Add Commit.ExtractSignature to get signature data from a commit

This method returns the signature data along with the rest of the commit
without this header field, ready to be fed to whatever authentication
method the caller prefers.

LibGit2Sharp.Tests/CommitFixture.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Text;
 using LibGit2Sharp.Core;
 using LibGit2Sharp.Tests.TestHelpers;
 using Xunit;
@@ -1055,5 +1056,75 @@ public void CanPrettifyAMessage()
             Assert.Equal(expected, Commit.PrettifyMessage(input, '#'));
             Assert.Equal(expected, Commit.PrettifyMessage(input.Replace('#', ';'), ';'));
         }
+
+        [Fact]
+        public void CanExtractSignatureFromCommit()
+        {
+            string commitData = @"tree 6b79e22d69bf46e289df0345a14ca059dfc9bdf6
+parent 34734e478d6cf50c27c9d69026d93974d052c454
+author Ben Burkert <[email protected]> 1358451456 -0800
+committer Ben Burkert <[email protected]> 1358451456 -0800
+gpgsig -----BEGIN PGP SIGNATURE-----
+ Version: GnuPG v1.4.12 (Darwin)
+ 
+ iQIcBAABAgAGBQJQ+FMIAAoJEH+LfPdZDSs1e3EQAJMjhqjWF+WkGLHju7pTw2al
+ o6IoMAhv0Z/LHlWhzBd9e7JeCnanRt12bAU7yvYp9+Z+z+dbwqLwDoFp8LVuigl8
+ JGLcnwiUW3rSvhjdCp9irdb4+bhKUnKUzSdsR2CK4/hC0N2i/HOvMYX+BRsvqweq
+ AsAkA6dAWh+gAfedrBUkCTGhlNYoetjdakWqlGL1TiKAefEZrtA1TpPkGn92vbLq
+ SphFRUY9hVn1ZBWrT3hEpvAIcZag3rTOiRVT1X1flj8B2vGCEr3RrcwOIZikpdaW
+ who/X3xh/DGbI2RbuxmmJpxxP/8dsVchRJJzBwG+yhwU/iN3MlV2c5D69tls/Dok
+ 6VbyU4lm/ae0y3yR83D9dUlkycOnmmlBAHKIZ9qUts9X7mWJf0+yy2QxJVpjaTGG
+ cmnQKKPeNIhGJk2ENnnnzjEve7L7YJQF6itbx5VCOcsGh3Ocb3YR7DMdWjt7f8pu
+ c6j+q1rP7EpE2afUN/geSlp5i3x8aXZPDj67jImbVCE/Q1X9voCtyzGJH7MXR0N9
+ ZpRF8yzveRfMH8bwAJjSOGAFF5XkcR/RNY95o+J+QcgBLdX48h+ZdNmUf6jqlu3J
+ 7KmTXXQcOVpN6dD3CmRFsbjq+x6RHwa8u1iGn+oIkX908r97ckfB/kHKH7ZdXIJc
+ cpxtDQQMGYFpXK/71stq
+ =ozeK
+ -----END PGP SIGNATURE-----
+
+a simple commit which works
+";
+
+            string signatureData = @"-----BEGIN PGP SIGNATURE-----
+Version: GnuPG v1.4.12 (Darwin)
+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+=ozeK
+-----END PGP SIGNATURE-----";
+
+            string signedData = @"tree 6b79e22d69bf46e289df0345a14ca059dfc9bdf6
+parent 34734e478d6cf50c27c9d69026d93974d052c454
+author Ben Burkert <[email protected]> 1358451456 -0800
+committer Ben Burkert <[email protected]> 1358451456 -0800
+
+a simple commit which works
+";
+
+            string repoPath = InitNewRepository();
+            using (var repo = new Repository(repoPath))
+            {
+                var odb = repo.ObjectDatabase;
+                var signedId = odb.Write<Commit>(Encoding.UTF8.GetBytes(commitData));
+
+                // Look up the commit to make sure we wrote something valid
+                var commit = repo.Lookup<Commit>(signedId);
+                Assert.Equal("a simple commit which works\n", commit.Message);
+
+                var signatureInfo = Commit.ExtractSignature(repo, signedId, "gpgsig");
+                Assert.Equal(signedData, signatureInfo.SignedData);
+                Assert.Equal(signatureData, signatureInfo.Signature);
+            }
+        }
     }
 }

LibGit2Sharp/Commit.cs

@@ -138,6 +138,32 @@ private string DebuggerDisplay
             }
         }
 
+        /// <summary>
+        /// Extract the signature data from this commit
+        /// </summary>
+        /// <returns>The signature and the signed data</returns>
+        /// <param name="repo">The repository in which the object lives</param>
+        /// <param name="id">The commit to extract the signature from</param>
+        /// <param name="field">The header field which contains the signature; use null for the default of "gpgsig"</param>
+        public static SignatureInfo ExtractSignature(Repository repo, ObjectId id, string field)
+        {
+            return Proxy.git_commit_extract_signature(repo.Handle, id, field);
+        }
+
+        /// <summary>
+        /// Extract the signature data from this commit
+        /// <para>
+        /// The overload uses the default header field "gpgsig"
+        /// </para>
+        /// </summary>
+        /// <returns>The signature and the signed data</returns>
+        /// <param name="repo">The repository in which the object lives</param>
+        /// <param name="id">The commit to extract the signature from</param>
+        public static SignatureInfo ExtractSignature(Repository repo, ObjectId id)
+        {
+            return Proxy.git_commit_extract_signature(repo.Handle, id, null);
+        }
+
         private class ParentsCollection : ICollection<Commit>
         {
             private readonly Lazy<ICollection<Commit>> _parents;

LibGit2Sharp/Core/NativeMethods.cs

@@ -337,6 +337,14 @@ internal static extern unsafe int git_commit_create_from_ids(
         [DllImport(libgit2)]
         internal static extern unsafe git_oid* git_commit_tree_id(git_object* commit);
 
+        [DllImport(libgit2)]
+        internal static extern unsafe int git_commit_extract_signature(
+            GitBuf signature,
+            GitBuf signed_data,
+            git_repository *repo,
+            ref GitOid commit_id,
+            [MarshalAs(UnmanagedType.CustomMarshaler, MarshalCookie = UniqueId.UniqueIdentifier, MarshalTypeRef = typeof(StrictUtf8Marshaler))] string field);
+
         [DllImport(libgit2)]
         internal static extern unsafe int git_config_delete_entry(
             git_config* cfg,

LibGit2Sharp/Core/Proxy.cs

@@ -426,6 +426,22 @@ public static unsafe ObjectId git_commit_tree_id(ObjectHandle obj)
             return ObjectId.BuildFromPtr(NativeMethods.git_commit_tree_id(obj));
         }
 
+        public static unsafe SignatureInfo git_commit_extract_signature(RepositoryHandle repo, ObjectId id, string field)
+        {
+            using (var signature = new GitBuf())
+            using (var signedData = new GitBuf())
+            {
+                var oid = id.Oid;
+                Ensure.ZeroResult(NativeMethods.git_commit_extract_signature(signature, signedData, repo, ref oid, field));
+
+                return new SignatureInfo()
+                {
+                    Signature = LaxUtf8Marshaler.FromNative(signature.ptr, signature.size.ConvertToInt()),
+                    SignedData = LaxUtf8Marshaler.FromNative(signedData.ptr, signedData.size.ConvertToInt()),
+                };
+            }
+        }
+
         #endregion
 
         #region git_config_

LibGit2Sharp/LibGit2Sharp.csproj

@@ -355,6 +355,7 @@
     <Compile Include="Commands\Stage.cs" />
     <Compile Include="Commands\Remove.cs" />
     <Compile Include="Commands\Checkout.cs" />
+    <Compile Include="SignatureInfo.cs" />
   </ItemGroup>
   <ItemGroup>
     <CodeAnalysisDictionary Include="CustomDictionary.xml" />

LibGit2Sharp/SignatureInfo.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace LibGit2Sharp
+{
+    /// <summary>
+    /// Structure for holding a signature extracted from a commit or a tag
+    /// </summary>
+    public struct SignatureInfo
+    {
+        /// <summary>
+        /// The signature data, PGP/GPG or otherwise.
+        /// </summary>
+        public string Signature;
+        /// <summary>
+        /// The data which was signed. The object contents without the signature part.
+        /// </summary>
+        public string SignedData;
+    }
+}
+