feat: reload TLS certificates when their files change (#625)

pkg/server/tlscert/reloader.go

@@ -0,0 +1,116 @@
+// Package tlscert provides X509 key pairs that follow changes to their files on disk.
+package tlscert
+
+import (
+	"crypto/tls"
+	"fmt"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/rs/zerolog"
+)
+
+// retryInterval is the minimum time between attempts to reload a key pair that failed to load
+// while its files are unchanged, such as when a chmod or chown fixes its permissions.
+const retryInterval = 30 * time.Second
+
+// fileState identifies a version of a file by its modification time and size.
+type fileState struct {
+	modTime time.Time
+	size    int64
+}
+
+// Reloader serves a certificate and private key loaded from files, reloading them when either
+// file changes. It is safe for concurrent use.
+type Reloader struct {
+	certFile      string           // X509 public certificate file.
+	keyFile       string           // X509 private key file.
+	logger        zerolog.Logger   // Logger for reload events.
+	retryInterval time.Duration    // Minimum time between retries of a failed, unchanged pair.
+	now           func() time.Time // Clock used to schedule retries.
+
+	mu        sync.Mutex       // Guards the fields below.
+	cert      *tls.Certificate // Last successfully loaded key pair.
+	certState fileState        // State of certFile at the last load attempt.
+	keyState  fileState        // State of keyFile at the last load attempt.
+	failedAt  time.Time        // Time of the last failed load attempt, zero after a success.
+}
+
+// NewReloader loads the key pair from certFile and keyFile, returning an error if it cannot be
+// loaded.
+func NewReloader(certFile, keyFile string, logger zerolog.Logger) (*Reloader, error) {
+	r := &Reloader{
+		certFile:      certFile,
+		keyFile:       keyFile,
+		logger:        logger,
+		retryInterval: retryInterval,
+		now:           time.Now,
+	}
+	certState, keyState, err := r.stat()
+	if err != nil {
+		return nil, err
+	}
+	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
+	if err != nil {
+		return nil, err
+	}
+	r.cert = &cert
+	r.certState = certState
+	r.keyState = keyState
+	return r, nil
+}
+
+// GetCertificate returns the current key pair, reloading it first if either file changed since
+// the last load attempt, or if the last attempt failed and retryInterval has elapsed. Failures
+// are logged and the previous key pair is served. crypto/tls calls it on full handshakes only.
+func (r *Reloader) GetCertificate(*tls.ClientHelloInfo) (*tls.Certificate, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// On error, stat returns zero states, so repeated stat failures count as unchanged files.
+	certState, keyState, err := r.stat()
+	changed := certState != r.certState || keyState != r.keyState
+	retryDue := !r.failedAt.IsZero() && r.now().Sub(r.failedAt) >= r.retryInterval
+	if !changed && !retryDue {
+		return r.cert, nil
+	}
+
+	r.certState = certState
+	r.keyState = keyState
+	if err == nil {
+		var cert tls.Certificate
+		cert, err = tls.LoadX509KeyPair(r.certFile, r.keyFile)
+		if err == nil {
+			r.cert = &cert
+			r.failedAt = time.Time{}
+			r.logger.Info().Msg("Reloaded X509 KeyPair")
+			return r.cert, nil
+		}
+	}
+	r.failedAt = r.now()
+	r.logger.Warn().Err(err).Msg("Failed to reload X509 KeyPair, serving previous certificate")
+	return r.cert, nil
+}
+
+// stat returns the current state of the certificate and key files, or zero states and an error
+// if either cannot be read.
+func (r *Reloader) stat() (certState, keyState fileState, err error) {
+	certState, err = statFile(r.certFile)
+	if err != nil {
+		return fileState{}, fileState{}, err
+	}
+	keyState, err = statFile(r.keyFile)
+	if err != nil {
+		return fileState{}, fileState{}, err
+	}
+	return certState, keyState, nil
+}
+
+func statFile(name string) (fileState, error) {
+	info, err := os.Stat(name)
+	if err != nil {
+		return fileState{}, fmt.Errorf("stat %s: %w", name, err)
+	}
+	return fileState{modTime: info.ModTime(), size: info.Size()}, nil
+}