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// GoToSocial
// Copyright (C) GoToSocial Authors admin@gotosocial.org
// SPDX-License-Identifier: AGPL-3.0-or-later
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
/*
The code in this file is adapted from MIT-licensed code in github.com/go-chi/chi. Thanks chi (thi)!
See: https://github.com/go-chi/chi/blob/e6baba61759b26ddf7b14d1e02d1da81a4d76c08/middleware/throttle.go
And: https://github.com/sponsors/pkieltyka
*/
package middleware
import (
"net/http"
"runtime"
"strconv"
"sync/atomic"
"time"
"github.com/gin-gonic/gin"
apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
)
// token represents a request that is being processed.
type token struct{}
// Throttle returns a gin middleware that performs throttling of incoming requests,
// ensuring that only a certain number of requests are handled concurrently, to reduce
// congestion of the server.
//
// Limits are configured using available CPUs and the given cpuMultiplier value.
// Open request limit is available CPUs * multiplier; backlog limit is limit * multiplier.
//
// Example values for multiplier 8:
//
// 1 cpu = 08 open, 064 backlog
// 2 cpu = 16 open, 128 backlog
// 4 cpu = 32 open, 256 backlog
//
// Example values for multiplier 4:
//
// 1 cpu = 04 open, 016 backlog
// 2 cpu = 08 open, 032 backlog
// 4 cpu = 16 open, 064 backlog
//
// Callers will first attempt to get a backlog token. Once they have that, they will
// wait in the backlog queue until they can get a token to allow their request to be
// processed.
//
// If the backlog queue is full, the request context is closed, or the caller has been
// waiting in the backlog for too long, this function will abort the request chain,
// write a JSON error into the response, set an appropriate Retry-After value, and set
// the HTTP response code to 503: Service Unavailable.
//
// If the multiplier is <= 0, a noop middleware will be returned instead.
//
// RetryAfter determines the Retry-After header value to be sent to throttled requests.
//
// Useful links:
//
// - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
// - https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/503
func Throttle(cpuMultiplier int, retryAfter time.Duration) gin.HandlerFunc {
if cpuMultiplier <= 0 {
// throttling is disabled, return a noop middleware
return func(c *gin.Context) {}
}
if retryAfter < 0 {
retryAfter = 0
}
var (
limit = runtime.GOMAXPROCS(0) * cpuMultiplier
queueLimit = limit * cpuMultiplier
tokens = make(chan token, limit)
requestCount = atomic.Int64{}
retryAfterStr = strconv.FormatUint(uint64(retryAfter/time.Second), 10) // #nosec G115 -- Checked right above
)
// prefill token channel
for i := 0; i < limit; i++ {
tokens <- token{}
}
return func(c *gin.Context) {
// Always decrement request counter.
defer func() { requestCount.Add(-1) }()
// Increment request count.
n := requestCount.Add(1)
// Check whether the request
// count is over queue limit.
if n > int64(queueLimit) {
c.Header("Retry-After", retryAfterStr)
apiutil.Data(c,
http.StatusTooManyRequests,
apiutil.AppJSON,
apiutil.ErrorCapacityExceeded,
)
c.Abort()
return
}
// Sit and wait in the
// queue for free token.
select {
case <-c.Request.Context().Done():
// request context has
// been canceled already.
return
case tok := <-tokens:
// caller has successfully
// received a token, allowing
// request to be processed.
defer func() {
// when we're finished, return
// this token to the bucket.
tokens <- tok
}()
// Process
// request!
c.Next()
}
}
}
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