Do not throw error on Pop when stack size is 1 in lenient mode and fix #973

src/UglyToad.PdfPig.Tests/Graphics/Operations/SpecialGraphicsState/PopTests.cs

@@ -14,7 +14,7 @@
             Assert.Equal("Q", Pop.Value.Operator);
         }
 
-        [Fact]
+        [Fact(Skip = "The stack size check has been moved out of the Pop Operation, and is now in BaseStreamProcessor.PopState().")]
         public void CannotPopWithSingleFrame()
         {
             Action action = () => Pop.Value.Run(context);

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

@@ -7,11 +7,35 @@
 
     public class GithubIssuesTests
     {
+        [Fact]
+        public void Issue973()
+        {
+            var path = IntegrationHelpers.GetSpecificTestDocumentPath("JD5008.pdf");
+
+            // Lenient parsing ON
+            using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = true }))
+            {
+                var page = document.GetPage(2);
+                Assert.NotNull(page);
+                Assert.Equal(2, page.Number);
+                Assert.NotEmpty(page.Letters);
+            }
+
+            // Lenient parsing OFF
+            using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = false }))
+            {
+                var exception = Assert.Throws<InvalidOperationException>(() => document.GetPage(2));
+                Assert.Equal("Cannot execute a pop of the graphics state stack, it would leave the stack empty.", exception.Message);
+            }
+        }
+
+
         [Fact]
         public void Issue959()
         {
-            // Lenient parsing ON
             var path = IntegrationHelpers.GetSpecificTestDocumentPath("algo.pdf");
+
+            // Lenient parsing ON
             using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = true }))
             {
                 for (int i = 1; i <= document.NumberOfPages; ++i)

src/UglyToad.PdfPig/Graphics/BaseStreamProcessor.cs

@@ -195,7 +195,21 @@
         /// <inheritdoc/>
         public virtual void PopState()
         {
-            GraphicsStack.Pop();
+            if (StackSize > 1)
+            {
+                GraphicsStack.Pop();
+            }
+            else
+            {
+                const string error = "Cannot execute a pop of the graphics state stack, it would leave the stack empty.";
+                ParsingOptions.Logger.Error(error);
+
+                if (!ParsingOptions.UseLenientParsing)
+                {
+                    throw new InvalidOperationException(error);
+                }
+            }
+
             ActiveExtendedGraphicsStateFont = null;
         }
 

src/UglyToad.PdfPig/Graphics/Operations/SpecialGraphicsState/Pop.cs

@@ -1,6 +1,5 @@
 namespace UglyToad.PdfPig.Graphics.Operations.SpecialGraphicsState
 {
-    using System;
     using System.IO;
 
     /// <inheritdoc />
@@ -29,15 +28,7 @@
         /// <inheritdoc />
         public void Run(IOperationContext operationContext)
         {
-            var currentStackSize = operationContext.StackSize;
-            if (currentStackSize > 1)
-            {
-                operationContext.PopState();
-            }
-            else
-            {
-                throw new InvalidOperationException("Cannot execute a pop of the graphics state stack, it would leave the stack empty.");
-            }
+            operationContext.PopState();
         }
 
         /// <inheritdoc />