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, 11 insertions, 0 deletions

diff --git a/git.go b/git.go
index e13c7e8..cdcf6bb 100644
--- a/git.go
+++ b/git.go
@@ -330,6 +330,17 @@ func ucbool(b bool) C.uint {
 	return C.uint(0)
 }
 
+func setCallbackError(errorMessage **C.char, err error) C.int {
+	if err != nil {
+		*errorMessage = C.CString(err.Error())
+		if gitError, ok := err.(*GitError); ok {
+			return C.int(gitError.Code)
+		}
+		return C.int(ErrorCodeUser)
+	}
+	return C.int(ErrorCodeOK)
+}
+
 func Discover(start string, across_fs bool, ceiling_dirs []string) (string, error) {
 	ceildirs := C.CString(strings.Join(ceiling_dirs, string(C.GIT_PATH_LIST_SEPARATOR)))
 	defer C.free(unsafe.Pointer(ceildirs))