Do not throw exception when lenient parsing in GetExtendedGraphicsStateDictionary() and improve StackDictionary.TryGetValue() to not throw on empty

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

@@ -7,6 +7,33 @@
 
     public class GithubIssuesTests
     {
+        [Fact]
+        public void Issue953()
+        {
+            // NB: We actually do not fix issue 953 here, but another bug found with the same document.
+            var path = IntegrationHelpers.GetSpecificTestDocumentPath("FailedToParseContentForPage32.pdf");
+
+            // Lenient parsing ON + Skip missing fonts
+            using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = true, SkipMissingFonts = true}))
+            {
+                var page = document.GetPage(33);
+                Assert.Equal(33, page.Number);
+                Assert.Equal(792, page.Height);
+                Assert.Equal(612, page.Width);
+            }
+
+            // Lenient parsing ON + Do not Skip missing fonts
+            using (var document = PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = true, SkipMissingFonts = false }))
+            {
+                var pageException = Assert.Throws<InvalidOperationException>(() =>  document.GetPage(33));
+                Assert.Equal("Could not find the font with name /TT4 in the resource store. It has not been loaded yet.", pageException.Message);
+            }
+
+            var docException = Assert.Throws<PdfDocumentFormatException>(() =>
+                PdfDocument.Open(path, new ParsingOptions() { UseLenientParsing = false, SkipMissingFonts = false }));
+            Assert.Equal("Could not find dictionary associated with reference in pages kids array: 102 0.", docException.Message);
+        }
+
         [Fact]
         public void Issue987()
         {

src/UglyToad.PdfPig/Content/IResourceStore.cs

@@ -35,7 +35,7 @@
         /// <summary>
         /// Get the extended graphics state dictionary corresponding to the name.
         /// </summary>
-        DictionaryToken GetExtendedGraphicsStateDictionary(NameToken name);
+        DictionaryToken? GetExtendedGraphicsStateDictionary(NameToken name);
 
         /// <summary>
         /// Get the font from the <see cref="IndirectReferenceToken"/>.

src/UglyToad.PdfPig/Content/ResourceStore.cs

@@ -336,8 +336,14 @@
             return DirectObjectFinder.TryGet(new IndirectReferenceToken(indirectReference), scanner, out stream);
         }
 
-        public DictionaryToken GetExtendedGraphicsStateDictionary(NameToken name)
+        public DictionaryToken? GetExtendedGraphicsStateDictionary(NameToken name)
         {
+            if (parsingOptions.UseLenientParsing && !extendedGraphicsStates.ContainsKey(name))
+            {
+                parsingOptions.Logger.Error($"The graphic state dictionary does not contain the key '{name}'.");
+                return null;
+            }
+
             return extendedGraphicsStates[name];
         }
 

src/UglyToad.PdfPig/Util/StackDictionary.cs

@@ -5,7 +5,7 @@ namespace UglyToad.PdfPig.Util
     using System.Collections.Generic;
     using System.Diagnostics.CodeAnalysis;
 
-    internal class StackDictionary<K, V> where K : notnull
+    internal sealed class StackDictionary<K, V> where K : notnull
     {
         private readonly List<Dictionary<K, V>> values = new List<Dictionary<K, V>>();
 
@@ -13,6 +13,11 @@ namespace UglyToad.PdfPig.Util
         {
             get
             {
+                if (values.Count == 0)
+                {
+                    throw new InvalidOperationException($"Cannot get item from empty stack, call {nameof(Push)} before use.");
+                }
+
                 if (TryGetValue(key, out var result))
                 {
                     return result;
@@ -35,7 +40,8 @@ namespace UglyToad.PdfPig.Util
         {
             if (values.Count == 0)
             {
-                throw new InvalidOperationException($"Cannot get item from empty stack, call {nameof(Push)} before use.");
+                result = default!;
+                return false;
             }
 
             for (var i = values.Count - 1; i >= 0; i--)