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idle.go
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idle.go
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/*
*
* Copyright 2023 gRPC 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 grpc
import (
"fmt"
"math"
"sync"
"sync/atomic"
"time"
)
// For overriding in unit tests.
var timeAfterFunc = func(d time.Duration, f func()) *time.Timer {
return time.AfterFunc(d, f)
}
// idlenessEnforcer is the functionality provided by grpc.ClientConn to enter
// and exit from idle mode.
type idlenessEnforcer interface {
exitIdleMode() error
enterIdleMode() error
}
// idlenessManager defines the functionality required to track RPC activity on a
// channel.
type idlenessManager interface {
onCallBegin() error
onCallEnd()
close()
}
type noopIdlenessManager struct{}
func (noopIdlenessManager) onCallBegin() error { return nil }
func (noopIdlenessManager) onCallEnd() {}
func (noopIdlenessManager) close() {}
// idlenessManagerImpl implements the idlenessManager interface. It uses atomic
// operations to synchronize access to shared state and a mutex to guarantee
// mutual exclusion in a critical section.
type idlenessManagerImpl struct {
// State accessed atomically.
lastCallEndTime int64 // Unix timestamp in nanos; time when the most recent RPC completed.
activeCallsCount int32 // Count of active RPCs; -math.MaxInt32 means channel is idle or is trying to get there.
activeSinceLastTimerCheck int32 // Boolean; True if there was an RPC since the last timer callback.
closed int32 // Boolean; True when the manager is closed.
// Can be accessed without atomics or mutex since these are set at creation
// time and read-only after that.
enforcer idlenessEnforcer // Functionality provided by grpc.ClientConn.
timeout int64 // Idle timeout duration nanos stored as an int64.
// idleMu is used to guarantee mutual exclusion in two scenarios:
// - Opposing intentions:
// - a: Idle timeout has fired and handleIdleTimeout() is trying to put
// the channel in idle mode because the channel has been inactive.
// - b: At the same time an RPC is made on the channel, and onCallBegin()
// is trying to prevent the channel from going idle.
// - Competing intentions:
// - The channel is in idle mode and there are multiple RPCs starting at
// the same time, all trying to move the channel out of idle. Only one
// of them should succeed in doing so, while the other RPCs should
// piggyback on the first one and be successfully handled.
idleMu sync.RWMutex
actuallyIdle bool
timer *time.Timer
}
// newIdlenessManager creates a new idleness manager implementation for the
// given idle timeout.
func newIdlenessManager(enforcer idlenessEnforcer, idleTimeout time.Duration) idlenessManager {
if idleTimeout == 0 {
return noopIdlenessManager{}
}
i := &idlenessManagerImpl{
enforcer: enforcer,
timeout: int64(idleTimeout),
}
i.timer = timeAfterFunc(idleTimeout, i.handleIdleTimeout)
return i
}
// resetIdleTimer resets the idle timer to the given duration. This method
// should only be called from the timer callback.
func (i *idlenessManagerImpl) resetIdleTimer(d time.Duration) {
i.idleMu.Lock()
defer i.idleMu.Unlock()
if i.timer == nil {
// Only close sets timer to nil. We are done.
return
}
// It is safe to ignore the return value from Reset() because this method is
// only ever called from the timer callback, which means the timer has
// already fired.
i.timer.Reset(d)
}
// handleIdleTimeout is the timer callback that is invoked upon expiry of the
// configured idle timeout. The channel is considered inactive if there are no
// ongoing calls and no RPC activity since the last time the timer fired.
func (i *idlenessManagerImpl) handleIdleTimeout() {
if i.isClosed() {
return
}
if atomic.LoadInt32(&i.activeCallsCount) > 0 {
i.resetIdleTimer(time.Duration(i.timeout))
return
}
// There has been activity on the channel since we last got here. Reset the
// timer and return.
if atomic.LoadInt32(&i.activeSinceLastTimerCheck) == 1 {
// Set the timer to fire after a duration of idle timeout, calculated
// from the time the most recent RPC completed.
atomic.StoreInt32(&i.activeSinceLastTimerCheck, 0)
i.resetIdleTimer(time.Duration(atomic.LoadInt64(&i.lastCallEndTime) + i.timeout - time.Now().UnixNano()))
return
}
// This CAS operation is extremely likely to succeed given that there has
// been no activity since the last time we were here. Setting the
// activeCallsCount to -math.MaxInt32 indicates to onCallBegin() that the
// channel is either in idle mode or is trying to get there.
if !atomic.CompareAndSwapInt32(&i.activeCallsCount, 0, -math.MaxInt32) {
// This CAS operation can fail if an RPC started after we checked for
// activity at the top of this method, or one was ongoing from before
// the last time we were here. In both case, reset the timer and return.
i.resetIdleTimer(time.Duration(i.timeout))
return
}
// Now that we've set the active calls count to -math.MaxInt32, it's time to
// actually move to idle mode.
if i.tryEnterIdleMode() {
// Successfully entered idle mode. No timer needed until we exit idle.
return
}
// Failed to enter idle mode due to a concurrent RPC that kept the channel
// active, or because of an error from the channel. Undo the attempt to
// enter idle, and reset the timer to try again later.
atomic.AddInt32(&i.activeCallsCount, math.MaxInt32)
i.resetIdleTimer(time.Duration(i.timeout))
}
// tryEnterIdleMode instructs the channel to enter idle mode. But before
// that, it performs a last minute check to ensure that no new RPC has come in,
// making the channel active.
//
// Return value indicates whether or not the channel moved to idle mode.
//
// Holds idleMu which ensures mutual exclusion with exitIdleMode.
func (i *idlenessManagerImpl) tryEnterIdleMode() bool {
i.idleMu.Lock()
defer i.idleMu.Unlock()
if atomic.LoadInt32(&i.activeCallsCount) != -math.MaxInt32 {
// We raced and lost to a new RPC. Very rare, but stop entering idle.
return false
}
if atomic.LoadInt32(&i.activeSinceLastTimerCheck) == 1 {
// An very short RPC could have come in (and also finished) after we
// checked for calls count and activity in handleIdleTimeout(), but
// before the CAS operation. So, we need to check for activity again.
return false
}
// No new RPCs have come in since we last set the active calls count value
// -math.MaxInt32 in the timer callback. And since we have the lock, it is
// safe to enter idle mode now.
if err := i.enforcer.enterIdleMode(); err != nil {
logger.Errorf("Failed to enter idle mode: %v", err)
return false
}
// Successfully entered idle mode.
i.actuallyIdle = true
return true
}
// onCallBegin is invoked at the start of every RPC.
func (i *idlenessManagerImpl) onCallBegin() error {
if i.isClosed() {
return nil
}
if atomic.AddInt32(&i.activeCallsCount, 1) > 0 {
// Channel is not idle now. Set the activity bit and allow the call.
atomic.StoreInt32(&i.activeSinceLastTimerCheck, 1)
return nil
}
// Channel is either in idle mode or is in the process of moving to idle
// mode. Attempt to exit idle mode to allow this RPC.
if err := i.exitIdleMode(); err != nil {
// Undo the increment to calls count, and return an error causing the
// RPC to fail.
atomic.AddInt32(&i.activeCallsCount, -1)
return err
}
atomic.StoreInt32(&i.activeSinceLastTimerCheck, 1)
return nil
}
// exitIdleMode instructs the channel to exit idle mode.
//
// Holds idleMu which ensures mutual exclusion with tryEnterIdleMode.
func (i *idlenessManagerImpl) exitIdleMode() error {
i.idleMu.Lock()
defer i.idleMu.Unlock()
if !i.actuallyIdle {
// This can happen in two scenarios:
// - handleIdleTimeout() set the calls count to -math.MaxInt32 and called
// tryEnterIdleMode(). But before the latter could grab the lock, an RPC
// came in and onCallBegin() noticed that the calls count is negative.
// - Channel is in idle mode, and multiple new RPCs come in at the same
// time, all of them notice a negative calls count in onCallBegin and get
// here. The first one to get the lock would got the channel to exit idle.
//
// Either way, nothing to do here.
return nil
}
if err := i.enforcer.exitIdleMode(); err != nil {
return fmt.Errorf("channel failed to exit idle mode: %v", err)
}
// Undo the idle entry process. This also respects any new RPC attempts.
atomic.AddInt32(&i.activeCallsCount, math.MaxInt32)
i.actuallyIdle = false
// Start a new timer to fire after the configured idle timeout.
i.timer = timeAfterFunc(time.Duration(i.timeout), i.handleIdleTimeout)
return nil
}
// onCallEnd is invoked at the end of every RPC.
func (i *idlenessManagerImpl) onCallEnd() {
if i.isClosed() {
return
}
// Record the time at which the most recent call finished.
atomic.StoreInt64(&i.lastCallEndTime, time.Now().UnixNano())
// Decrement the active calls count. This count can temporarily go negative
// when the timer callback is in the process of moving the channel to idle
// mode, but one or more RPCs come in and complete before the timer callback
// can get done with the process of moving to idle mode.
atomic.AddInt32(&i.activeCallsCount, -1)
}
func (i *idlenessManagerImpl) isClosed() bool {
return atomic.LoadInt32(&i.closed) == 1
}
func (i *idlenessManagerImpl) close() {
atomic.StoreInt32(&i.closed, 1)
i.idleMu.Lock()
i.timer.Stop()
i.timer = nil
i.idleMu.Unlock()
}