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certwatcher.go
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certwatcher.go
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// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: Copyright (c) 2024 Matthew Penner
package certwatcher
import (
"context"
"crypto/tls"
"crypto/x509"
"errors"
"fmt"
"log/slog"
"sync/atomic"
"time"
"github.com/fsnotify/fsnotify"
"go.opentelemetry.io/otel"
"go.opentelemetry.io/otel/metric"
"github.com/matthewpi/certwatcher/internal/ocsp"
"github.com/matthewpi/certwatcher/internal/sets"
"github.com/matthewpi/certwatcher/internal/wait"
)
// Options controls options for a [Watcher]. Changes to Options are ignored
// after being provided to a [Watcher].
type Options struct {
// Debounce is the duration to wait before triggering a reload, it's purpose
// is to ensure that if multiple watched files are updated during it's
// duration, that multiple full reloads are not triggered.
Debounce time.Duration
// DontStaple controls whether OCSP stapling should be disabled when loading
// certificates, by default it is enabled.
DontStaple bool
// Logger to use for the [Watcher] instance.
Logger *slog.Logger
}
// Watcher watches for changes to TLS certificate files on disk and attempts to
// automatically reload the certificate. This is used to allow the graceful
// rotation of certificates.
//
// Watcher is also capable of performing OCSP stapling when (re)loading
// certificates.
type Watcher struct {
options Options
certPath string
keyPath string
cert atomic.Pointer[tls.Certificate]
debounce debounced
fsWatcher *fsnotify.Watcher
logger *slog.Logger
meter metric.Meter
reconfigureTotalCounter metric.Int64Counter
reconfigureErrorCounter metric.Int64Counter
}
// New creates a new certwatcher [Watcher], capable of reloading certificates on
// the fly.
//
// After calling New, you will want to configure it with Watcher.Reconfigure()
// and then run Watcher.Start().
func New(options Options) (*Watcher, error) {
d := options.Debounce
if d < 10*time.Millisecond {
d = 100 * time.Millisecond
}
w := &Watcher{
options: options,
logger: options.Logger,
meter: otel.Meter("github.com/matthewpi/certwatcher"),
debounce: debounce(d),
}
if w.logger == nil {
w.logger = slog.Default()
}
var err error
w.reconfigureTotalCounter, err = w.meter.Int64Counter("certwatcher.reconfigure.total")
if err != nil {
return nil, fmt.Errorf("certwatcher: failed to create otel meter: %w", err)
}
w.reconfigureErrorCounter, err = w.meter.Int64Counter("certwatcher.reconfigure.errors")
if err != nil {
return nil, fmt.Errorf("certwatcher: failed to create otel meter: %w", err)
}
w.fsWatcher, err = fsnotify.NewWatcher()
if err != nil {
return nil, fmt.Errorf("certwatcher: failed to create fswatcher: %w", err)
}
return w, nil
}
// GetCertificate satisfies tls.Config#GetCertificate. This function should be
// used on a tls.Config to use the certificate loaded by certwatcher.
func (w *Watcher) GetCertificate(_ *tls.ClientHelloInfo) (*tls.Certificate, error) {
return w.cert.Load(), nil
}
// GetClientCertificate satisfies tls.Config#GetClientCertificate. This function
// should be used on a tls.Config to use the certificate loaded by certwatcher.
func (w *Watcher) GetClientCertificate(_ *tls.CertificateRequestInfo) (*tls.Certificate, error) {
return w.cert.Load(), nil
}
// Reconfigure reconfigures the [Watcher] to watch the given `certPath` and
// `keyPath`.
//
// This method is both used for initial configuration and for reconfiguration
// if the certificate paths need to be changed (e.g. config hot-reloading).
func (w *Watcher) Reconfigure(ctx context.Context, certPath, keyPath string) error {
// Load the certificate from disk.
w.reconfigureTotalCounter.Add(ctx, 1)
cert, err := tls.LoadX509KeyPair(certPath, keyPath)
if err != nil {
w.reconfigureErrorCounter.Add(ctx, 1)
return fmt.Errorf("certwatcher: failed to load x509 certificate: %w", err)
}
if cert.Leaf == nil {
cert.Leaf, err = x509.ParseCertificate(cert.Certificate[0])
if err != nil {
return fmt.Errorf("certwatcher: failed to parse leaf certificate: %w", err)
}
}
fields := make([]slog.Attr, 2, 4)
fields[0] = slog.String("cert_path", certPath)
fields[1] = slog.String("key_path", keyPath)
now := time.Now()
notBefore := cert.Leaf.NotBefore
if !notBefore.IsZero() && now.Before(notBefore) {
w.logger.LogAttrs(ctx, slog.LevelWarn, "certificate isn't valid yet", fields...)
}
notAfter := cert.Leaf.NotAfter
if !notAfter.IsZero() && now.After(notAfter) {
w.logger.LogAttrs(ctx, slog.LevelWarn, "certificate has expired", fields...)
}
fields = append(
fields,
slog.String("not_before", notBefore.Format(time.DateTime)),
slog.String("not_after", notAfter.Format(time.DateTime)),
)
// Attempt to staple the certificate only if we were able to load its leaf.
cert, err = w.staple(ctx, cert)
if err != nil {
w.logger.LogAttrs(ctx, slog.LevelWarn, "failed to staple certificate", slog.Any("err", err))
}
// Update the certificate we are serving.
// We use swap, so we can display a helpful message if this is the first
// time a certificate was loaded or if it was a reload.
if wCert := w.cert.Swap(&cert); wCert == nil {
w.logger.LogAttrs(ctx, slog.LevelInfo, "certificate loaded", fields...)
} else {
w.logger.LogAttrs(ctx, slog.LevelInfo, "certificate reloaded", fields...)
}
// Configure the filesystem watcher based off the paths given, this will
// ensure the paths are watched properly. If the paths are changed, this
// will remove the old paths and start watching only the new ones.
return w.configureFsWatcher(ctx, certPath, keyPath)
}
// staple attempts to perform OCSP stapling on the supplied certificate.
func (w *Watcher) staple(ctx context.Context, cert tls.Certificate) (tls.Certificate, error) {
// If stapling is disabled, return the supplied certificate.
if w.options.DontStaple {
return cert, nil
}
// If the certificate is already stapled, return it.
if cert.OCSPStaple != nil {
w.logger.DebugContext(ctx, "certificate was already stapled")
return cert, nil
}
// Attempt to staple the certificate.
s := ocsp.Stapler{Certificate: cert}
w.logger.LogAttrs(
ctx,
slog.LevelInfo,
"attempting to staple certificate...",
slog.Any("ocsp_servers", cert.Leaf.OCSPServer),
slog.Any("issuing_certificate_url", cert.Leaf.IssuingCertificateURL),
)
if err := s.Staple(ctx); err != nil {
if errors.Is(err, ocsp.ErrNoOCSPServer) {
w.logger.InfoContext(ctx, "certificate has no ocsp servers, cannot staple")
return cert, nil
}
return cert, fmt.Errorf("certwatcher: error stapling certificate: %w", err)
}
// Update the certificate with the stapled one.
cert = s.Certificate
if cert.OCSPStaple == nil {
w.logger.WarnContext(ctx, "certificate was not stapled")
} else {
w.logger.InfoContext(ctx, "stapled certificate")
}
return cert, nil
}
// configureFsWatcher configures the fswatcher to watch the given paths. If any
// files are already being watched, they will be removed from the watcher before
// watching the newly provided paths.
func (w *Watcher) configureFsWatcher(ctx context.Context, certPath, keyPath string) error {
// Skip any configuration if the paths haven't changed.
// (Initial configuration will have w.certPath and w.keyPath as empty)
if w.certPath == certPath && w.keyPath == keyPath {
return nil
}
// Before trying to configure the filesystem watcher, make sure it exists.
if w.fsWatcher == nil {
return nil
}
// Stop watching any existing files.
if wl := w.fsWatcher.WatchList(); len(wl) > 0 {
if err := w.remove(ctx, wl...); err != nil {
return err
}
}
// Start watching the new files.
if err := w.add(ctx, certPath, keyPath); err != nil {
return err
}
// Update the paths stored on the struct, these are used to detect if the
// paths we are watching have changed to avoid unnecessarily reconfiguring
// the filesystem watcher.
w.certPath = certPath
w.keyPath = keyPath
return nil
}
// add attempts to add paths to the fsnotify watcher in a consistent way.
func (w *Watcher) add(ctx context.Context, paths ...string) error {
return w.forPaths(ctx, w.fsWatcher.Add, paths...)
}
// remove attempts to remove paths to the fsnotify watcher in a consistent way.
func (w *Watcher) remove(ctx context.Context, paths ...string) error {
return w.forPaths(ctx, w.fsWatcher.Remove, paths...)
}
// forPaths is used to consistently do actions with paths and the watcher.
//
// Its job is to handle inconsistency with paths that may be in the process of
// being added or removed from the filesystem and may cause errors if we try
// to add or remove from the watcher.
//
// To do this, we use a Set to hold the list of paths we need to perform an
// action on. Then we use a backoff to retry the action multiple times until
// a timeout is reached.
func (w *Watcher) forPaths(ctx context.Context, fn func(string) error, paths ...string) error {
set := sets.New(paths...)
var watchErr error
err := wait.PollUntilContextTimeout(
ctx,
1*time.Second,
10*time.Second,
true,
func(_ context.Context) (done bool, err error) {
for _, f := range set.UnsortedList() {
if err := fn(f); err != nil {
watchErr = err
// We want to keep trying, so don't return the error.
return false, nil //nolint:nilerr
}
// We've successfully done what we needed to do with the path,
// remove it from the set.
set.Delete(f)
}
return true, nil
},
)
if err != nil {
return errors.Join(err, watchErr)
}
return nil
}
// Start starts listening for fsnotify events and will automatically reload the
// certificate when necessary.
func (w *Watcher) Start(ctx context.Context) {
if w.fsWatcher == nil {
panic("fsWatcher is nil")
return
}
// Close the filesystem watcher whenever the context is canceled.
defer w.fsWatcher.Close()
// Run the watcher, blocks until context is cancelled or until the
// filesystem watcher is closed.
w.watch(ctx)
}
// watch watches for incoming events from fsnotify and passes them off to
// handleEvent.
func (w *Watcher) watch(ctx context.Context) {
for {
select {
case <-ctx.Done():
return
case event, ok := <-w.fsWatcher.Events:
if !ok {
return
}
w.handleEvent(ctx, event)
case err, ok := <-w.fsWatcher.Errors:
if !ok {
return
}
w.logger.LogAttrs(ctx, slog.LevelError, "an error occurred while watching files", slog.Any("err", err))
}
}
}
// handleEvent handles incoming fsnotify events to detect when we need to reload
// the watched certificate files.
func (w *Watcher) handleEvent(ctx context.Context, event fsnotify.Event) {
// Filter operations that may modify the file's content.
switch {
case event.Op.Has(fsnotify.Create):
case event.Op.Has(fsnotify.Write):
case event.Op.Has(fsnotify.Remove):
default:
// Explicitly ignore all other event types.
return
}
// If the file was removed, re-watch it.
if event.Op.Has(fsnotify.Remove) {
// The handles a case where a file is deleted, then replaced with new
// content.
if err := w.fsWatcher.Add(event.Name); err != nil {
w.logger.LogAttrs(ctx, slog.LevelError, "failed to re-watch file", slog.Any("err", err))
}
}
// Debounce the reload to prevent multiple back-to-back events from
// triggering more than one reload in a small time-frame.
w.debounce(func() {
w.logger.LogAttrs(ctx, slog.LevelInfo, "reloading...")
if err := w.Reconfigure(ctx, w.certPath, w.keyPath); err != nil {
w.logger.LogAttrs(ctx, slog.LevelError, "failed to reload certificate", slog.Any("err", err))
}
})
}