Improve check for circular reference and throw exception

src/UglyToad.PdfPig.Tests/Integration/XObjectFormTests.cs

@@ -1,16 +1,37 @@
 namespace UglyToad.PdfPig.Tests.Integration
 {
+    using UglyToad.PdfPig.Core;
     using Xunit;
 
     public class XObjectFormTests
     {
         [Fact]
-        public void CanReadDocumentWithoutStackOverflow()
+        public void CanReadDocumentWithoutStackOverflowIssue671()
         {
             using (var document = PdfDocument.Open(IntegrationHelpers.GetDocumentPath("issue_671")))
             {
                 var page = document.GetPage(1);
             }
         }
+
+        [Fact]
+        public void CanReadDocumentThrowsIssue671()
+        {
+            using (var document = PdfDocument.Open(IntegrationHelpers.GetDocumentPath("issue_671"), ParsingOptions.LenientParsingOff))
+            {
+                var exception = Assert.Throws<PdfDocumentFormatException>(() => document.GetPage(1));
+                Assert.Contains("is referencing itself which can cause unexpected behaviour", exception.Message);
+            }
+        }
+
+        [Fact]
+        public void CanReadDocumentMOZILLA_3136_0()
+        {
+            // This document does not actually contain circular references
+            using (var document = PdfDocument.Open(IntegrationHelpers.GetDocumentPath("MOZILLA-3136-0"), ParsingOptions.LenientParsingOff))
+            {
+                var page = document.GetPage(1);
+            }
+        }
     }
 }

src/UglyToad.PdfPig/Graphics/ContentStreamProcessor.cs

@@ -603,10 +603,18 @@ private void ProcessFormXObject(StreamToken formStream, NameToken xObjectName)
             // 3. We don't respect clipping currently.
 
             // 4. Paint the objects.
-            if (parsingOptions.UseLenientParsing && xObjectName != null && operations.OfType<InvokeNamedXObject>()?.Any(o => o.Name == xObjectName) == true)
+            bool hasCircularReference = HasFormXObjectCircularReference(formStream, xObjectName, operations);
+            if (hasCircularReference)
             {
-                operations = operations.Where(o => o is not InvokeNamedXObject xo || xo.Name != xObjectName).ToArray();
-                parsingOptions.Logger.Warn($"An XObject form named '{xObjectName}' is referencing itself which can cause unexpected behaviour. The self reference was removed from the operations before further processing.");
+                if (parsingOptions.UseLenientParsing)
+                {
+                    operations = operations.Where(o => o is not InvokeNamedXObject xo || xo.Name != xObjectName).ToArray();
+                    parsingOptions.Logger.Warn($"An XObject form named '{xObjectName}' is referencing itself which can cause unexpected behaviour. The self reference was removed from the operations before further processing.");
+                }
+                else
+                {
+                    throw new PdfDocumentFormatException($"An XObject form named '{xObjectName}' is referencing itself which can cause unexpected behaviour.");
+                }
             }
 
             ProcessOperations(operations);
@@ -620,6 +628,20 @@ private void ProcessFormXObject(StreamToken formStream, NameToken xObjectName)
             }
         }
 
+        /// <summary>
+        /// Check for circular reference in the XObject form.
+        /// </summary>
+        /// <param name="formStream">The original form stream.</param>
+        /// <param name="xObjectName">The form's name.</param>
+        /// <param name="operations">The form operations parsed from original form stream.</param>
+        private bool HasFormXObjectCircularReference(StreamToken formStream, NameToken xObjectName, IReadOnlyList<IGraphicsStateOperation> operations)
+        {
+            return xObjectName != null
+                && operations.OfType<InvokeNamedXObject>()?.Any(o => o.Name == xObjectName) == true // operations contain another form with same name
+                && resourceStore.TryGetXObject(xObjectName, out var result)
+                && result.Data.SequenceEqual(formStream.Data); // The form contained in the operations has identical data to current form
+        }
+
         public void BeginSubpath()
         {
             if (CurrentPath == null)