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, 8 insertions, 1 deletions

diff --git a/reset.go b/reset.go
index b3ecff4..155b58b 100644
--- a/reset.go
+++ b/reset.go
@@ -17,8 +17,15 @@ const (
 func (r *Repository) ResetToCommit(commit *Commit, resetType ResetType, opts *CheckoutOptions) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
-	ret := C.git_reset(r.ptr, commit.ptr, C.git_reset_t(resetType), opts.toC())
 
+	var err error
+	cOpts := opts.toC(&err)
+	defer freeCheckoutOptions(cOpts)
+
+	ret := C.git_reset(r.ptr, commit.ptr, C.git_reset_t(resetType), cOpts)
+	if ret == C.int(ErrorCodeUser) && err != nil {
+		return err
+	}
 	if ret < 0 {
 		return MakeGitError(ret)
 	}