e: IOException,
  )

  /**
   * Called when the HTTP response was successfully returned by the remote server. The callback may
   * proceed to read the response body with [Response.body]. The response is still live until its
   * response body is [closed][ResponseBody]. The recipient of the callback may consume the response
   * body on another thread.
   *

// license that can be found in the LICENSE file.

// Tests CPU profiling.

//go:build ignore

package main

/*
#include <pthread.h>

void go_callback();
void go_callback2();

static void *thr(void *arg) {
    go_callback();
    return 0;
}

static void foo() {
    pthread_t th;
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setstacksize(&attr, 256 << 10);

		}
	}
	return nil
}

func (p *processor) Before(name string) *callback {
	return &callback{before: name, processor: p}
}

func (p *processor) After(name string) *callback {
	return &callback{after: name, processor: p}
}

func (p *processor) Match(fc func(*DB) bool) *callback {
	return &callback{match: fc, processor: p}
}

func (p *processor) Register(name string, fn func(*DB)) error {

// Running under TSAN, this fails with "signal handler
// spoils errno".

/*
#include <pthread.h>
#include <signal.h>
#include <stdlib.h>

void go_callback();

static void *thr(void *arg) {
	int i;
	for (i = 0; i < 10; i++)
		go_callback();
	return 0;
}

static void *sendthr(void *arg) {
	pthread_t th = *(pthread_t*)arg;
	while (1) {
		int r = pthread_kill(th, SIGWINCH);
		if (r < 0)

    boolean isCancellationRequested();

    /**
     * Adds a callback that will be executed when cancel event is triggered.
     * It can be run synchronously if the token is already cancelled.
     *
     * @param cancellationHandler callback to be executed when cancel is requested.
     * @return current state of cancellation request before callback was added.
     */
    boolean addCallback(Runnable cancellationHandler);

        if (queryLambda != null) {
            queryLambda.callback(cq);
        }
        FilterAggregationBuilder builder = regFilterA(name, cq.getQuery());
        if (opLambda != null) {
            opLambda.callback(builder);
        }
        if (aggsLambda != null) {
            RoleCA ca = new RoleCA();
            aggsLambda.callback(ca);
            ca.getAggregationBuilderList().forEach(builder::subAggregation);

            Consumer<RESPONSE> callback;
            while ((callback = doneCallbacks.poll()) != null) {
                executeCallbacks.add(callback);
            }
        }
        if (executeCallbacks.size() > 0) {
            try {
                executeCallbacks.stream().forEach(callback -> callback.accept(response));
            } catch (final Exception ignore) {}
        }

## Die normale **FastAPI**-Anwendung

Sehen wir uns zunächst an, wie die normale API-Anwendung aussehen würde, bevor wir den Callback hinzufügen.

Sie verfügt über eine *Pfadoperation*, die einen `Invoice`-Body empfängt, und einen Query-Parameter `callback_url`, der die URL für den Callback enthält.

func TestCallbacks(t *testing.T) {
	type callback struct {
		name    string
		before  string
		after   string
		remove  bool
		replace bool
		err     string
		match   func(*gorm.DB) bool
		h       func(*gorm.DB)
	}

	datas := []struct {
		callbacks []callback
		err       string
		results   []string
	}{
		{
			callbacks: []callback{{h: c1}, {h: c2}, {h: c3}, {h: c4}, {h: c5}},

## The normal **FastAPI** app

Let's first see how the normal API app would look like before adding the callback.

It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

This part is pretty normal, most of the code is probably already familiar to you: