feat(client): reload plugin certificates on frpc reload (#5576)

The https2http and https2https plugins read their certificate and key
once when the proxy starts, so a certificate that an external tool renews
on disk is only picked up after the proxy itself is recreated. frpc
reload rebuilt proxies only when their configuration changed, and a
rebuild closes the work connections that are already in use.

Let the plugin own the refresh. The HTTPS plugin server keeps the active
keypair and the last refresh error in one immutable snapshot published
through an atomic pointer, so a TLS handshake only loads that pointer and
never touches the disk. Reload rereads both files, and when their bytes
match the published pair it returns without parsing or replacing
anything; otherwise it parses the pair and swaps the snapshot in. A read
or parse failure keeps the previous certificate serving and records the
error next to it.

frpc reload collects the reloadable plugins of the live proxies and
applies the new configuration inside the same reload transaction that
also serializes control handoff, so a reload that races a reconnect
still refreshes the plugins of the published control. The certificate
files are read only after that transaction has been released. The config
store API keeps applying configuration without refreshing certificates,
because only an explicit reload may reread files. Proxy status reports a
refresh failure next to the still running proxy instead of dropping its
remote address.

No configuration field is added, and frps and the wire protocol are
unchanged. Without a reload nothing changes: certificates are still read
once at startup, generated self-signed certificates still never reload,
and a proxy whose configuration changed still goes through the existing
rebuild path.

pkg/plugin/client/internal/httpsserver/server_test.go

@@ -0,0 +1,153 @@
+// Copyright 2026 The frp 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.
+
+//go:build !frps
+
+package httpsserver
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/pem"
+	"io"
+	"math/big"
+	"net"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func certificatePEM(t *testing.T, serial int64) ([]byte, []byte) {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	require.NoError(t, err)
+	template := &x509.Certificate{
+		SerialNumber: big.NewInt(serial),
+		DNSNames:     []string{"localhost"},
+		NotBefore:    time.Now().Add(-time.Hour),
+		NotAfter:     time.Now().Add(time.Hour),
+		KeyUsage:     x509.KeyUsageDigitalSignature,
+	}
+	der, err := x509.CreateCertificate(rand.Reader, template, template, &key.PublicKey, key)
+	require.NoError(t, err)
+	keyDER, err := x509.MarshalPKCS8PrivateKey(key)
+	require.NoError(t, err)
+	return pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der}),
+		pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: keyDER})
+}
+
+func handshake(t *testing.T, cfg *tls.Config) (*tls.Conn, *tls.Conn) {
+	t.Helper()
+	clientRaw, serverRaw := net.Pipe()
+	t.Cleanup(func() { _ = clientRaw.Close(); _ = serverRaw.Close() })
+	require.NoError(t, clientRaw.SetDeadline(time.Now().Add(5*time.Second)))
+	require.NoError(t, serverRaw.SetDeadline(time.Now().Add(5*time.Second)))
+	client := tls.Client(clientRaw, &tls.Config{InsecureSkipVerify: true})
+	server := tls.Server(serverRaw, cfg)
+	done := make(chan error, 1)
+	go func() { done <- server.Handshake() }()
+	require.NoError(t, client.Handshake())
+	require.NoError(t, <-done)
+	return client, server
+}
+
+func TestReloadCertificate(t *testing.T) {
+	dir := t.TempDir()
+	crtPath, keyPath := filepath.Join(dir, "tls.crt"), filepath.Join(dir, "tls.key")
+	certPEM, keyPEM := certificatePEM(t, 1)
+	require.NoError(t, os.WriteFile(crtPath, certPEM, 0o600))
+	require.NoError(t, os.WriteFile(keyPath, keyPEM, 0o600))
+	s, err := New(nil, crtPath, keyPath, nil)
+	require.NoError(t, err)
+	originalState := s.state.Load()
+	original := originalState.cert
+	oldClient, oldServer := handshake(t, s.TLSConfig)
+	require.Equal(t, int64(1), oldClient.ConnectionState().PeerCertificates[0].SerialNumber.Int64())
+
+	require.NoError(t, s.Reload())
+	require.Same(t, originalState, s.state.Load(), "unchanged bytes must not parse or publish a new pair")
+	require.NoError(t, s.ReloadError())
+
+	for _, tc := range []struct {
+		name, path string
+		data       []byte
+	}{
+		{"missing certificate", crtPath, nil},
+		{"missing key", keyPath, nil},
+		{"invalid certificate", crtPath, []byte("invalid")},
+		{"invalid key", keyPath, []byte("invalid")},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.data == nil {
+				require.NoError(t, os.Remove(tc.path))
+			} else {
+				require.NoError(t, os.WriteFile(tc.path, tc.data, 0o600))
+			}
+			require.Error(t, s.Reload())
+			require.Error(t, s.ReloadError())
+			require.Same(t, original, s.state.Load().cert)
+			client, _ := handshake(t, s.TLSConfig)
+			require.Equal(t, int64(1), client.ConnectionState().PeerCertificates[0].SerialNumber.Int64())
+			require.NoError(t, os.WriteFile(crtPath, certPEM, 0o600))
+			require.NoError(t, os.WriteFile(keyPath, keyPEM, 0o600))
+			require.NoError(t, s.Reload())
+			require.NoError(t, s.ReloadError())
+			require.Same(t, original, s.state.Load().cert)
+		})
+	}
+	// Even a key-only byte change must be noticed, regardless of parsed identity.
+	require.NoError(t, os.WriteFile(keyPath, append(keyPEM, '\n'), 0o600))
+	require.NoError(t, s.Reload())
+	require.NotSame(t, original, s.state.Load().cert)
+
+	nextCert, nextKey := certificatePEM(t, 2)
+	require.NoError(t, os.WriteFile(crtPath, nextCert, 0o600))
+	beforeMismatch := s.state.Load().cert
+	require.Error(t, s.Reload(), "mismatched pair must retain the active certificate")
+	require.Same(t, beforeMismatch, s.state.Load().cert)
+	require.NoError(t, os.WriteFile(keyPath, nextKey, 0o600))
+	beforeReload, _ := handshake(t, s.TLSConfig)
+	require.Equal(t, int64(1), beforeReload.ConnectionState().PeerCertificates[0].SerialNumber.Int64())
+	require.NoError(t, s.Reload())
+	newClient, _ := handshake(t, s.TLSConfig)
+	require.Equal(t, int64(2), newClient.ConnectionState().PeerCertificates[0].SerialNumber.Int64())
+
+	require.NoError(t, oldClient.SetDeadline(time.Now().Add(5*time.Second)))
+	require.NoError(t, oldServer.SetDeadline(time.Now().Add(5*time.Second)))
+	done := make(chan error, 1)
+	go func() { _, err := oldServer.Write([]byte("still connected")); done <- err }()
+	data := make([]byte, len("still connected"))
+	_, err = io.ReadFull(oldClient, data)
+	require.NoError(t, err)
+	require.NoError(t, <-done)
+	require.Equal(t, "still connected", string(data))
+}
+
+func TestReloadCertificateKeepsSelfSigned(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "missing")
+	for _, paths := range [][2]string{{"", ""}, {missing, ""}, {"", missing}} {
+		s, err := New(nil, paths[0], paths[1], nil)
+		require.NoError(t, err)
+		original := &s.TLSConfig.Certificates[0]
+		require.Nil(t, s.TLSConfig.GetCertificate)
+		require.NoError(t, s.Reload())
+		require.Same(t, original, &s.TLSConfig.Certificates[0])
+	}
+}