Add a test and some comments as to the ugly pointer arithmetic

repository.go

@@ -412,12 +412,19 @@ func (v *Repository) CreateCommitFromIds(
 
 	nparents := len(parents)
 	if nparents > 0 {
-		parentsarg = (**C.git_oid)(C.malloc(C.size_t(unsafe.Sizeof(uintptr(0)) * uintptr(nparents))))
+		// All this awful pointer arithmetic is needed to avoid passing a Go
+		// pointer to Go pointer into C. Other methods (like CreateCommits) are
+		// fine without this workaround because they are just passing Go pointers
+		// to C pointers, but arrays-of-pointers-to-git_oid are a bit special since
+		// both the array and the objects are allocated from Go.
+		var emptyOidPtr *C.git_oid
+		sizeofOidPtr := unsafe.Sizeof(emptyOidPtr)
+		parentsarg = (**C.git_oid)(C.calloc(C.size_t(uintptr(nparents)), C.size_t(sizeofOidPtr)))
 		defer C.free(unsafe.Pointer(parentsarg))
 		parentsptr := uintptr(unsafe.Pointer(parentsarg))
 		for _, v := range parents {
 			*(**C.git_oid)(unsafe.Pointer(parentsptr)) = v.toC()
-			parentsptr += unsafe.Sizeof(uintptr(0))
+			parentsptr += sizeofOidPtr
 		}
 	}
 

repository_test.go

@@ -0,0 +1,42 @@
+package git
+
+import (
+	"testing"
+	"time"
+)
+
+func TestCreateCommitFromIds(t *testing.T) {
+	t.Parallel()
+	repo := createTestRepo(t)
+	defer cleanupTestRepo(t, repo)
+
+	loc, err := time.LoadLocation("Europe/Berlin")
+	checkFatal(t, err)
+	sig := &Signature{
+		Name:  "Rand Om Hacker",
+		Email: "random@hacker.com",
+		When:  time.Date(2013, 03, 06, 14, 30, 0, 0, loc),
+	}
+
+	idx, err := repo.Index()
+	checkFatal(t, err)
+	err = idx.AddByPath("README")
+	checkFatal(t, err)
+	err = idx.Write()
+	checkFatal(t, err)
+	treeId, err := idx.WriteTree()
+	checkFatal(t, err)
+
+	message := "This is a commit\n"
+	tree, err := repo.LookupTree(treeId)
+	checkFatal(t, err)
+	expectedCommitId, err := repo.CreateCommit("HEAD", sig, sig, message, tree)
+	checkFatal(t, err)
+
+	commitId, err := repo.CreateCommitFromIds("", sig, sig, message, treeId)
+	checkFatal(t, err)
+
+	if !expectedCommitId.Equal(commitId) {
+		t.Errorf("mismatched commit ids, expected %v, got %v", expectedCommitId.String(), commitId.String())
+	}
+}