Refactor all callbacks (#700)

This change is a preparation for another change that makes all callback
types return a Go error instead of an error code / an integer. That is
going to make make things a lot more idiomatic.

The reason this change is split is threefold:

a) This change is mostly mechanical and should contain no semantic
   changes.
b) This change is backwards-compatible (in the Go API compatibility
   sense of the word), and thus can be backported to all other releases.
c) It makes the other change a bit smaller and more focused on just one
   thing.

Concretely, this change makes all callbacks populate a Go error when
they fail. If the callback is invoked from the same stack as the
function to which it was passed (e.g. for `Tree.Walk`), it will preserve
the error object directly into a struct that also holds the callback
function. Otherwise if the callback is pased to one func and will be
invoked when run from another one (e.g. for `Repository.InitRebase`),
the error string is saved into the libgit2 thread-local storage and then
re-created as a `GitError`.

1 files changed, 9 insertions, 4 deletions

diff --git a/patch.go b/patch.go
index 4c6648e..6441751 100644
--- a/patch.go
+++ b/patch.go
@@ -77,17 +77,22 @@ func (v *Repository) PatchFromBuffers(oldPath, newPath string, oldBuf, newBuf []
 	cNewPath := C.CString(newPath)
 	defer C.free(unsafe.Pointer(cNewPath))
 
-	copts := diffOptionsToC(opts, v)
+	var err error
+	copts := opts.toC(v, &err)
 	defer freeDiffOptions(copts)
 
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	ecode := C.git_patch_from_buffers(&patchPtr, oldPtr, C.size_t(len(oldBuf)), cOldPath, newPtr, C.size_t(len(newBuf)), cNewPath, copts)
+	ret := C.git_patch_from_buffers(&patchPtr, oldPtr, C.size_t(len(oldBuf)), cOldPath, newPtr, C.size_t(len(newBuf)), cNewPath, copts)
 	runtime.KeepAlive(oldBuf)
 	runtime.KeepAlive(newBuf)
-	if ecode < 0 {
-		return nil, MakeGitError(ecode)
+	if ret == C.int(ErrorCodeUser) && err != nil {
+		return nil, err
+	}
+	if ret < 0 {
+		return nil, MakeGitError(ret)
 	}
+
 	return newPatchFromC(patchPtr), nil
 }