outposts: implement general paginator for list API requests (#10619)

* outposts: implement general paginator

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* migrate LDAP

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* change main outpost refresh logic to use paginator everywhere

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* add comments to understand anything

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* actually use paginator everywhere

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

---------

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

internal/outpost/ak/api_utils.go

@@ -0,0 +1,64 @@
+package ak
+
+import (
+	"errors"
+	"net/http"
+
+	log "github.com/sirupsen/logrus"
+	"goauthentik.io/api/v3"
+)
+
+// Generic interface that mimics a generated request by the API client
+// Requires mainly `Treq` which will be the actual request type, and
+// `Tres` which is the response type
+type PaginatorRequest[Treq any, Tres any] interface {
+	Page(page int32) Treq
+	PageSize(size int32) Treq
+	Execute() (Tres, *http.Response, error)
+}
+
+// Generic interface that mimics a generated response by the API client
+type PaginatorResponse[Tobj any] interface {
+	GetResults() []Tobj
+	GetPagination() api.Pagination
+}
+
+// Paginator options for page size
+type PaginatorOptions struct {
+	PageSize int
+	Logger   *log.Entry
+}
+
+// Automatically fetch all objects from an API endpoint using the pagination
+// data received from the server.
+func Paginator[Tobj any, Treq any, Tres PaginatorResponse[Tobj]](
+	req PaginatorRequest[Treq, Tres],
+	opts PaginatorOptions,
+) ([]Tobj, error) {
+	fetchOffset := func(page int32) (Tres, error) {
+		req.Page(page)
+		req.PageSize(int32(opts.PageSize))
+		res, _, err := req.Execute()
+		if err != nil {
+			opts.Logger.WithError(err).WithField("page", page).Warning("failed to fetch page")
+		}
+		return res, err
+	}
+	var page int32 = 1
+	errs := make([]error, 0)
+	objects := make([]Tobj, 0)
+	for {
+		apiObjects, err := fetchOffset(page)
+		if err != nil {
+			errs = append(errs, err)
+			continue
+		}
+		objects = append(objects, apiObjects.GetResults()...)
+		if apiObjects.GetPagination().Next > 0 {
+			page += 1
+		} else {
+			break
+		}
+	}
+	return objects, errors.Join(errs...)
+}