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/*
* 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 io.trino.execution;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.errorprone.annotations.ThreadSafe;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import io.airlift.log.Logger;
import io.trino.spi.TrinoException;
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Predicate;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static io.trino.spi.StandardErrorCode.SERVER_SHUTTING_DOWN;
import static java.util.Objects.requireNonNull;
/**
* Simple state machine which holds a single state. Callers can register for state change events.
*/
@ThreadSafe
public class StateMachine
{
private static final Logger log = Logger.get(StateMachine.class);
private final String name;
private final Executor executor;
private final Object lock = new Object();
private final Set terminalStates;
@GuardedBy("lock")
private volatile T state;
@GuardedBy("lock")
private final List> stateChangeListeners = new ArrayList<>();
private final AtomicReference> futureStateChange = new AtomicReference<>(new FutureStateChange<>());
/**
* Creates a state machine with the specified initial state and no terminal states.
*
* @param name name of this state machine to use in debug statements
* @param executor executor for firing state change events; must not be a same thread executor
* @param initialState the initial state
*/
public StateMachine(String name, Executor executor, T initialState)
{
this(name, executor, initialState, ImmutableSet.of());
}
/**
* Creates a state machine with the specified initial state and terminal states.
*
* @param name name of this state machine to use in debug statements
* @param executor executor for firing state change events; must not be a same thread executor
* @param initialState the initial state
* @param terminalStates the terminal states
*/
public StateMachine(String name, Executor executor, T initialState, Iterable terminalStates)
{
this.name = requireNonNull(name, "name is null");
this.executor = requireNonNull(executor, "executor is null");
this.state = requireNonNull(initialState, "initialState is null");
this.terminalStates = ImmutableSet.copyOf(requireNonNull(terminalStates, "terminalStates is null"));
}
// state changes are atomic and state is volatile, so a direct read is safe here
@SuppressWarnings("FieldAccessNotGuarded")
public T get()
{
return state;
}
/**
* Sets the state.
* If the new state does not {@code .equals()} the current state, listeners and waiters will be notified.
*
* @return the old state
* @throws IllegalStateException if state change would cause a transition from a terminal state
*/
public T set(T newState)
{
T oldState = trySet(newState);
checkState(oldState.equals(newState) || !isTerminalState(oldState), "%s cannot transition from %s to %s", name, state, newState);
return oldState;
}
/**
* Tries to change the state. State will not change if the new state {@code .equals()} the current state,
* of if the current state is a terminal state. If the state changed, listeners and waiters will be notified.
*
* @return the state before the possible state change
*/
public T trySet(T newState)
{
checkState(!Thread.holdsLock(lock), "Cannot set state while holding the lock");
requireNonNull(newState, "newState is null");
T oldState;
FutureStateChange futureStateChange;
ImmutableList> stateChangeListeners;
synchronized (lock) {
if (state.equals(newState) || isTerminalState(state)) {
return state;
}
oldState = state;
state = newState;
futureStateChange = this.futureStateChange.getAndSet(new FutureStateChange<>());
stateChangeListeners = ImmutableList.copyOf(this.stateChangeListeners);
// if we are now in a terminal state, free the listeners since this will be the last notification
if (isTerminalState(state)) {
this.stateChangeListeners.clear();
}
}
fireStateChanged(newState, futureStateChange, stateChangeListeners);
return oldState;
}
/**
* Sets the state if the current state satisfies the specified predicate.
* If the new state does not {@code .equals()} the current state, listeners and waiters will be notified.
*
* @return true if the state is set
*/
public boolean setIf(T newState, Predicate predicate)
{
checkState(!Thread.holdsLock(lock), "Cannot set state while holding the lock");
requireNonNull(newState, "newState is null");
while (true) {
// check if the current state passes the predicate
T currentState = get();
// change to same state is not a change, and does not notify the notify listeners
if (currentState.equals(newState)) {
return false;
}
// do not call predicate while holding the lock
if (!predicate.test(currentState)) {
return false;
}
// if state did not change while, checking the predicate, apply the new state
if (compareAndSet(currentState, newState)) {
return true;
}
}
}
/**
* Sets the state if the current state {@code .equals()} the specified expected state.
* If the new state does not {@code .equals()} the current state, listeners and waiters will be notified.
*
* @return true if the state is set
*/
public boolean compareAndSet(T expectedState, T newState)
{
checkState(!Thread.holdsLock(lock), "Cannot set state while holding the lock");
requireNonNull(expectedState, "expectedState is null");
requireNonNull(newState, "newState is null");
FutureStateChange futureStateChange;
ImmutableList> stateChangeListeners;
synchronized (lock) {
if (!state.equals(expectedState)) {
return false;
}
// change to same state is not a change, and does not notify the notify listeners
if (state.equals(newState)) {
return false;
}
checkState(!isTerminalState(state), "%s cannot transition from %s to %s", name, state, newState);
state = newState;
futureStateChange = this.futureStateChange.getAndSet(new FutureStateChange<>());
stateChangeListeners = ImmutableList.copyOf(this.stateChangeListeners);
// if we are now in a terminal state, free the listeners since this will be the last notification
if (isTerminalState(state)) {
this.stateChangeListeners.clear();
}
}
fireStateChanged(newState, futureStateChange, stateChangeListeners);
return true;
}
private void fireStateChanged(T newState, FutureStateChange futureStateChange, List> stateChangeListeners)
{
checkState(!Thread.holdsLock(lock), "Cannot fire state change event while holding the lock");
requireNonNull(newState, "newState is null");
// always fire listener callbacks from a different thread
safeExecute(() -> {
checkState(!Thread.holdsLock(lock), "Cannot notify while holding the lock");
try {
futureStateChange.complete(newState);
}
catch (Throwable e) {
log.error(e, "Error setting future state for %s", name);
}
for (StateChangeListener stateChangeListener : stateChangeListeners) {
fireStateChangedListener(newState, stateChangeListener);
}
});
}
private void fireStateChangedListener(T newState, StateChangeListener stateChangeListener)
{
try {
stateChangeListener.stateChanged(newState);
}
catch (Throwable e) {
log.error(e, "Error notifying state change listener for %s", name);
}
}
/**
* Gets a future that completes when the state is no longer {@code .equals()} to {@code currentState)}.
*/
public ListenableFuture getStateChange(T currentState)
{
checkState(!Thread.holdsLock(lock), "Cannot wait for state change while holding the lock");
requireNonNull(currentState, "currentState is null");
synchronized (lock) {
// return a completed future if the state has already changed, or we are in a terminal state
if (!state.equals(currentState) || isTerminalState(state)) {
return immediateFuture(state);
}
return futureStateChange.get().createNewListener();
}
}
/**
* Adds a listener to be notified when the state instance changes according to {@code .equals()}.
* Listener is always notified asynchronously using a dedicated notification thread pool so, care should
* be taken to avoid leaking {@code this} when adding a listener in a constructor. Additionally, it is
* possible notifications are observed out of order due to the asynchronous execution. The listener is
* immediately notified immediately of the current state.
*/
public void addStateChangeListener(StateChangeListener stateChangeListener)
{
requireNonNull(stateChangeListener, "stateChangeListener is null");
boolean inTerminalState;
T currentState;
synchronized (lock) {
currentState = state;
inTerminalState = isTerminalState(currentState);
if (!inTerminalState) {
stateChangeListeners.add(stateChangeListener);
}
}
// fire state change listener with the current state
// always fire listener callbacks from a different thread
safeExecute(() -> fireStateChangedListener(currentState, stateChangeListener));
}
@VisibleForTesting
boolean isTerminalState(T state)
{
return terminalStates.contains(state);
}
@VisibleForTesting
List> getStateChangeListeners()
{
synchronized (lock) {
return ImmutableList.copyOf(stateChangeListeners);
}
}
public interface StateChangeListener
{
void stateChanged(T newState);
}
@Override
public String toString()
{
return get().toString();
}
private void safeExecute(Runnable command)
{
try {
executor.execute(command);
}
catch (RejectedExecutionException e) {
if (executor instanceof ExecutorService service && service.isShutdown()) {
throw new TrinoException(SERVER_SHUTTING_DOWN, "Server is shutting down", e);
}
throw e;
}
}
}
