Fix append mode for hybrid-reference documents

iText used to refer to regular xref section by /Prev key in the xref
stream which is "not meaningful in hybrid-reference files"
according to the specification. In full compression mode after the
changes in this commit first an xref stream is written and then a regular
xref section referring to the xref stream. For safety, regular objects,
not the ones from object streams, are also enumerated in the last regular
xref section of the stamped document.

DEVSIX-2080

Autoported commit.
Original commit hash: [68411e67b]

Author: Snipx

itext.tests/itext.sign.tests/itext/signatures/sign/PdfSignatureAppearanceTest.cs

@@ -41,6 +41,7 @@ source product.
 address: [email protected]
 */
 using System;
+using System.Collections.Generic;
 using System.IO;
 using Org.BouncyCastle.Crypto;
 using Org.BouncyCastle.X509;
@@ -160,6 +161,52 @@ public virtual void TextAutoscaleTest06() {
             AssertAppearanceFontSize(dest, 6.26f);
         }
 
+        /// <exception cref="System.IO.IOException"/>
+        /// <exception cref="Org.BouncyCastle.Security.GeneralSecurityException"/>
+        /// <exception cref="System.Exception"/>
+        [NUnit.Framework.Test]
+        public virtual void TestSigningInAppendModeWithHybridDocument() {
+            String src = sourceFolder + "hybrid.pdf";
+            String dest = destinationFolder + "signed_hybrid.pdf";
+            String cmp = sourceFolder + "cmp_signed_hybrid.pdf";
+            PdfSigner signer = new PdfSigner(new PdfReader(src), new FileStream(dest, FileMode.Create), new StampingProperties
+                ().UseAppendMode());
+            PdfSignatureAppearance appearance = signer.GetSignatureAppearance();
+            appearance.SetLayer2FontSize(13.8f).SetPageRect(new Rectangle(36, 748, 200, 100)).SetPageNumber(1).SetReason
+                ("Test").SetLocation("Nagpur");
+            signer.SetFieldName("Sign1");
+            signer.SetCertificationLevel(PdfSigner.NOT_CERTIFIED);
+            IExternalSignature pks = new PrivateKeySignature(pk, DigestAlgorithms.SHA256);
+            signer.SignDetached(pks, chain, null, null, null, 0, PdfSigner.CryptoStandard.CADES);
+            // Make sure iText can open the document
+            new PdfDocument(new PdfReader(dest)).Close();
+            // Assert that the document can be rendered correctly
+            NUnit.Framework.Assert.IsNull(new CompareTool().CompareVisually(dest, cmp, destinationFolder, "diff_", GetIgnoredAreaTestMap
+                (new Rectangle(36, 748, 200, 100))));
+        }
+
+        /// <exception cref="Org.BouncyCastle.Security.GeneralSecurityException"/>
+        /// <exception cref="System.IO.IOException"/>
+        /// <exception cref="System.Exception"/>
+        [NUnit.Framework.Test]
+        public virtual void FontColorTest01() {
+            String fileName = "fontColorTest01.pdf";
+            String dest = destinationFolder + fileName;
+            Rectangle rect = new Rectangle(36, 648, 100, 50);
+            String src = sourceFolder + "simpleDocument.pdf";
+            PdfSigner signer = new PdfSigner(new PdfReader(src), new FileStream(dest, FileMode.Create), new StampingProperties
+                ());
+            // Creating the appearance
+            signer.GetSignatureAppearance().SetLayer2FontColor(ColorConstants.RED).SetLayer2Text("Verified and signed by me."
+                ).SetPageRect(rect);
+            signer.SetFieldName("Signature1");
+            // Creating the signature
+            IExternalSignature pks = new PrivateKeySignature(pk, DigestAlgorithms.SHA256);
+            signer.SignDetached(pks, chain, null, null, null, 0, PdfSigner.CryptoStandard.CADES);
+            NUnit.Framework.Assert.IsNull(new CompareTool().CompareVisually(dest, sourceFolder + "cmp_" + fileName, destinationFolder
+                , "diff_"));
+        }
+
         /// <exception cref="System.IO.IOException"/>
         /// <exception cref="Org.BouncyCastle.Security.GeneralSecurityException"/>
         private void TestSignatureAppearanceAutoscale(String dest, Rectangle rect, PdfSignatureAppearance.RenderingMode
@@ -198,26 +245,10 @@ private static void AssertAppearanceFontSize(String filename, float expectedFont
                 .Format("Font size: exptected {0}, found {1}", expectedFontSize, fontSize));
         }
 
-        /// <exception cref="Org.BouncyCastle.Security.GeneralSecurityException"/>
-        /// <exception cref="System.IO.IOException"/>
-        /// <exception cref="System.Exception"/>
-        [NUnit.Framework.Test]
-        public virtual void FontColorTest01() {
-            String fileName = "fontColorTest01.pdf";
-            String dest = destinationFolder + fileName;
-            Rectangle rect = new Rectangle(36, 648, 100, 50);
-            String src = sourceFolder + "simpleDocument.pdf";
-            PdfSigner signer = new PdfSigner(new PdfReader(src), new FileStream(dest, FileMode.Create), new StampingProperties
-                ());
-            // Creating the appearance
-            signer.GetSignatureAppearance().SetLayer2FontColor(ColorConstants.RED).SetLayer2Text("Verified and signed by me."
-                ).SetPageRect(rect);
-            signer.SetFieldName("Signature1");
-            // Creating the signature
-            IExternalSignature pks = new PrivateKeySignature(pk, DigestAlgorithms.SHA256);
-            signer.SignDetached(pks, chain, null, null, null, 0, PdfSigner.CryptoStandard.CADES);
-            NUnit.Framework.Assert.IsNull(new CompareTool().CompareVisually(dest, sourceFolder + "cmp_" + fileName, destinationFolder
-                , "diff_"));
+        private static IDictionary<int, IList<Rectangle>> GetIgnoredAreaTestMap(Rectangle ignoredArea) {
+            IDictionary<int, IList<Rectangle>> result = new Dictionary<int, IList<Rectangle>>();
+            result.Put(1, JavaUtil.ArraysAsList(ignoredArea));
+            return result;
         }
     }
 }

itext.tests/itext.sign.tests/resources/itext/signatures/sign/PdfSignatureAppearanceTest/cmp_signed_hybrid.pdf

@@ -216,41 +216,14 @@ protected internal virtual void WriteXrefTableAndTrailer(PdfDocument document, P
                     }
                 }
             }
-            IList<int> sections = new List<int>();
-            int first = 0;
-            int len = 0;
-            for (int i = 0; i < Size(); i++) {
-                PdfIndirectReference reference = xref[i];
-                if (document.properties.appendMode && reference != null && !reference.CheckState(PdfObject.MODIFIED)) {
-                    reference = null;
-                }
-                if (reference == null) {
-                    if (len > 0) {
-                        sections.Add(first);
-                        sections.Add(len);
-                    }
-                    len = 0;
-                }
-                else {
-                    if (len > 0) {
-                        len++;
-                    }
-                    else {
-                        first = i;
-                        len = 1;
-                    }
-                }
-            }
-            if (len > 0) {
-                sections.Add(first);
-                sections.Add(len);
-            }
+            IList<int> sections = CreateSections(document, false);
             if (document.properties.appendMode && sections.Count == 0) {
                 // no modifications.
                 xref = null;
                 return;
             }
             long startxref = writer.GetCurrentPos();
+            long xRefStmPos = -1;
             if (writer.IsFullCompression()) {
                 PdfStream xrefStream = (PdfStream)new PdfStream().MakeIndirect(document);
                 xrefStream.MakeIndirect(document);
@@ -269,15 +242,16 @@ protected internal virtual void WriteXrefTableAndTrailer(PdfDocument document, P
                 foreach (int? section in sections) {
                     index.Add(new PdfNumber((int)section));
                 }
-                if (document.properties.appendMode) {
+                if (document.properties.appendMode && !document.reader.hybridXref) {
+                    // "not meaningful in hybrid-reference files"
                     PdfNumber lastXref = new PdfNumber(document.reader.GetLastXref());
                     xrefStream.Put(PdfName.Prev, lastXref);
                 }
                 xrefStream.Put(PdfName.Index, index);
                 iText.Kernel.Pdf.PdfXrefTable xrefTable = document.GetXref();
                 for (int k = 0; k < sections.Count; k += 2) {
-                    first = (int)sections[k];
-                    len = (int)sections[k + 1];
+                    int first = (int)sections[k];
+                    int len = (int)sections[k + 1];
                     for (int i = first; i < first + len; i++) {
                         PdfIndirectReference reference = xrefTable.Get(i);
                         if (reference.IsFree()) {
@@ -300,13 +274,23 @@ protected internal virtual void WriteXrefTableAndTrailer(PdfDocument document, P
                     }
                 }
                 xrefStream.Flush();
+                xRefStmPos = startxref;
             }
-            else {
+            // For documents with hybrid cross-reference table, i.e. containing xref streams as well as regular xref sections,
+            // we write additional regular xref section at the end of the document because the /Prev reference from
+            // xref stream to a regular xref section doesn't seem to be valid
+            bool needsRegularXref = !writer.IsFullCompression() || document.properties.appendMode && document.reader.hybridXref;
+            if (needsRegularXref) {
+                startxref = writer.GetCurrentPos();
                 writer.WriteString("xref\n");
                 iText.Kernel.Pdf.PdfXrefTable xrefTable = document.GetXref();
+                if (xRefStmPos != -1) {
+                    // Get rid of all objects from object stream. This is done for hybrid documents
+                    sections = CreateSections(document, true);
+                }
                 for (int k = 0; k < sections.Count; k += 2) {
-                    first = (int)sections[k];
-                    len = (int)sections[k + 1];
+                    int first = (int)sections[k];
+                    int len = (int)sections[k + 1];
                     writer.WriteInteger(first).WriteSpace().WriteInteger(len).WriteByte((byte)'\n');
                     for (int i = first; i < first + len; i++) {
                         PdfIndirectReference reference = xrefTable.Get(i);
@@ -330,6 +314,9 @@ protected internal virtual void WriteXrefTableAndTrailer(PdfDocument document, P
                 trailer.Remove(PdfName.Length);
                 trailer.Put(PdfName.Size, new PdfNumber(this.Size()));
                 trailer.Put(PdfName.ID, fileId);
+                if (xRefStmPos != -1) {
+                    trailer.Put(PdfName.XRefStm, new PdfNumber(xRefStmPos));
+                }
                 if (crypto != null) {
                     trailer.Put(PdfName.Encrypt, crypto);
                 }
@@ -357,6 +344,40 @@ internal virtual void Clear() {
             count = 1;
         }
 
+        private IList<int> CreateSections(PdfDocument document, bool dropObjectsFromObjectStream) {
+            IList<int> sections = new List<int>();
+            int first = 0;
+            int len = 0;
+            for (int i = 0; i < Size(); i++) {
+                PdfIndirectReference reference = xref[i];
+                if (document.properties.appendMode && reference != null && (!reference.CheckState(PdfObject.MODIFIED) || dropObjectsFromObjectStream
+                     && reference.GetObjStreamNumber() != 0)) {
+                    reference = null;
+                }
+                if (reference == null) {
+                    if (len > 0) {
+                        sections.Add(first);
+                        sections.Add(len);
+                    }
+                    len = 0;
+                }
+                else {
+                    if (len > 0) {
+                        len++;
+                    }
+                    else {
+                        first = i;
+                        len = 1;
+                    }
+                }
+            }
+            if (len > 0) {
+                sections.Add(first);
+                sections.Add(len);
+            }
+            return sections;
+        }
+
         /// <summary>Gets size of the offset.</summary>
         /// <remarks>Gets size of the offset. Max size is 2^40, i.e. 1 Tb.</remarks>
         private int GetOffsetSize(long startxref) {

itext.tests/itext.sign.tests/resources/itext/signatures/sign/PdfSignatureAppearanceTest/hybrid.pdf

@@ -1 +1 @@
-1a4910165bfaddba81a44cf05371fadaa19ff8b8
+68411e67bd5a0cbf9cdff2a8cc2e59aaa67f8bb5