// Package admit is the protocol between the weave CNI plugin and weave-npc's egress agent: at CNI ADD the
// plugin tells the agent which pod got which IP, and the agent answers once the pod's egress rules are in
// place, so an EgressService member's first packet already leaves from the Service's address (or not at all).
//
// It depends on the standard library only: the plugin binary must not grow a Kubernetes client.
package admit

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net"
	"net/http"
	"strings"
	"time"
)

// SocketPath is where the agent listens (a hostPath directory shared with the host, where the plugin runs).
const SocketPath = "/run/weave-npc/egress.sock"

// Request is one pod's admission: the IP the plugin just allocated to it.
type Request struct {
	Namespace string `json:"namespace"`
	Name      string `json:"name"`
	IP        string `json:"ip"`
}

// Response says whether the pod is an EgressService member (its rules are installed either way when 200).
type Response struct {
	Member bool `json:"member"`
}

// Timeout bounds one admission (the agent may have to fetch the pod from the API server).
const Timeout = 20 * time.Second

// PodFromArgs reads the pod's namespace and name from CNI_ARGS ("K8S_POD_NAMESPACE=..;K8S_POD_NAME=..").
func PodFromArgs(args string) (namespace, name string) {
	for _, kv := range strings.Split(args, ";") {
		k, v, ok := strings.Cut(kv, "=")
		if !ok {
			continue
		}
		switch k {
		case "K8S_POD_NAMESPACE":
			namespace = v
		case "K8S_POD_NAME":
			name = v
		}
	}
	return namespace, name
}

// Admit asks the agent at socket to admit req, and returns only once the agent has installed the pod's rules.
func Admit(ctx context.Context, socket string, req Request) (Response, error) {
	var resp Response
	body, err := json.Marshal(req)
	if err != nil {
		return resp, err
	}
	client := &http.Client{Transport: &http.Transport{
		DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
			var d net.Dialer
			return d.DialContext(ctx, "unix", socket)
		},
	}}
	ctx, cancel := context.WithTimeout(ctx, Timeout)
	defer cancel()
	hreq, err := http.NewRequestWithContext(ctx, http.MethodPost, "http://weave-npc/admit", bytes.NewReader(body))
	if err != nil {
		return resp, err
	}
	hreq.Header.Set("Content-Type", "application/json")
	hresp, err := client.Do(hreq)
	if err != nil {
		return resp, fmt.Errorf("egress admission (weave-npc at %s): %v", socket, err)
	}
	defer hresp.Body.Close()
	out, _ := io.ReadAll(io.LimitReader(hresp.Body, 1<<16))
	if hresp.StatusCode != http.StatusOK {
		return resp, fmt.Errorf("egress admission refused (%s): %s", hresp.Status, strings.TrimSpace(string(out)))
	}
	if err := json.Unmarshal(out, &resp); err != nil {
		return resp, fmt.Errorf("egress admission: bad answer: %v", err)
	}
	return resp, nil
}