1// Copyright 2021 The Prometheus Authors
2// This code is partly borrowed from Caddy:
3//    Copyright 2015 Matthew Holt and The Caddy Authors
4// Licensed under the Apache License, Version 2.0 (the "License");
5// you may not use this file except in compliance with the License.
6// You may obtain a copy of the License at
7//
8// http://www.apache.org/licenses/LICENSE-2.0
9//
10// Unless required by applicable law or agreed to in writing, software
11// distributed under the License is distributed on an "AS IS" BASIS,
12// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13// See the License for the specific language governing permissions and
14// limitations under the License.
15
16package web
17
18import (
19	weakrand "math/rand"
20	"sync"
21	"time"
22)
23
24var cacheSize = 100
25
26func init() {
27	weakrand.Seed(time.Now().UnixNano())
28}
29
30type cache struct {
31	cache map[string]bool
32	mtx   sync.Mutex
33}
34
35// newCache returns a cache that contains a mapping of plaintext passwords
36// to their hashes (with random eviction). This can greatly improve the
37// performance of traffic-heavy servers that use secure password hashing
38// algorithms, with the downside that plaintext passwords will be stored in
39// memory for a longer time (this should not be a problem as long as your
40// machine is not compromised, at which point all bets are off, since basicauth
41// necessitates plaintext passwords being received over the wire anyway).
42func newCache() *cache {
43	return &cache{
44		cache: make(map[string]bool),
45	}
46}
47
48func (c *cache) get(key string) (bool, bool) {
49	c.mtx.Lock()
50	defer c.mtx.Unlock()
51	v, ok := c.cache[key]
52	return v, ok
53}
54
55func (c *cache) set(key string, value bool) {
56	c.mtx.Lock()
57	defer c.mtx.Unlock()
58	c.makeRoom()
59	c.cache[key] = value
60}
61
62func (c *cache) makeRoom() {
63	if len(c.cache) < cacheSize {
64		return
65	}
66	// We delete more than just 1 entry so that we don't have
67	// to do this on every request; assuming the capacity of
68	// the cache is on a long tail, we can save a lot of CPU
69	// time by doing a whole bunch of deletions now and then
70	// we won't have to do them again for a while.
71	numToDelete := len(c.cache) / 10
72	if numToDelete < 1 {
73		numToDelete = 1
74	}
75	for deleted := 0; deleted <= numToDelete; deleted++ {
76		// Go maps are "nondeterministic" not actually random,
77		// so although we could just chop off the "front" of the
78		// map with less code, this is a heavily skewed eviction
79		// strategy; generating random numbers is cheap and
80		// ensures a much better distribution.
81		rnd := weakrand.Intn(len(c.cache))
82		i := 0
83		for key := range c.cache {
84			if i == rnd {
85				delete(c.cache, key)
86				break
87			}
88			i++
89		}
90	}
91}
92