Rate limit the unimplemented syscall event handler.

This introduces two new types of Emitters:
1. MultiEmitter, which will forward events to other registered Emitters, and
2. RateLimitedEmitter, which will forward events to a wrapped Emitter, subject
	to given rate limits.

The methods in the eventchannel package itself act like a multiEmitter, but is
not actually an Emitter. Now we have a DefaultEmitter, and the methods in
eventchannel simply forward calls to the DefaultEmitter.

The unimplemented syscall handler now uses a RateLimetedEmitter that wraps the
DefaultEmitter.

PiperOrigin-RevId: 260612770

1 files changed, 54 insertions, 0 deletions

diff --git a/pkg/eventchannel/rate.go b/pkg/eventchannel/rate.go
new file mode 100644
index 000000000..179226c92
--- /dev/null
+++ b/pkg/eventchannel/rate.go
@@ -0,0 +1,54 @@
+// Copyright 2019 The gVisor 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 eventchannel
+
+import (
+	"github.com/golang/protobuf/proto"
+	"golang.org/x/time/rate"
+)
+
+// rateLimitedEmitter wraps an emitter and limits events to the given limits.
+// Events that would exceed the limit are discarded.
+type rateLimitedEmitter struct {
+	inner   Emitter
+	limiter *rate.Limiter
+}
+
+// RateLimitedEmitterFrom creates a new event channel emitter that wraps the
+// existing emitter and enforces rate limits. The limits are imposed via a
+// token bucket, with `maxRate` events per second, with burst size of `burst`
+// events. See the golang.org/x/time/rate package and
+// https://en.wikipedia.org/wiki/Token_bucket for more information about token
+// buckets generally.
+func RateLimitedEmitterFrom(inner Emitter, maxRate float64, burst int) Emitter {
+	return &rateLimitedEmitter{
+		inner:   inner,
+		limiter: rate.NewLimiter(rate.Limit(maxRate), burst),
+	}
+}
+
+// Emit implements EventEmitter.Emit.
+func (rle *rateLimitedEmitter) Emit(msg proto.Message) (bool, error) {
+	if !rle.limiter.Allow() {
+		// Drop event.
+		return false, nil
+	}
+	return rle.inner.Emit(msg)
+}
+
+// Close implements EventEmitter.Close.
+func (rle *rateLimitedEmitter) Close() error {
+	return rle.inner.Close()
+}