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Remove internal StateLocker implementation (#688)
Fixes #657 With the changes in #667 and #669 to use a plain-old-mutex for concurrent access of Histogram and MinMaxSumCount aggregators, the StateLocker implementation is no longer used in the project. Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>
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// Copyright The OpenTelemetry Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package internal
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import (
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"runtime"
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"sync"
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"sync/atomic"
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)
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// StateLocker implements a two state lock algorithm that enabled lock free operations inside a state
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// and a global lock for switching between states. At every time, only one state is active and one cold state.
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// States are represented by int numbers 0 and 1.
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//
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// This was inspired by the algorithm used on the prometheus client library that can be found at:
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// https://github.com/prometheus/client_golang/blob/e7776d2c54305c1b62fdb113b5a7e9b944c5c27e/prometheus/histogram.go#L227
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//
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// To execute operations within the same state, call `Start()` before the operation and call `End(idx)`
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// to end this operation. The `idx` argument of `End()` is the index of the active state when the operation
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// started and it is returned by the `Start()` method. It is recommended to defer the call to `End(idx)`.
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//
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// One can change the active state by calling `SwapActiveState(fn)`. `fn` is a function that will be executed *before*
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// switching the active state. Operations such as preparing the new state shall be called by this function. This will
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// wait in-flight operations to end.
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//
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// Example workflow:
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// 1. State 0 is active.
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// 1.1 Operations to the active state can happen with `Start()` and `End(idx)` methods.
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// 2. Call to `SwitchState(fn)`
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// 2.1 run `fn` function to prepare the new state
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// 2.2 make state 1 active
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// 2.3 wait in-flight operations of the state 0 to end.
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// 3. State 1 is now active and every new operation are executed in it.
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//
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// `SwitchState(fn)` are synchronized with a mutex that can be access with the `Lock()` and `Unlock()` methods.
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// Access to the cold state must also be synchronized to ensure the cold state is not in the middle of state switch
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// since that could represent an invalid state.
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//
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type StateLocker struct {
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countsAndActiveIdx uint64
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finishedOperations [2]uint64
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sync.Mutex
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}
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// Start an operation that will happen on a state. The current active state is returned.
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// A call to `End(idx int)` must happens for every `Start()` call.
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func (c *StateLocker) Start() int {
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n := atomic.AddUint64(&c.countsAndActiveIdx, 1)
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return int(n >> 63)
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}
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// End an operation that happened to the idx state.
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func (c *StateLocker) End(idx int) {
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atomic.AddUint64(&c.finishedOperations[idx], 1)
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}
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// ColdIdx returns the index of the cold state.
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func (c *StateLocker) ColdIdx() int {
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return int((^c.countsAndActiveIdx) >> 63)
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}
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// SwapActiveState swaps the cold and active states.
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//
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// This will wait all for in-flight operations that are happening to the current
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// active state to end, this ensure that all access to this state will be consistent.
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//
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// This is synchronized by a mutex.
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func (c *StateLocker) SwapActiveState(beforeFn func()) {
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c.Lock()
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defer c.Unlock()
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if beforeFn != nil {
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// prepare the state change
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beforeFn()
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}
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// Adding 1<<63 switches the active index (from 0 to 1 or from 1 to 0)
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// without touching the count bits.
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n := atomic.AddUint64(&c.countsAndActiveIdx, 1<<63)
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// count represents how many operations have started *before* the state change.
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count := n & ((1 << 63) - 1)
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activeFinishedOperations := &c.finishedOperations[n>>63]
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// coldFinishedOperations are the number of operations that have *ended* on the previous state.
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coldFinishedOperations := &c.finishedOperations[(^n)>>63]
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// Await all cold writers to finish writing, when coldFinishedOperations == count, all in-flight operations
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// have finished and we can cleanly end the state change.
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for count != atomic.LoadUint64(coldFinishedOperations) {
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runtime.Gosched() // Let observations get work done.
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}
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// Make sure that the new state keeps the same count of *ended* operations.
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atomic.AddUint64(activeFinishedOperations, count)
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atomic.StoreUint64(coldFinishedOperations, 0)
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}
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@ -1,102 +0,0 @@
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// Copyright The OpenTelemetry Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package internal
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import (
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"testing"
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"time"
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"github.com/stretchr/testify/require"
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)
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func TestInflightOperationMustEndBeforeSwap(t *testing.T) {
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var swapped bool
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ch := make(chan struct{})
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l := StateLocker{}
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op1 := l.Start()
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go func() {
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l.SwapActiveState(func() {})
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swapped = true
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ch <- struct{}{}
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}()
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require.False(t, swapped, "Swap should wait the end of the in-flight operation.")
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l.End(op1)
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select {
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case <-ch:
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require.True(t, swapped, "Swap should've been completed. ")
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case <-time.After(50 * time.Millisecond):
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t.Fatal("Swap was not concluded after 50 milliseconds.")
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}
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}
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func TestEnsureIndexIsConsistent(t *testing.T) {
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l := StateLocker{}
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op1 := l.Start()
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l.End(op1)
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l.SwapActiveState(func() {})
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op2 := l.Start()
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l.End(op2)
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op3 := l.Start()
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l.End(op3)
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l.SwapActiveState(func() {})
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op4 := l.Start()
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l.End(op4)
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require.Equal(t, op1, op4, "two operations separated by two swaps should have the same index.")
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require.Equal(t, op2, op3, "two operations with no swap in between should have the same index.")
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require.Equal(t, 0, op1, "first index should be 0")
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require.Equal(t, 1, op2, "second index should be 1")
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}
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func TestTwoSwapsCanHappenWithoutOperationsInBetween(t *testing.T) {
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l := StateLocker{}
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require.Equal(t, 1, l.ColdIdx(), "first cold index should be 1")
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l.SwapActiveState(func() {})
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require.Equal(t, 0, l.ColdIdx(), "second cold index should be 0")
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l.SwapActiveState(func() {})
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require.Equal(t, 1, l.ColdIdx(), "third cold index should be 1")
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}
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func BenchmarkStateLocker_StartEnd(b *testing.B) {
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l := StateLocker{}
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b.ReportAllocs()
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for i := 0; i < b.N; i++ {
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l.End(l.Start())
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}
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}
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func BenchmarkStateLocker_SwapActiveState(b *testing.B) {
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b.ReportAllocs()
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for i := 0; i < b.N; i++ {
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l := StateLocker{}
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l.SwapActiveState(func() {})
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}
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}
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