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, 17 insertions, 18 deletions

diff --git a/odb.go b/odb.go
index d313e3b..2d052ac 100644
--- a/odb.go
+++ b/odb.go
@@ -175,35 +175,33 @@ func (v *Odb) Read(oid *Oid) (obj *OdbObject, err error) {
 }
 
 type OdbForEachCallback func(id *Oid) error
-
-type foreachData struct {
-	callback OdbForEachCallback
-	err      error
+type odbForEachCallbackData struct {
+	callback    OdbForEachCallback
+	errorTarget *error
 }
 
-//export odbForEachCb
-func odbForEachCb(id *C.git_oid, handle unsafe.Pointer) int {
-	data, ok := pointerHandles.Get(handle).(*foreachData)
-
+//export odbForEachCallback
+func odbForEachCallback(id *C.git_oid, handle unsafe.Pointer) C.int {
+	data, ok := pointerHandles.Get(handle).(*odbForEachCallbackData)
 	if !ok {
 		panic("could not retrieve handle")
 	}
 
 	err := data.callback(newOidFromC(id))
 	if err != nil {
-		data.err = err
-		return C.GIT_EUSER
+		*data.errorTarget = err
+		return C.int(ErrorCodeUser)
 	}
 
-	return 0
+	return C.int(ErrorCodeOK)
 }
 
 func (v *Odb) ForEach(callback OdbForEachCallback) error {
-	data := foreachData{
-		callback: callback,
-		err:      nil,
+	var err error
+	data := odbForEachCallbackData{
+		callback:    callback,
+		errorTarget: &err,
 	}
-
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
@@ -212,9 +210,10 @@ func (v *Odb) ForEach(callback OdbForEachCallback) error {
 
 	ret := C._go_git_odb_foreach(v.ptr, handle)
 	runtime.KeepAlive(v)
-	if ret == C.GIT_EUSER {
-		return data.err
-	} else if ret < 0 {
+	if ret == C.int(ErrorCodeUser) && err != nil {
+		return err
+	}
+	if ret < 0 {
 		return MakeGitError(ret)
 	}