/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package certificate

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	cryptorand "crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"sync"
	"time"

	"github.com/golang/glog"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/fields"
	"k8s.io/apimachinery/pkg/util/wait"
	"k8s.io/apimachinery/pkg/watch"
	"k8s.io/client-go/util/cert"
	certificates "k8s.io/kubernetes/pkg/apis/certificates/v1beta1"
	certificatesclient "k8s.io/kubernetes/pkg/client/clientset_generated/clientset/typed/certificates/v1beta1"
)

const (
	syncPeriod = 1 * time.Hour
)

// Manager maintains and updates the certificates in use by this certificate
// manager. In the background it communicates with the API server to get new
// certificates for certificates about to expire.
type Manager interface {
	// Start the API server status sync loop.
	Start()
	// GetCertificate gets the current certificate from the certificate
	// manager. This function matches the signature required by
	// tls.Config.GetCertificate so it can be passed as TLS configuration. A
	// TLS server will automatically call back here to get the correct
	// certificate when establishing each new connection.
	GetCertificate(clientHello *tls.ClientHelloInfo) (*tls.Certificate, error)
}

// Store is responsible for getting and updating the current certificate.
// Depending on the concrete implementation, the backing store for this
// behavior may vary.
type Store interface {
	// Current returns the currently selected certificate.
	Current() (*tls.Certificate, error)
	// Update accepts the PEM data for the cert/key pair and makes the new
	// cert/key pair the 'current' pair, that will be returned by future calls
	// to Current().
	Update(cert, key []byte) (*tls.Certificate, error)
}

type manager struct {
	certSigningRequestClient certificatesclient.CertificateSigningRequestInterface
	template                 *x509.CertificateRequest
	usages                   []certificates.KeyUsage
	certStore                Store
	certAccessLock           sync.RWMutex
	cert                     *tls.Certificate
	shouldRotatePercent      uint
}

// NewManager returns a new certificate manager. A certificate manager is
// responsible for being the authoritative source of certificates in the
// Kubelet and handling updates due to rotation.
func NewManager(
	certSigningRequestClient certificatesclient.CertificateSigningRequestInterface,
	template *x509.CertificateRequest,
	usages []certificates.KeyUsage,
	certificateStore Store,
	certRotationPercent uint) (Manager, error) {

	cert, err := certificateStore.Current()
	if err != nil {
		return nil, err
	}

	if certRotationPercent > 100 {
		certRotationPercent = 100
	}

	m := manager{
		certSigningRequestClient: certSigningRequestClient,
		template:                 template,
		usages:                   usages,
		certStore:                certificateStore,
		cert:                     cert,
		shouldRotatePercent:      certRotationPercent,
	}

	return &m, nil
}

// GetCertificate returns the certificate that should be used with TLS
// connections.  The value returned by this function will change over time as
// the certificate is rotated. If a reference to this method is passed directly
// into the TLS options for a connection, certificate rotation will be handled
// correctly by the underlying go libraries.
//
//    tlsOptions := &server.TLSOptions{
//        ...
//        GetCertificate: certificateManager.GetCertificate
//        ...
//    }
//
func (m *manager) GetCertificate(clientHello *tls.ClientHelloInfo) (*tls.Certificate, error) {
	m.certAccessLock.RLock()
	defer m.certAccessLock.RUnlock()
	return m.cert, nil
}

// Start will start the background work of rotating the certificates.
func (m *manager) Start() {
	if m.shouldRotatePercent < 1 {
		glog.V(2).Infof("Certificate rotation is not enabled.")
		return
	}

	// Certificate rotation depends on access to the API server certificate
	// signing API, so don't start the certificate manager if we don't have a
	// client. This will happen on the master, where the kubelet is responsible
	// for bootstrapping the pods of the master components.
	if m.certSigningRequestClient == nil {
		glog.V(2).Infof("Certificate rotation is not enabled, no connection to the apiserver.")
		return
	}

	glog.V(2).Infof("Certificate rotation is enabled.")
	go wait.Forever(func() {
		for range time.Tick(syncPeriod) {
			err := m.rotateCerts()
			if err != nil {
				glog.Errorf("Could not rotate certificates: %v", err)
			}
		}
	}, 0)
}

// shouldRotate looks at how close the current certificate is to expiring and
// decides if it is time to rotate or not.
func (m *manager) shouldRotate() bool {
	m.certAccessLock.RLock()
	defer m.certAccessLock.RUnlock()
	notAfter := m.cert.Leaf.NotAfter
	total := notAfter.Sub(m.cert.Leaf.NotBefore)
	remaining := notAfter.Sub(time.Now())
	return remaining < 0 || uint(remaining*100/total) < m.shouldRotatePercent
}

func (m *manager) rotateCerts() error {
	if !m.shouldRotate() {
		return nil
	}

	csrPEM, keyPEM, err := m.generateCSR()
	if err != nil {
		return err
	}

	// Call the Certificate Signing Request API to get a certificate for the
	// new private key.
	crtPEM, err := requestCertificate(m.certSigningRequestClient, csrPEM, m.usages)
	if err != nil {
		return fmt.Errorf("unable to get a new key signed: %v", err)
	}

	cert, err := m.certStore.Update(crtPEM, keyPEM)
	if err != nil {
		return fmt.Errorf("unable to store the new cert/key pair: %v", err)
	}

	m.certAccessLock.Lock()
	defer m.certAccessLock.Unlock()
	m.cert = cert
	return nil
}

func (m *manager) generateCSR() (csrPEM []byte, keyPEM []byte, err error) {
	// Generate a new private key.
	privateKey, err := ecdsa.GenerateKey(elliptic.P256(), cryptorand.Reader)
	if err != nil {
		return nil, nil, fmt.Errorf("unable to generate a new private key: %v", err)
	}
	der, err := x509.MarshalECPrivateKey(privateKey)
	if err != nil {
		return nil, nil, fmt.Errorf("unable to marshal the new key to DER: %v", err)
	}

	keyPEM = pem.EncodeToMemory(&pem.Block{Type: cert.ECPrivateKeyBlockType, Bytes: der})

	csrPEM, err = cert.MakeCSRFromTemplate(privateKey, m.template)
	if err != nil {
		return nil, nil, fmt.Errorf("unable to create a csr from the private key: %v", err)
	}
	return csrPEM, keyPEM, nil
}

// requestCertificate will create a certificate signing request using the PEM
// encoded CSR and send it to API server, then it will watch the object's
// status, once approved by API server, it will return the API server's issued
// certificate (pem-encoded). If there is any errors, or the watch timeouts, it
// will return an error.
//
// NOTE This is a copy of a function with the same name in
// k8s.io/kubernetes/pkg/kubelet/util/csr/csr.go, changing only the package that
// CertificateSigningRequestInterface and KeyUsage are imported from.
func requestCertificate(client certificatesclient.CertificateSigningRequestInterface, csrData []byte, usages []certificates.KeyUsage) (certData []byte, err error) {
	req, err := client.Create(&certificates.CertificateSigningRequest{
		// Username, UID, Groups will be injected by API server.
		TypeMeta:   metav1.TypeMeta{Kind: "CertificateSigningRequest"},
		ObjectMeta: metav1.ObjectMeta{GenerateName: "csr-"},

		Spec: certificates.CertificateSigningRequestSpec{
			Request: csrData,
			Usages:  usages,
		},
	})
	if err != nil {
		return nil, fmt.Errorf("cannot create certificate signing request: %v", err)
	}

	// Make a default timeout = 3600s.
	var defaultTimeoutSeconds int64 = 3600
	certWatch, err := client.Watch(metav1.ListOptions{
		Watch:          true,
		TimeoutSeconds: &defaultTimeoutSeconds,
		FieldSelector:  fields.OneTermEqualSelector("metadata.name", req.Name).String(),
	})
	if err != nil {
		return nil, fmt.Errorf("cannot watch on the certificate signing request: %v", err)
	}
	defer certWatch.Stop()
	ch := certWatch.ResultChan()

	for {
		event, ok := <-ch
		if !ok {
			break
		}

		if event.Type == watch.Modified || event.Type == watch.Added {
			if event.Object.(*certificates.CertificateSigningRequest).UID != req.UID {
				continue
			}
			status := event.Object.(*certificates.CertificateSigningRequest).Status
			for _, c := range status.Conditions {
				if c.Type == certificates.CertificateDenied {
					return nil, fmt.Errorf("certificate signing request is not approved, reason: %v, message: %v", c.Reason, c.Message)
				}
				if c.Type == certificates.CertificateApproved && status.Certificate != nil {
					return status.Certificate, nil
				}
			}
		}
	}

	return nil, fmt.Errorf("watch channel closed")
}