Add more clear exception for an attempt to place elements via Document on flushed pages

DEVSIX-2533

Autoported commit.
Original commit hash: [e86e59f65]

Author: yulian-gaponenko

itext.tests/itext.layout.tests/itext/layout/PositioningTest.cs

@@ -42,6 +42,7 @@ source product.
 */
 using System;
 using iText.IO.Image;
+using iText.Kernel;
 using iText.Kernel.Colors;
 using iText.Kernel.Geom;
 using iText.Kernel.Pdf;
@@ -313,6 +314,28 @@ public virtual void ShowTextAlignedTest03() {
                 , "diff"));
         }
 
+        /// <exception cref="System.IO.IOException"/>
+        [NUnit.Framework.Test]
+        public virtual void ShowTextAlignedOnFlushedPageTest01() {
+            NUnit.Framework.Assert.That(() =>  {
+                String outFileName = destinationFolder + "showTextAlignedOnFlushedPageTest01.pdf";
+                PdfDocument pdfDoc = new PdfDocument(new PdfWriter(outFileName));
+                Document doc = new Document(pdfDoc);
+                Paragraph p = new Paragraph();
+                for (int i = 0; i < 1000; ++i) {
+                    p.Add("abcdefghijklkmnopqrstuvwxyz");
+                }
+                doc.Add(p);
+                // First page will be flushed by now, because immediateFlush is set to false by default.
+                int pageNumberToDrawTextOn = 1;
+                doc.ShowTextAligned(new Paragraph("Hello Bruno on page 1!"), 36, 36, pageNumberToDrawTextOn, TextAlignment
+                    .LEFT, VerticalAlignment.TOP, 0);
+                doc.Close();
+            }
+            , NUnit.Framework.Throws.InstanceOf<PdfException>().With.Message.EqualTo(PdfException.CannotDrawElementsOnAlreadyFlushedPages))
+;
+        }
+
         private void DrawCross(PdfCanvas canvas, float x, float y) {
             DrawLine(canvas, x - 50, y, x + 50, y);
             DrawLine(canvas, x, y - 50, x, y + 50);

itext/itext.kernel/itext/kernel/PdfException.cs

@@ -116,6 +116,8 @@ public class PdfException : Exception {
 
         public const String CannotCreatePdfStreamByInputStreamWithoutPdfDocument = "Cannot create pdfstream by InputStream without PdfDocument.";
 
+        public const String CannotDrawElementsOnAlreadyFlushedPages = "Cannot draw elements on already flushed pages.";
+
         public const String CannotGetContentBytes = "Cannot get content bytes.";
 
         public const String CannotGetPdfStreamBytes = "Cannot get PdfStream bytes.";

itext/itext.layout/itext/layout/renderer/DocumentRenderer.cs

@@ -43,6 +43,7 @@ source product.
 */
 using System;
 using System.Collections.Generic;
+using iText.Kernel;
 using iText.Kernel.Geom;
 using iText.Kernel.Pdf;
 using iText.Kernel.Pdf.Canvas;
@@ -124,6 +125,9 @@ protected internal override void FlushSingleRenderer(IRenderer resultRenderer) {
                 PdfDocument pdfDocument = document.GetPdfDocument();
                 EnsureDocumentHasNPages(pageNum, null);
                 PdfPage correspondingPage = pdfDocument.GetPage(pageNum);
+                if (correspondingPage.IsFlushed()) {
+                    throw new PdfException(PdfException.CannotDrawElementsOnAlreadyFlushedPages);
+                }
                 bool wrapOldContent = pdfDocument.GetReader() != null && pdfDocument.GetWriter() != null && correspondingPage
                     .GetContentStreamCount() > 0 && correspondingPage.GetLastContentStream().GetLength() > 0 && !wrappedContentPage
                     .Contains(pageNum) && pdfDocument.GetNumberOfPages() >= pageNum;

port-hash

@@ -1 +1 @@
-badd958c6688332e7e47b8f3d687e4a4377dfbc6
+e86e59f65f536fe9aba17be6711990dad3a0adf8