Enhance error handling when native methods return 0 or positive values as success signals

Usually libgit2 API returns negative values for errors and 0 (zero) when the methods executed successfully. In some rare cases, some native methods may additionally return positive values. This fix cope with this.

2 files changed, 21 insertions, 5 deletions

diff --git a/LibGit2Sharp/Core/Ensure.cs b/LibGit2Sharp/Core/Ensure.cs
index bbd4c31a..8d943cfd 100644
--- a/LibGit2Sharp/Core/Ensure.cs
+++ b/LibGit2Sharp/Core/Ensure.cs
@@ -38,11 +38,24 @@ namespace LibGit2Sharp.Core
 
         /// <summary>
         ///   Check that the result of a C call was successful
+        /// <para>
+        /// This usually means that the method is expected to return 0.
+        /// In some rare cases, some methods may return negative values for errors and
+        /// positive values carrying information. Those positive values should be interpreted
+        /// as successful calls as well.
+        /// </para>
         /// </summary>
-        /// <param name = "result">The result.</param>
-        public static void Success(int result)
+        /// <param name = "result">The result to examine.</param>
+        /// <param name="allowPositiveResult">False to only allow success when comparing against 0, 
+        /// True when positive values are allowed as well.</param>
+        public static void Success(int result, bool allowPositiveResult = false)
         {
-            if (result == (int) GitErrorCode.GIT_SUCCESS)
+            if (result == (int)GitErrorCode.GIT_SUCCESS)
+            {
+                return;
+            }
+
+            if (allowPositiveResult && result > (int)GitErrorCode.GIT_SUCCESS)
             {
                 return;
             }
@@ -65,7 +78,7 @@ namespace LibGit2Sharp.Core
             {
                 return;
             }
-            
+
             throw new ArgumentException(argumentName);
         }
     }
diff --git a/LibGit2Sharp/Index.cs b/LibGit2Sharp/Index.cs
index cf1c811b..de822896 100644
--- a/LibGit2Sharp/Index.cs
+++ b/LibGit2Sharp/Index.cs
@@ -30,7 +30,10 @@ namespace LibGit2Sharp
             {
                 Ensure.ArgumentNotNullOrEmptyString(path, "path");
 
-                return this[NativeMethods.git_index_find(handle, path)];
+                int res = NativeMethods.git_index_find(handle, path);
+                Ensure.Success(res, true);
+
+                return this[res];
             }
         }