/*
 *
 * Copyright 2023 gRPC 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 advancedtls

import (
	"crypto/x509"
	"fmt"
	"os"
	"sync"
	"time"
)

const defaultCRLRefreshDuration = 1 * time.Hour

// CRLProvider is the interface to be implemented to enable custom CRL provider
// behavior.
//
// The interface defines how gRPC gets CRLs from the provider during handshakes,
// but doesn't prescribe a specific way to load and store CRLs. Such
// implementations can be used in RevocationConfig of advancedtls.ClientOptions
// and/or advancedtls.ServerOptions.
// Please note that checking CRLs is being directly on the path of connection
// establishment, so implementations of the CRL function need to be fast, and
// slow things such as file IO should be done asynchronously.
// TODO(erm-g): Add link to related gRFC once it's ready.
// Please refer to https://github.com/grpc/proposal/ for more details.
type CRLProvider interface {
	// CRL accepts x509 Cert and returns back related CRL struct. The CRL struct
	// can be nil, can contain empty or non-empty list of revkoed certificates.
	// Callers are expected to use the returned value as read-only.
	CRL(cert *x509.Certificate) (*CRL, error)
}

// StaticCRLProvider implements CRLProvider interface by accepting raw content
// of CRL files at creation time and storing parsed CRL structs  in-memory.
type StaticCRLProvider struct {
	crls map[string]*CRL
}

// MakeStaticCRLProvider processes raw content of CRL files, adds parsed CRL
// structs into in-memory, and returns a new instance of the StaticCRLProvider.
func MakeStaticCRLProvider(rawCRLs [][]byte) *StaticCRLProvider {
	p := StaticCRLProvider{}
	p.crls = make(map[string]*CRL)
	for idx, rawCRL := range rawCRLs {
		cRL, err := NewCRL(rawCRL)
		if err != nil {
			grpclogLogger.Warningf("Can't parse raw CRL number %v from the slice: %v", idx, err)
			continue
		}
		p.addCRL(cRL)
	}
	return &p
}

// AddCRL adds/updates provided CRL to in-memory storage.
func (p *StaticCRLProvider) addCRL(crl *CRL) {
	key := crl.certList.Issuer.ToRDNSequence().String()
	p.crls[key] = crl
}

// CRL returns CRL struct if it was previously loaded by calling AddCRL.
func (p *StaticCRLProvider) CRL(cert *x509.Certificate) (*CRL, error) {
	return p.crls[cert.Issuer.ToRDNSequence().String()], nil
}

// Options represents a data structure holding a
// configuration for FileWatcherCRLProvider.
type Options struct {
	CRLDirectory               string          // Path of the directory containing CRL files
	RefreshDuration            time.Duration   // Time interval between CRLDirectory scans
	crlReloadingFailedCallback func(err error) // Custom callback executed when a CRL file can’t be processed
}

// FileWatcherCRLProvider implements the CRLProvider interface by periodically scanning
// CRLDirectory (see Options) and storing CRL structs in-memory
type FileWatcherCRLProvider struct {
	crls map[string]*CRL
	opts Options
	mu   sync.Mutex
	done chan struct{}
}

// MakeFileWatcherCRLProvider returns a new instance of the
// FileWatcherCRLProvider. It uses Options to validate and apply configuration
// required for creating a new instance.
func MakeFileWatcherCRLProvider(o Options) (*FileWatcherCRLProvider, error) {
	if err := o.validate(); err != nil {
		return nil, err
	}
	done := make(chan struct{})
	provider := &FileWatcherCRLProvider{
		crls: make(map[string]*CRL),
		opts: o,
		done: done,
	}
	go provider.run()
	return provider, nil
}

func (o *Options) validate() error {
	// Checks relates to CRLDirectory.
	if o.CRLDirectory == "" {
		return fmt.Errorf("advancedtls: CRLDirectory needs to be specified")
	}
	fileInfo, err := os.Stat(o.CRLDirectory)
	if err != nil {
		if os.IsNotExist(err) {
			return fmt.Errorf("advancedtls: CRLDirectory %v does not exist", o.CRLDirectory)
		}
		return err
	}
	if !fileInfo.IsDir() {
		return fmt.Errorf("advancedtls: CRLDirectory %v is not a directory", o.CRLDirectory)
	}
	_, err = os.Open(o.CRLDirectory)
	if err != nil {
		if os.IsPermission(err) {
			return fmt.Errorf("advancedtls: CRLDirectory %v is not readable", o.CRLDirectory)
		}
		return err
	}
	// Checks related to RefreshDuration.
	if o.RefreshDuration <= 0 || o.RefreshDuration < time.Second {
		o.RefreshDuration = defaultCRLRefreshDuration
		grpclogLogger.Warningf("RefreshDuration must larger then 1 second: provided value %v, default value will be used %v", o.RefreshDuration, defaultCRLRefreshDuration)
	}
	return nil
}

// Start starts watching the directory for CRL files and updates the provider accordingly.
func (p *FileWatcherCRLProvider) run() {
	ticker := time.NewTicker(p.opts.RefreshDuration)
	defer ticker.Stop()
	p.ScanCRLDirectory()

	for {
		select {
		case <-p.done:
			ticker.Stop()
			return
		case <-ticker.C:
			p.ScanCRLDirectory()
		}
	}
}

// Close stops the background refresh of CRLDirectory of FileWatcherCRLProvider
func (p *FileWatcherCRLProvider) Close() {
	close(p.done)
}

// ScanCRLDirectory starts the process of scanning Options.CRLDirectory and
// updating in-memory storage of CRL structs. Please note that the same method is
// called periodically by run goroutine.
// TODO(erm-g): Add link to related gRFC once it's ready.
// Please refer to https://github.com/grpc/proposal/ for edge case details.
func (p *FileWatcherCRLProvider) ScanCRLDirectory() {
	dir, err := os.Open(p.opts.CRLDirectory)
	if err != nil {
		grpclogLogger.Errorf("Can't open CRLDirectory %v", p.opts.CRLDirectory, err)
		if p.opts.crlReloadingFailedCallback != nil {
			p.opts.crlReloadingFailedCallback(err)
		}
	}
	defer dir.Close()

	files, err := dir.ReadDir(0)
	if err != nil {
		grpclogLogger.Errorf("Can't access files under CRLDirectory %v", p.opts.CRLDirectory, err)
		if p.opts.crlReloadingFailedCallback != nil {
			p.opts.crlReloadingFailedCallback(err)
		}
	}

	tempCRLs := make(map[string]*CRL)
	successCounter := 0
	failCounter := 0
	for _, file := range files {
		filePath := fmt.Sprintf("%s/%s", p.opts.CRLDirectory, file.Name())
		crl, err := ReadCRLFile(filePath)
		if err != nil {
			failCounter++
			grpclogLogger.Warningf("Can't add CRL from file %v under CRLDirectory %v", filePath, p.opts.CRLDirectory, err)
			if p.opts.crlReloadingFailedCallback != nil {
				p.opts.crlReloadingFailedCallback(err)
			}
			continue
		}
		tempCRLs[crl.certList.Issuer.ToRDNSequence().String()] = crl
		successCounter++
	}
	// Only if all the files are processed successful we can swap maps (there
	// might be deletions of entries in this case
	if len(files) == successCounter {
		p.mu.Lock()
		defer p.mu.Unlock()
		p.crls = make(map[string]*CRL)
		for key, value := range tempCRLs {
			p.crls[key] = value
		}
		grpclogLogger.Infof("Scan of CRLDirectory %v completed, %v files found and processed successfully, in-memory CRL storage flushed and repopulated", p.opts.CRLDirectory, len(files))
	} else {
		// Since some of the files failed we can only add/update entries in the map
		p.mu.Lock()
		defer p.mu.Unlock()
		for key, value := range tempCRLs {
			p.crls[key] = value
		}
		grpclogLogger.Infof("Scan of CRLDirectory %v completed, %v files found, %v files processing failed, %v entries of in-memory CRL storage added/updated", p.opts.CRLDirectory, len(files), failCounter, successCounter)
	}
}

// CRL retrieves the CRL associated with the given certificate's issuer DN from
// in-memory if it was previously loaded during CRLDirectory scan.
func (p *FileWatcherCRLProvider) CRL(cert *x509.Certificate) (*CRL, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.crls[cert.Issuer.ToRDNSequence().String()], nil
}