perf(actions): cache successful runner token verification (#39934)

Every Actions runner RPC (`FetchTask` polling, `UpdateTask`,
`UpdateLog`, ...) goes through the `withRunner` interceptor, which
verifies the runner token with PBKDF2 (`auth_model.HashToken`, ~2 ms).
In a 60s CPU profile of a production instance with many runners, this
accounted for ~49% of all CPU samples.

- Cache successful verifications in a dedicated LRU of up to 10000
runners, keyed by the stored salt and hash and holding only a SHA-256
digest of the token. A hit costs ~100 ns.
- The runner is still loaded from the DB on every RPC and failures are
never cached, so deleted runners and wrong tokens never pass.
- The shared `auth_model.TokenCache` is not used because its default 20
entries miss on every poll once more runners poll in turn.

---------

Co-authored-by: silverwind <me@silverwind.io>

routers/api/actions/runner/interceptor.go

@@ -5,6 +5,7 @@ package runner
 
 import (
 	"context"
+	"crypto/sha256"
 	"errors"
 	"strings"
 	"time"
@@ -17,6 +18,7 @@ import (
 	"gitea.dev/modules/util"
 
 	"connectrpc.com/connect/v2"
+	lru "github.com/hashicorp/golang-lru/v2"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 )
@@ -26,6 +28,10 @@ const (
 	tokenHeaderKey = protocol.TokenHeader
 )
 
+type runnerTokenHash struct{ salt, hash string }
+
+var verifiedRunnerTokens, _ = lru.New[runnerTokenHash, [sha256.Size]byte](10000) // not auth_model.TokenCache, whose small default size thrashes once more runners poll in turn
+
 // withErrorMessages sends plain errors to runners as CodeUnknown with their message, like connect v1 did.
 func withErrorMessages(next connect.ServerFunc) connect.ServerFunc {
 	return func(ctx context.Context, spec connect.Spec, stream connect.ServerStream) error {
@@ -57,7 +63,7 @@ func withRunner(next connect.ServerFunc) connect.ServerFunc {
 			}
 			return status.Error(codes.Internal, err.Error())
 		}
-		if !util.CryptoConstTimeEqual(runner.TokenHash, auth_model.HashToken(token, runner.TokenSalt)) {
+		if !verifyRunnerToken(runner, token) {
 			return errUnregisteredRunner
 		}
 
@@ -88,6 +94,18 @@ func withRunner(next connect.ServerFunc) connect.ServerFunc {
 	}
 }
 
+func verifyRunnerToken(runner *actions_model.ActionRunner, token string) bool {
+	key, digest := runnerTokenHash{runner.TokenSalt, runner.TokenHash}, sha256.Sum256([]byte(token))
+	if cached, ok := verifiedRunnerTokens.Get(key); ok && util.CryptoConstTimeEqual(cached[:], digest[:]) {
+		return true
+	}
+	if !util.CryptoConstTimeEqual(runner.TokenHash, auth_model.HashToken(token, runner.TokenSalt)) {
+		return false
+	}
+	verifiedRunnerTokens.Add(key, digest)
+	return true
+}
+
 func getMethodName(spec connect.Spec) string {
 	splits := strings.Split(spec.Procedure, "/")
 	if len(splits) > 0 {

routers/api/actions/runner/interceptor_test.go

@@ -0,0 +1,26 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package runner
+
+import (
+	"crypto/sha256"
+	"testing"
+
+	actions_model "gitea.dev/models/actions"
+	auth_model "gitea.dev/models/auth"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestVerifyRunnerToken(t *testing.T) {
+	verifiedRunnerTokens.Purge()
+	runner := &actions_model.ActionRunner{TokenSalt: "salt", TokenHash: auth_model.HashToken("token", "salt")}
+	assert.True(t, verifyRunnerToken(runner, "token"))
+	assert.True(t, verifyRunnerToken(runner, "token"))
+	assert.False(t, verifyRunnerToken(runner, "wrong"))
+
+	other := &actions_model.ActionRunner{TokenSalt: "salt", TokenHash: auth_model.HashToken("other", "salt")}
+	assert.False(t, verifyRunnerToken(other, "token"))
+	assert.Equal(t, [][sha256.Size]byte{sha256.Sum256([]byte("token"))}, verifiedRunnerTokens.Values())
+}