
+            bool archiveEpilogueRequiresUpdate = _entries.Count == 0;
 
             foreach (ZipArchiveEntry entry in _entries)
             {
-                entry.WriteCentralDirectoryFileHeader();
+                // The central directory needs to be rewritten if its position has moved, if there's a new entry in the archive, or if the entry might be different.
+                bool centralDirectoryEntryRequiresUpdate = startOfCentralDirectory != _centralDirectoryStart
+                    | (!entry.OriginallyInArchive || (entry.OriginallyInArchive && entry.OffsetOfLocalHeader >= dynamicDirtyStartingOffset));
+
+                entry.WriteCentralDirectoryFileHeader(centralDirectoryEntryRequiresUpdate);
+                archiveEpilogueRequiresUpdate |= centralDirectoryEntryRequiresUpdate;
             }
 
             long sizeOfCentralDirectory = _archiveStream.Position - startOfCentralDirectory;
 
-            WriteArchiveEpilogue(startOfCentralDirectory, sizeOfCentralDirectory);
+            WriteArchiveEpilogue(startOfCentralDirectory, sizeOfCentralDirectory, archiveEpilogueRequiresUpdate);
+
+            // If entries have been removed and new (smaller) ones added, there could be empty space at the end of the file.
+            // Shrink the file to reclaim this space.
+            if (_mode == ZipArchiveMode.Update && _archiveStream.Position != _archiveStream.Length)
+            {
+                _archiveStream.SetLength(_archiveStream.Position);
+            }
         }
 
         // writes eocd, and if needed, zip 64 eocd, zip64 eocd locator
         // should only throw an exception in extremely exceptional cases because it is called from dispose
-        private void WriteArchiveEpilogue(long startOfCentralDirectory, long sizeOfCentralDirectory)
+        private void WriteArchiveEpilogue(long startOfCentralDirectory, long sizeOfCentralDirectory, bool centralDirectoryChanged)
         {
             // determine if we need Zip 64
             if (startOfCentralDirectory >= uint.MaxValue
@@ -680,12 +767,37 @@ private void WriteArchiveEpilogue(long startOfCentralDirectory, long sizeOfCentr
             {
                 // if we need zip 64, write zip 64 eocd and locator
                 long zip64EOCDRecordStart = _archiveStream.Position;
-                Zip64EndOfCentralDirectoryRecord.WriteBlock(_archiveStream, _entries.Count, startOfCentralDirectory, sizeOfCentralDirectory);
-                Zip64EndOfCentralDirectoryLocator.WriteBlock(_archiveStream, zip64EOCDRecordStart);
+
+                if (centralDirectoryChanged)
+                {
+                    Zip64EndOfCentralDirectoryRecord.WriteBlock(_archiveStream, _entries.Count, startOfCentralDirectory, sizeOfCentralDirectory);
+                    Zip64EndOfCentralDirectoryLocator.WriteBlock(_archiveStream, zip64EOCDRecordStart);
+                }
+                else
+                {
+                    _archiveStream.Seek(Zip64EndOfCentralDirectoryRecord.TotalSize, SeekOrigin.Current);
+                    _archiveStream.Seek(Zip64EndOfCentralDirectoryLocator.TotalSize, SeekOrigin.Current);
+                }
             }
 
             // write normal eocd
-            ZipEndOfCentralDirectoryBlock.WriteBlock(_archiveStream, _entries.Count, startOfCentralDirectory, sizeOfCentralDirectory, _archiveComment);
+            if (centralDirectoryChanged | (Changed != ChangeState.Unchanged))
+            {
+                ZipEndOfCentralDirectoryBlock.WriteBlock(_archiveStream, _entries.Count, startOfCentralDirectory, sizeOfCentralDirectory, _archiveComment);
+            }
+            else
+            {
+                _archiveStream.Seek(ZipEndOfCentralDirectoryBlock.TotalSize + _archiveComment.Length, SeekOrigin.Current);
+            }
+        }
+
+        [Flags]
+        internal enum ChangeState
+        {
+            Unchanged = 0x0,
+            FixedLengthMetadata = 0x1,
+            DynamicLengthMetadata = 0x2,
+            StoredData = 0x4
         }
     }
 }