Improve HasFormXObjectCircularReference and fix #1250

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

@@ -11,6 +11,31 @@
 
     public class GithubIssuesTests
     {
+        [Fact]
+        public void Issues1250()
+        {
+            // Issue comes from HasFormXObjectCircularReference
+            var path = IntegrationHelpers.GetDocumentPath("SPE8EF26T0545.pdf");
+            using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = true }))
+            {
+                var page = document.GetPage(1);
+                Assert.NotNull(page);
+                Assert.NotEmpty(page.Letters);
+
+                page = document.GetPage(7);
+                Assert.NotNull(page);
+                Assert.NotEmpty(page.Letters);
+            }
+
+            // Ensure still no StackOverflowException
+            using (var document = PdfDocument.Open(IntegrationHelpers.GetDocumentPath("issue_671")))
+            {
+                var page = document.GetPage(1);
+                Assert.NotNull(page);
+                Assert.NotEmpty(page.Letters);
+            }
+        }
+
         [Fact]
         public void Issues1248()
         {
@@ -29,7 +54,7 @@
                 }
             }
         }
-
+        
         [Fact]
         public void Issues1238()
         {

src/UglyToad.PdfPig/Graphics/BaseStreamProcessor.cs

@@ -587,7 +587,8 @@
             if (hasCircularReference)
             {
                 if (ParsingOptions.UseLenientParsing)
-                {
+                {
+                    // TODO - We might be removing too much, good for the moment. See Issues1250() for examples
                     operations = operations.Where(o => o is not InvokeNamedXObject xo || xo.Name != xObjectName)
                         .ToArray();
                     ParsingOptions.Logger.Warn(
@@ -618,14 +619,56 @@
         /// <param name="xObjectName">The form's name.</param>
         /// <param name="operations">The form operations parsed from original form stream.</param>
         protected virtual bool HasFormXObjectCircularReference(StreamToken formStream,
-            NameToken xObjectName,
+            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.Span.SequenceEqual(formStream.Data.Span); // The form contained in the operations has identical data to current form
+            if (xObjectName is null)
+            {
+                return false;
+            }
+            
+            if (operations.OfType<InvokeNamedXObject>()?.Any(o => o.Name == xObjectName) != true)
+            {
+                return false;
+            }
+
+            if (!TryGetXObjectToken(formStream, xObjectName, PdfScanner, out var t1))
+            {
+                return false;
+            }
+
+            if (!ResourceStore.TryGetXObject(xObjectName, out var resourceStream))
+            {
+                return false;
+            }
+
+            if (!TryGetXObjectToken(resourceStream, xObjectName, PdfScanner, out var t2))
+            {
+                return false;
+            }
+            
+            if (t1 is null || t2 is null)
+            {
+                return false;
+            }
+
+            return t1.Equals(t2);
+
+            static bool TryGetXObjectToken(StreamToken streamToken, NameToken xObjectName, IPdfTokenScanner scanner, out IToken? token)
+            {
+                token = null;
+                if (!streamToken.StreamDictionary.TryGet<DictionaryToken>(NameToken.Resources, scanner, out var formResources))
+                {
+                    return false;
+                }
+
+                if (!formResources.TryGet<DictionaryToken>(NameToken.Xobject, out var xObjectBase) || !xObjectBase.TryGet(xObjectName, out token))
+                {
+                    return false;
+                }
+
+                return token is not null;
+            }
         }
 
         /// <inheritdoc/>