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// Copyright 2021 The Prometheus Authors
// This code is partly borrowed from Caddy:
// Copyright 2015 Matthew Holt and The Caddy 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 web
import (
weakrand "math/rand"
"sync"
)
var cacheSize = 100
type cache struct {
cache map[string]bool
mtx sync.Mutex
}
// newCache returns a cache that contains a mapping of plaintext passwords
// to their hashes (with random eviction). This can greatly improve the
// performance of traffic-heavy servers that use secure password hashing
// algorithms, with the downside that plaintext passwords will be stored in
// memory for a longer time (this should not be a problem as long as your
// machine is not compromised, at which point all bets are off, since basicauth
// necessitates plaintext passwords being received over the wire anyway).
func newCache() *cache {
return &cache{
cache: make(map[string]bool),
}
}
func (c *cache) get(key string) (bool, bool) {
c.mtx.Lock()
defer c.mtx.Unlock()
v, ok := c.cache[key]
return v, ok
}
func (c *cache) set(key string, value bool) {
c.mtx.Lock()
defer c.mtx.Unlock()
c.makeRoom()
c.cache[key] = value
}
func (c *cache) makeRoom() {
if len(c.cache) < cacheSize {
return
}
// We delete more than just 1 entry so that we don't have
// to do this on every request; assuming the capacity of
// the cache is on a long tail, we can save a lot of CPU
// time by doing a whole bunch of deletions now and then
// we won't have to do them again for a while.
numToDelete := len(c.cache) / 10
if numToDelete < 1 {
numToDelete = 1
}
for deleted := 0; deleted <= numToDelete; deleted++ {
// Go maps are "nondeterministic" not actually random,
// so although we could just chop off the "front" of the
// map with less code, this is a heavily skewed eviction
// strategy; generating random numbers is cheap and
// ensures a much better distribution.
rnd := weakrand.Intn(len(c.cache))
i := 0
for key := range c.cache {
if i == rnd {
delete(c.cache, key)
break
}
i++
}
}
}