1// Copyright 2016 The Prometheus Authors
2// Licensed under the Apache License, Version 2.0 (the "License");
3// you may not use this file except in compliance with the License.
4// You may obtain a copy of the License at
5//
6//     http://www.apache.org/licenses/LICENSE-2.0
7//
8// Unless required by applicable law or agreed to in writing, software
9// distributed under the License is distributed on an "AS IS" BASIS,
10// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
11// See the License for the specific language governing permissions and
12// limitations under the License.
13
14package gate
15
16import "context"
17
18// A Gate controls the maximum number of concurrently running and waiting queries.
19type Gate struct {
20	ch chan struct{}
21}
22
23// New returns a query gate that limits the number of queries
24// being concurrently executed.
25func New(length int) *Gate {
26	return &Gate{
27		ch: make(chan struct{}, length),
28	}
29}
30
31// Start blocks until the gate has a free spot or the context is done.
32func (g *Gate) Start(ctx context.Context) error {
33	select {
34	case <-ctx.Done():
35		return ctx.Err()
36	case g.ch <- struct{}{}:
37		return nil
38	}
39}
40
41// Done releases a single spot in the gate.
42func (g *Gate) Done() {
43	select {
44	case <-g.ch:
45	default:
46		panic("gate.Done: more operations done than started")
47	}
48}
49