Added logging of json snippets on json deserialization errors.

src/NzbDrone.Common/Serializer/Json.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Reflection;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
 using Newtonsoft.Json.Serialization;
@@ -14,13 +15,13 @@ namespace NzbDrone.Common.Serializer
         static Json()
         {
             SerializerSetting = new JsonSerializerSettings
-                        {
-                            DateTimeZoneHandling = DateTimeZoneHandling.Utc,
-                            NullValueHandling = NullValueHandling.Ignore,
-                            Formatting = Formatting.Indented,
-                            DefaultValueHandling = DefaultValueHandling.Include,
-                            ContractResolver = new CamelCasePropertyNamesContractResolver()
-                        };
+            {
+                DateTimeZoneHandling = DateTimeZoneHandling.Utc,
+                NullValueHandling = NullValueHandling.Ignore,
+                Formatting = Formatting.Indented,
+                DefaultValueHandling = DefaultValueHandling.Include,
+                ContractResolver = new CamelCasePropertyNamesContractResolver()
+            };
 
 
             SerializerSetting.Converters.Add(new StringEnumConverter { CamelCaseText = true });
@@ -34,12 +35,61 @@ namespace NzbDrone.Common.Serializer
 
         public static T Deserialize<T>(string json) where T : new()
         {
-            return JsonConvert.DeserializeObject<T>(json, SerializerSetting);
+            try
+            {
+                return JsonConvert.DeserializeObject<T>(json, SerializerSetting);
+            }
+            catch (JsonReaderException ex)
+            {
+                throw DetailedJsonReaderException(ex, json);
+            }
         }
 
         public static object Deserialize(string json, Type type)
         {
-            return JsonConvert.DeserializeObject(json, type, SerializerSetting);
+            try
+            {
+                return JsonConvert.DeserializeObject(json, type, SerializerSetting);
+            }
+            catch (JsonReaderException ex)
+            {
+                throw DetailedJsonReaderException(ex, json);
+            }
+        }
+
+        private static JsonReaderException DetailedJsonReaderException(JsonReaderException ex, string json)
+        {
+            var lineNumber = ex.LineNumber == 0 ? 0 : (ex.LineNumber - 1);
+            var linePosition = ex.LinePosition;
+
+            var lines = json.Split('\n');
+            if (lineNumber >= 0 && lineNumber < lines.Length &&
+                linePosition >= 0 && linePosition < lines[lineNumber].Length)
+            {
+                var line = lines[lineNumber];
+                var start = Math.Max(0, linePosition - 20);
+                var end = Math.Min(line.Length, linePosition + 20);
+
+                var snippetBefore = line.Substring(start, linePosition - start);
+                var snippetAfter = line.Substring(linePosition, end - linePosition);
+                var message = ex.Message + " (Json snippet '" + snippetBefore + "<--error-->" + snippetAfter + "')";
+
+                // Not risking updating JSON.net from 9.x to 10.x just to get this as public ctor.
+                var ctor = typeof(JsonReaderException).GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, new Type[] { typeof(string), typeof(Exception), typeof(string), typeof(int), typeof(int) }, null);
+                if (ctor != null)
+                {
+                    return (JsonReaderException)ctor.Invoke(new object[] { message, ex, ex.Path, ex.LineNumber, linePosition });
+                }
+
+                // JSON.net 10.x ctor in case we update later.
+                ctor = typeof(JsonReaderException).GetConstructor(BindingFlags.Public | BindingFlags.Instance, null, new Type[] { typeof(string), typeof(string), typeof(int), typeof(int), typeof(Exception) }, null);
+                if (ctor != null)
+                {
+                    return (JsonReaderException)ctor.Invoke(new object[] { message, ex.Path, ex.LineNumber, linePosition, ex });
+                }
+            }
+
+            return ex;
         }
 
         public static bool TryDeserialize<T>(string json, out T result) where T : new()
@@ -78,4 +128,4 @@ namespace NzbDrone.Common.Serializer
             Serialize(model, new StreamWriter(outputStream));
         }
     }
-}
+}

src/NzbDrone.Libraries.Test/JsonTests/JsonFixture.cs

@@ -1,3 +1,4 @@
+using System;
 using FluentAssertions;
 using NUnit.Framework;
 using NzbDrone.Common.Serializer;
@@ -24,5 +25,31 @@ namespace NzbDrone.Libraries.Test.JsonTests
 
             result.ShouldBeEquivalentTo(quality, o => o.IncludingAllRuntimeProperties());
         }
+
+        [Test]
+        public void should_log_start_snippet_on_failure()
+        {
+            try
+            {
+                Json.Deserialize<object>("asdfl kasjd fsdfs derers");
+            }
+            catch (Exception ex)
+            {
+                ex.Message.Should().Contain("snippet '<--error-->asdfl kasjd fsdfs de'");
+            }
+        }
+
+        [Test]
+        public void should_log_line_snippet_on_failure()
+        {
+            try
+            {
+                Json.Deserialize<object>("{ \"a\": \r\n\"b\",\r\n \"b\": \"c\", asdfl kasjd fsdfs derers vsdfsdf");
+            }
+            catch (Exception ex)
+            {
+                ex.Message.Should().Contain("snippet ' \"b\": \"c\", asdfl <--error-->kasjd fsdfs derers v'");
+            }
+        }
     }
 }