src/main/java/com/google/devtools/build/lib/dynamic/DynamicSpawnStrategy.java - bazel - Git at Google

 // Copyright 2018 The Bazel Authors. All rights reserved.
 //
 // 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 com.google.devtools.build.lib.dynamic;

 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Preconditions.checkState;

 import com.google.common.base.Throwables;
 import com.google.common.collect.ImmutableList;
 import com.google.common.flogger.GoogleLogger;
 import com.google.common.io.Files;
 import com.google.common.util.concurrent.ListenableFuture;
 import com.google.common.util.concurrent.ListeningExecutorService;
 import com.google.common.util.concurrent.MoreExecutors;
 import com.google.common.util.concurrent.SettableFuture;
 import com.google.devtools.build.lib.actions.ActionContext;
 import com.google.devtools.build.lib.actions.ActionExecutionContext;
 import com.google.devtools.build.lib.actions.DynamicStrategyRegistry;
 import com.google.devtools.build.lib.actions.ExecException;
 import com.google.devtools.build.lib.actions.SandboxedSpawnStrategy;
 import com.google.devtools.build.lib.actions.Spawn;
 import com.google.devtools.build.lib.actions.SpawnResult;
 import com.google.devtools.build.lib.actions.SpawnStrategy;
 import com.google.devtools.build.lib.exec.ExecutionPolicy;
 import com.google.devtools.build.lib.util.io.FileOutErr;
 import com.google.devtools.build.lib.vfs.Path;
 import java.io.IOException;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 import javax.annotation.Nullable;

 /**
  * A spawn strategy that speeds up incremental builds while not slowing down full builds.
  *
  * <p>This strategy tries to run spawn actions on the local and remote machine at the same time, and
  * picks the spawn action that completes first. This gives the benefits of remote execution on full
  * builds, and local execution on incremental builds.
  *
  * <p>One might ask, why we don't run spawns on the workstation all the time and just "spill over"
  * actions to remote execution when there are no local resources available. This would work, except
  * that the cost of transferring action inputs and outputs from the local machine to and from remote
  * executors over the network is way too high - there is no point in executing an action locally and
  * save 0.5s of time, when it then takes us 5 seconds to upload the results to remote executors for
  * another action that's scheduled to run there.
  */
 public class DynamicSpawnStrategy implements SpawnStrategy {

   private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();

   private final ListeningExecutorService executorService;
   private final DynamicExecutionOptions options;
   private final Function<Spawn, ExecutionPolicy> getExecutionPolicy;

   /**
    * Set to true by the first action that completes remotely. Until that happens, all local actions
    * are delayed by the amount given in {@link DynamicExecutionOptions#localExecutionDelay}.
    *
    * <p>This is a rather simple approach to make it possible to score a cache hit on remote
    * execution before even trying to start the action locally. This saves resources that would
    * otherwise be wasted by continuously starting and immediately killing local processes. One
    * possibility for improvement would be to establish a reporting mechanism from strategies back to
    * here, where we delay starting locally until the remote strategy tells us that the action isn't
    * a cache hit.
    */
   private final AtomicBoolean delayLocalExecution = new AtomicBoolean(false);

   /**
    * Constructs a {@code DynamicSpawnStrategy}.
    *
    * @param executorService an {@link ExecutorService} that will be used to run Spawn actions.
    */
   public DynamicSpawnStrategy(
       ExecutorService executorService,
       DynamicExecutionOptions options,
       Function<Spawn, ExecutionPolicy> getExecutionPolicy) {
     this.executorService = MoreExecutors.listeningDecorator(executorService);
     this.options = options;
     this.getExecutionPolicy = getExecutionPolicy;
   }

   /**
    * Cancels and waits for a branch (a spawn execution) to terminate.
    *
    * <p>This is intended to be used as the body of the {@link
    * SandboxedSpawnStrategy.StopConcurrentSpawns} lambda passed to the spawn runners. Each strategy
    * may call this at most once.
    *
    * @param branch the future running the spawn
    * @param branchDone semaphore that is expected to receive a permit once the future terminates
    *     (after {@link InterruptedException} bubbles up through its call stack)
    * @param cancellingStrategy identifier of the strategy that is performing the cancellation. Used
    *     to prevent cross-cancellations and to sanity-check that the same strategy doesn't issue the
    *     cancellation twice.
    * @param strategyThatCancelled name of the first strategy that executed this method, or a null
    *     reference if this is the first time this method is called. If not null, we expect the value
    *     referenced by this to be different than {@code cancellingStrategy}, or else we have a bug.
    * @throws InterruptedException if we get interrupted for any reason trying to cancel the future
    * @throws DynamicInterruptedException if we lost a race against another strategy trying to cancel
    *     us
    */
   private static void stopBranch(
       Future<ImmutableList<SpawnResult>> branch,
       Semaphore branchDone,
       String cancellingStrategy,
       AtomicReference<String> strategyThatCancelled)
       throws InterruptedException {
     // This multi-step, unlocked access to "strategyThatCancelled" is valid because, for a given
     // value of "cancellingStrategy", we do not expect concurrent calls to this method. (If there
     // are, we are in big trouble.)
     String current = strategyThatCancelled.get();
     if (current != null && current.equals(cancellingStrategy)) {
       throw new AssertionError("stopBranch called more than once by " + cancellingStrategy);
     } else {
       // Protect against the two branches from cancelling each other. The first branch to set the
       // reference to its own identifier wins and is allowed to issue the cancellation; the other
       // branch just has to give up execution.
       if (strategyThatCancelled.compareAndSet(null, cancellingStrategy)) {
         boolean cancelled = branch.cancel(true);
         checkState(cancelled, "Failed to cancel other branch from %s", cancellingStrategy);
         branchDone.acquire();
       } else {
         throw new DynamicInterruptedException(
             "Execution stopped because other strategy finished first");
       }
     }
   }

   /**
    * Waits for a branch (a spawn execution) to complete.
    *
    * @param branch the future running the spawn
    * @return the spawn result if the execution terminated successfully, or null if the branch was
    *     cancelled
    * @throws ExecException the execution error of the spawn if it failed
    * @throws InterruptedException if we get interrupted while waiting for completion
    */
   @Nullable
   private static ImmutableList<SpawnResult> waitBranch(Future<ImmutableList<SpawnResult>> branch)
       throws ExecException, InterruptedException {
     try {
       return branch.get();
     } catch (CancellationException e) {
       return null;
     } catch (ExecutionException e) {
       Throwable cause = e.getCause();
       if (cause instanceof ExecException) {
         throw (ExecException) cause;
       } else if (cause instanceof InterruptedException) {
         // If the branch was interrupted, it might be due to a user interrupt or due to our request
         // for cancellation. Assume the latter here because if this was actually a user interrupt,
         // our own get() would have been interrupted as well. It makes no sense to propagate the
         // interrupt status across threads.
         return null;
       } else {
         // Even though we cannot enforce this in the future's signature (but we do in Branch#call),
         // we only expect the exception types we validated above. Still, unchecked exceptions could
         // propagate, so just let them bubble up.
         Throwables.throwIfUnchecked(cause);
         throw new AssertionError("Unexpected exception type from strategy.exec()");
       }
     } catch (InterruptedException e) {
       branch.cancel(true);
       throw e;
     }
   }

   /**
    * Waits for the two branches of a spawn's execution to complete.
    *
    * <p>This guarantees that the two branches are stopped both on successful termination and on an
    * exception.
    *
    * @param branch1 the future running one side of the spawn (e.g. local). This future must cancel
    *     {@code branch2} at some point during its successful execution to guarantee termination. If
    *     we encounter an execution error, or if we are interrupted, then we handle such cancellation
    *     here.
    * @param branch2 the future running the other side of the spawn (e.g. remote). Same restrictions
    *     apply as in {@code branch1}, but in the symmetric direction.
    * @return the result of the branch that terminates first
    * @throws ExecException the execution error of the spawn that terminated first
    * @throws InterruptedException if we get interrupted while waiting for completion
    */
   private static ImmutableList<SpawnResult> waitBranches(
       Future<ImmutableList<SpawnResult>> branch1, Future<ImmutableList<SpawnResult>> branch2)
       throws ExecException, InterruptedException {
     ImmutableList<SpawnResult> result1;
     try {
       result1 = waitBranch(branch1);
     } catch (ExecException | InterruptedException | RuntimeException e) {
       branch2.cancel(true);
       throw e;
     }

     ImmutableList<SpawnResult> result2 = waitBranch(branch2);

     if (result2 != null && result1 != null) {
       throw new AssertionError("One branch did not cancel the other one");
     } else if (result2 != null) {
       return result2;
     } else if (result1 != null) {
       return result1;
     } else {
       throw new AssertionError("No branch completed, which might mean they cancelled each other");
     }
   }

   @Override
   public ImmutableList<SpawnResult> exec(
       final Spawn spawn, final ActionExecutionContext actionExecutionContext)
       throws ExecException, InterruptedException {
     ExecutionPolicy executionPolicy = getExecutionPolicy.apply(spawn);
     if (executionPolicy.canRunLocallyOnly()) {
       return runLocally(spawn, actionExecutionContext, null);
     }
     if (executionPolicy.canRunRemotelyOnly()) {
       return runRemotely(spawn, actionExecutionContext, null);
     }

     // Semaphores to track termination of each branch. These are necessary to wait for the branch to
     // finish its own cleanup (e.g. terminating subprocesses) once it has been cancelled.
     Semaphore localDone = new Semaphore(0);
     Semaphore remoteDone = new Semaphore(0);

     AtomicReference<String> strategyThatCancelled = new AtomicReference<>(null);
     SettableFuture<ImmutableList<SpawnResult>> remoteBranch = SettableFuture.create();

     AtomicBoolean localCanReportDone = new AtomicBoolean(false);
     AtomicBoolean remoteCanReportDone = new AtomicBoolean(false);

     ListenableFuture<ImmutableList<SpawnResult>> localBranch =
         executorService.submit(
             new Branch("local", actionExecutionContext) {
               @Override
               ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
                   throws InterruptedException, ExecException {
                 try {
                   if (!localCanReportDone.compareAndSet(false, true)) {
                     // If we ever get here, it's because we were cancelled and the listener ran
                     // first. Just make sure that's the case.
                     checkState(Thread.interrupted());
                     throw new InterruptedException();
                   }
                   if (delayLocalExecution.get()) {
                     Thread.sleep(options.localExecutionDelay);
                   }
                   return runLocally(
                       spawn,
                       context,
                       () -> stopBranch(remoteBranch, remoteDone, "local", strategyThatCancelled));
                 } finally {
                   localDone.release();
                 }
               }
             });
     localBranch.addListener(
         () -> {
           if (localCanReportDone.compareAndSet(false, true)) {
             localDone.release();
           }
           if (!localBranch.isCancelled()) {
             remoteBranch.cancel(true);
           }
         },
         MoreExecutors.directExecutor());

     remoteBranch.setFuture(
         executorService.submit(
             new Branch("remote", actionExecutionContext) {
               @Override
               public ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
                   throws InterruptedException, ExecException {
                 try {
                   if (!remoteCanReportDone.compareAndSet(false, true)) {
                     // If we ever get here, it's because we were cancelled and the listener ran
                     // first. Just make sure that's the case.
                     checkState(Thread.interrupted());
                     throw new InterruptedException();
                   }
                   ImmutableList<SpawnResult> spawnResults =
                       runRemotely(
                           spawn,
                           context,
                           () ->
                               stopBranch(localBranch, localDone, "remote", strategyThatCancelled));
                   delayLocalExecution.set(true);
                   return spawnResults;
                 } finally {
                   remoteDone.release();
                 }
               }
             }));
     remoteBranch.addListener(
         () -> {
           if (remoteCanReportDone.compareAndSet(false, true)) {
             remoteDone.release();
           }
           if (!remoteBranch.isCancelled()) {
             localBranch.cancel(true);
           }
         },
         MoreExecutors.directExecutor());

     try {
       return waitBranches(localBranch, remoteBranch);
     } finally {
       checkState(localBranch.isDone());
       checkState(remoteBranch.isDone());
     }
   }

   @Override
   public boolean canExec(Spawn spawn, ActionContext.ActionContextRegistry actionContextRegistry) {
     DynamicStrategyRegistry dynamicStrategyRegistry =
         actionContextRegistry.getContext(DynamicStrategyRegistry.class);
     for (SandboxedSpawnStrategy strategy :
         dynamicStrategyRegistry.getDynamicSpawnActionContexts(
             spawn, DynamicStrategyRegistry.DynamicMode.LOCAL)) {
       if (strategy.canExec(spawn, actionContextRegistry)) {
         return true;
       }
     }
     for (SandboxedSpawnStrategy strategy :
         dynamicStrategyRegistry.getDynamicSpawnActionContexts(
             spawn, DynamicStrategyRegistry.DynamicMode.REMOTE)) {
       if (strategy.canExec(spawn, actionContextRegistry)) {
         return true;
       }
     }
     return false;
   }

   @Override
   public void usedContext(ActionContext.ActionContextRegistry actionContextRegistry) {
     actionContextRegistry
         .getContext(DynamicStrategyRegistry.class)
         .notifyUsedDynamic(actionContextRegistry);
   }

   private static FileOutErr getSuffixedFileOutErr(FileOutErr fileOutErr, String suffix) {
     Path outDir = checkNotNull(fileOutErr.getOutputPath().getParentDirectory());
     String outBaseName = fileOutErr.getOutputPath().getBaseName();
     Path errDir = checkNotNull(fileOutErr.getErrorPath().getParentDirectory());
     String errBaseName = fileOutErr.getErrorPath().getBaseName();
     return new FileOutErr(
         outDir.getChild(outBaseName + suffix), errDir.getChild(errBaseName + suffix));
   }

   private static ImmutableList<SpawnResult> runLocally(
       Spawn spawn,
       ActionExecutionContext actionExecutionContext,
       @Nullable SandboxedSpawnStrategy.StopConcurrentSpawns stopConcurrentSpawns)
       throws ExecException, InterruptedException {
     DynamicStrategyRegistry dynamicStrategyRegistry =
         actionExecutionContext.getContext(DynamicStrategyRegistry.class);

     for (SandboxedSpawnStrategy strategy :
         dynamicStrategyRegistry.getDynamicSpawnActionContexts(
             spawn, DynamicStrategyRegistry.DynamicMode.LOCAL)) {
       if (strategy.canExec(spawn, actionExecutionContext)) {
         return strategy.exec(spawn, actionExecutionContext, stopConcurrentSpawns);
       }
     }
     throw new RuntimeException(
         "executorCreated not yet called or no default dynamic_local_strategy set");
   }

   private static ImmutableList<SpawnResult> runRemotely(
       Spawn spawn,
       ActionExecutionContext actionExecutionContext,
       @Nullable SandboxedSpawnStrategy.StopConcurrentSpawns stopConcurrentSpawns)
       throws ExecException, InterruptedException {
     DynamicStrategyRegistry dynamicStrategyRegistry =
         actionExecutionContext.getContext(DynamicStrategyRegistry.class);

     for (SandboxedSpawnStrategy strategy :
         dynamicStrategyRegistry.getDynamicSpawnActionContexts(
             spawn, DynamicStrategyRegistry.DynamicMode.REMOTE)) {
       if (strategy.canExec(spawn, actionExecutionContext)) {
         return strategy.exec(spawn, actionExecutionContext, stopConcurrentSpawns);
       }
     }
     throw new RuntimeException(
         "executorCreated not yet called or no default dynamic_remote_strategy set");
   }

   /**
    * Wraps the execution of a function that is supposed to execute a spawn via a strategy and only
    * updates the stdout/stderr files if this spawn succeeds.
    */
   private abstract static class Branch implements Callable<ImmutableList<SpawnResult>> {
     private final String name;
     private final ActionExecutionContext context;

     /**
      * Creates a new branch of dynamic execution.
      *
      * @param name a name to describe what this branch represents (e.g. {@code remote}). Used to
      *     qualify temporary files.
      * @param context the action execution context given to the dynamic strategy, used to obtain the
      *     final location of the stdout/stderr
      */
     Branch(String name, ActionExecutionContext context) {
       this.name = name;
       this.context = context;
     }

     /**
      * Moves a set of stdout/stderr files over another one. Errors during the move are logged and
      * swallowed.
      *
      * @param from the source location
      * @param to the target location
      */
     private static void moveFileOutErr(FileOutErr from, FileOutErr to) {
       try {
         if (from.getOutputPath().exists()) {
           Files.move(from.getOutputPath().getPathFile(), to.getOutputPath().getPathFile());
         }
         if (from.getErrorPath().exists()) {
           Files.move(from.getErrorPath().getPathFile(), to.getErrorPath().getPathFile());
         }
       } catch (IOException e) {
         logger.atWarning().withCause(e).log("Could not move action logs from execution");
       }
     }

     /**
      * Hook to execute a spawn using an arbitrary strategy.
      *
      * @param context the action execution context where the spawn can write its stdout/stderr. The
      *     location of these files is specific to this branch.
      * @return the spawn results if execution was successful
      * @throws InterruptedException if the branch was cancelled or an interrupt was caught
      * @throws ExecException if the spawn execution fails
      */
     abstract ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
         throws InterruptedException, ExecException;

     /**
      * Executes the {@link #callImpl} hook and handles stdout/stderr.
      *
      * @return the spawn results if execution was successful
      * @throws InterruptedException if the branch was cancelled or an interrupt was caught
      * @throws ExecException if the spawn execution fails
      */
     @Override
     public final ImmutableList<SpawnResult> call() throws InterruptedException, ExecException {
       FileOutErr fileOutErr = getSuffixedFileOutErr(context.getFileOutErr(), "." + name);

       ImmutableList<SpawnResult> results = null;
       ExecException exception = null;
       try {
         results = callImpl(context.withFileOutErr(fileOutErr));
       } catch (ExecException e) {
         exception = e;
       } finally {
         try {
           fileOutErr.close();
         } catch (IOException ignored) {
           // Nothing we can do here.
         }
       }

       moveFileOutErr(fileOutErr, context.getFileOutErr());

       if (exception != null) {
         throw exception;
       } else {
         checkNotNull(results);
         return results;
       }
     }
   }
 }
	// Copyright 2018 The Bazel Authors. All rights reserved.
	//
	// 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 com.google.devtools.build.lib.dynamic;

	import static com.google.common.base.Preconditions.checkNotNull;
	import static com.google.common.base.Preconditions.checkState;

	import com.google.common.base.Throwables;
	import com.google.common.collect.ImmutableList;
	import com.google.common.flogger.GoogleLogger;
	import com.google.common.io.Files;
	import com.google.common.util.concurrent.ListenableFuture;
	import com.google.common.util.concurrent.ListeningExecutorService;
	import com.google.common.util.concurrent.MoreExecutors;
	import com.google.common.util.concurrent.SettableFuture;
	import com.google.devtools.build.lib.actions.ActionContext;
	import com.google.devtools.build.lib.actions.ActionExecutionContext;
	import com.google.devtools.build.lib.actions.DynamicStrategyRegistry;
	import com.google.devtools.build.lib.actions.ExecException;
	import com.google.devtools.build.lib.actions.SandboxedSpawnStrategy;
	import com.google.devtools.build.lib.actions.Spawn;
	import com.google.devtools.build.lib.actions.SpawnResult;
	import com.google.devtools.build.lib.actions.SpawnStrategy;
	import com.google.devtools.build.lib.exec.ExecutionPolicy;
	import com.google.devtools.build.lib.util.io.FileOutErr;
	import com.google.devtools.build.lib.vfs.Path;
	import java.io.IOException;
	import java.util.concurrent.Callable;
	import java.util.concurrent.CancellationException;
	import java.util.concurrent.ExecutionException;
	import java.util.concurrent.ExecutorService;
	import java.util.concurrent.Future;
	import java.util.concurrent.Semaphore;
	import java.util.concurrent.atomic.AtomicBoolean;
	import java.util.concurrent.atomic.AtomicReference;
	import java.util.function.Function;
	import javax.annotation.Nullable;

	/**
	* A spawn strategy that speeds up incremental builds while not slowing down full builds.
	*
	* <p>This strategy tries to run spawn actions on the local and remote machine at the same time, and
	* picks the spawn action that completes first. This gives the benefits of remote execution on full
	* builds, and local execution on incremental builds.
	*
	* <p>One might ask, why we don't run spawns on the workstation all the time and just "spill over"
	* actions to remote execution when there are no local resources available. This would work, except
	* that the cost of transferring action inputs and outputs from the local machine to and from remote
	* executors over the network is way too high - there is no point in executing an action locally and
	* save 0.5s of time, when it then takes us 5 seconds to upload the results to remote executors for
	* another action that's scheduled to run there.
	*/
	public class DynamicSpawnStrategy implements SpawnStrategy {

	private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();

	private final ListeningExecutorService executorService;
	private final DynamicExecutionOptions options;
	private final Function<Spawn, ExecutionPolicy> getExecutionPolicy;

	/**
	* Set to true by the first action that completes remotely. Until that happens, all local actions
	* are delayed by the amount given in {@link DynamicExecutionOptions#localExecutionDelay}.
	*
	* <p>This is a rather simple approach to make it possible to score a cache hit on remote
	* execution before even trying to start the action locally. This saves resources that would
	* otherwise be wasted by continuously starting and immediately killing local processes. One
	* possibility for improvement would be to establish a reporting mechanism from strategies back to
	* here, where we delay starting locally until the remote strategy tells us that the action isn't
	* a cache hit.
	*/
	private final AtomicBoolean delayLocalExecution = new AtomicBoolean(false);

	/**
	* Constructs a {@code DynamicSpawnStrategy}.
	*
	* @param executorService an {@link ExecutorService} that will be used to run Spawn actions.
	*/
	public DynamicSpawnStrategy(
	ExecutorService executorService,
	DynamicExecutionOptions options,
	Function<Spawn, ExecutionPolicy> getExecutionPolicy) {
	this.executorService = MoreExecutors.listeningDecorator(executorService);
	this.options = options;
	this.getExecutionPolicy = getExecutionPolicy;
	}

	/**
	* Cancels and waits for a branch (a spawn execution) to terminate.
	*
	* <p>This is intended to be used as the body of the {@link
	* SandboxedSpawnStrategy.StopConcurrentSpawns} lambda passed to the spawn runners. Each strategy
	* may call this at most once.
	*
	* @param branch the future running the spawn
	* @param branchDone semaphore that is expected to receive a permit once the future terminates
	* (after {@link InterruptedException} bubbles up through its call stack)
	* @param cancellingStrategy identifier of the strategy that is performing the cancellation. Used
	* to prevent cross-cancellations and to sanity-check that the same strategy doesn't issue the
	* cancellation twice.
	* @param strategyThatCancelled name of the first strategy that executed this method, or a null
	* reference if this is the first time this method is called. If not null, we expect the value
	* referenced by this to be different than {@code cancellingStrategy}, or else we have a bug.
	* @throws InterruptedException if we get interrupted for any reason trying to cancel the future
	* @throws DynamicInterruptedException if we lost a race against another strategy trying to cancel
	* us
	*/
	private static void stopBranch(
	Future<ImmutableList<SpawnResult>> branch,
	Semaphore branchDone,
	String cancellingStrategy,
	AtomicReference<String> strategyThatCancelled)
	throws InterruptedException {
	// This multi-step, unlocked access to "strategyThatCancelled" is valid because, for a given
	// value of "cancellingStrategy", we do not expect concurrent calls to this method. (If there
	// are, we are in big trouble.)
	String current = strategyThatCancelled.get();
	if (current != null && current.equals(cancellingStrategy)) {
	throw new AssertionError("stopBranch called more than once by " + cancellingStrategy);
	} else {
	// Protect against the two branches from cancelling each other. The first branch to set the
	// reference to its own identifier wins and is allowed to issue the cancellation; the other
	// branch just has to give up execution.
	if (strategyThatCancelled.compareAndSet(null, cancellingStrategy)) {
	boolean cancelled = branch.cancel(true);
	checkState(cancelled, "Failed to cancel other branch from %s", cancellingStrategy);
	branchDone.acquire();
	} else {
	throw new DynamicInterruptedException(
	"Execution stopped because other strategy finished first");
	}
	}
	}

	/**
	* Waits for a branch (a spawn execution) to complete.
	*
	* @param branch the future running the spawn
	* @return the spawn result if the execution terminated successfully, or null if the branch was
	* cancelled
	* @throws ExecException the execution error of the spawn if it failed
	* @throws InterruptedException if we get interrupted while waiting for completion
	*/
	@Nullable
	private static ImmutableList<SpawnResult> waitBranch(Future<ImmutableList<SpawnResult>> branch)
	throws ExecException, InterruptedException {
	try {
	return branch.get();
	} catch (CancellationException e) {
	return null;
	} catch (ExecutionException e) {
	Throwable cause = e.getCause();
	if (cause instanceof ExecException) {
	throw (ExecException) cause;
	} else if (cause instanceof InterruptedException) {
	// If the branch was interrupted, it might be due to a user interrupt or due to our request
	// for cancellation. Assume the latter here because if this was actually a user interrupt,
	// our own get() would have been interrupted as well. It makes no sense to propagate the
	// interrupt status across threads.
	return null;
	} else {
	// Even though we cannot enforce this in the future's signature (but we do in Branch#call),
	// we only expect the exception types we validated above. Still, unchecked exceptions could
	// propagate, so just let them bubble up.
	Throwables.throwIfUnchecked(cause);
	throw new AssertionError("Unexpected exception type from strategy.exec()");
	}
	} catch (InterruptedException e) {
	branch.cancel(true);
	throw e;
	}
	}

	/**
	* Waits for the two branches of a spawn's execution to complete.
	*
	* <p>This guarantees that the two branches are stopped both on successful termination and on an
	* exception.
	*
	* @param branch1 the future running one side of the spawn (e.g. local). This future must cancel
	* {@code branch2} at some point during its successful execution to guarantee termination. If
	* we encounter an execution error, or if we are interrupted, then we handle such cancellation
	* here.
	* @param branch2 the future running the other side of the spawn (e.g. remote). Same restrictions
	* apply as in {@code branch1}, but in the symmetric direction.
	* @return the result of the branch that terminates first
	* @throws ExecException the execution error of the spawn that terminated first
	* @throws InterruptedException if we get interrupted while waiting for completion
	*/
	private static ImmutableList<SpawnResult> waitBranches(
	Future<ImmutableList<SpawnResult>> branch1, Future<ImmutableList<SpawnResult>> branch2)
	throws ExecException, InterruptedException {
	ImmutableList<SpawnResult> result1;
	try {
	result1 = waitBranch(branch1);
	} catch (ExecException \| InterruptedException \| RuntimeException e) {
	branch2.cancel(true);
	throw e;
	}

	ImmutableList<SpawnResult> result2 = waitBranch(branch2);

	if (result2 != null && result1 != null) {
	throw new AssertionError("One branch did not cancel the other one");
	} else if (result2 != null) {
	return result2;
	} else if (result1 != null) {
	return result1;
	} else {
	throw new AssertionError("No branch completed, which might mean they cancelled each other");
	}
	}

	@Override
	public ImmutableList<SpawnResult> exec(
	final Spawn spawn, final ActionExecutionContext actionExecutionContext)
	throws ExecException, InterruptedException {
	ExecutionPolicy executionPolicy = getExecutionPolicy.apply(spawn);
	if (executionPolicy.canRunLocallyOnly()) {
	return runLocally(spawn, actionExecutionContext, null);
	}
	if (executionPolicy.canRunRemotelyOnly()) {
	return runRemotely(spawn, actionExecutionContext, null);
	}

	// Semaphores to track termination of each branch. These are necessary to wait for the branch to
	// finish its own cleanup (e.g. terminating subprocesses) once it has been cancelled.
	Semaphore localDone = new Semaphore(0);
	Semaphore remoteDone = new Semaphore(0);

	AtomicReference<String> strategyThatCancelled = new AtomicReference<>(null);
	SettableFuture<ImmutableList<SpawnResult>> remoteBranch = SettableFuture.create();

	AtomicBoolean localCanReportDone = new AtomicBoolean(false);
	AtomicBoolean remoteCanReportDone = new AtomicBoolean(false);

	ListenableFuture<ImmutableList<SpawnResult>> localBranch =
	executorService.submit(
	new Branch("local", actionExecutionContext) {
	@Override
	ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
	throws InterruptedException, ExecException {
	try {
	if (!localCanReportDone.compareAndSet(false, true)) {
	// If we ever get here, it's because we were cancelled and the listener ran
	// first. Just make sure that's the case.
	checkState(Thread.interrupted());
	throw new InterruptedException();
	}
	if (delayLocalExecution.get()) {
	Thread.sleep(options.localExecutionDelay);
	}
	return runLocally(
	spawn,
	context,
	() -> stopBranch(remoteBranch, remoteDone, "local", strategyThatCancelled));
	} finally {
	localDone.release();
	}
	}
	});
	localBranch.addListener(
	() -> {
	if (localCanReportDone.compareAndSet(false, true)) {
	localDone.release();
	}
	if (!localBranch.isCancelled()) {
	remoteBranch.cancel(true);
	}
	},
	MoreExecutors.directExecutor());

	remoteBranch.setFuture(
	executorService.submit(
	new Branch("remote", actionExecutionContext) {
	@Override
	public ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
	throws InterruptedException, ExecException {
	try {
	if (!remoteCanReportDone.compareAndSet(false, true)) {
	// If we ever get here, it's because we were cancelled and the listener ran
	// first. Just make sure that's the case.
	checkState(Thread.interrupted());
	throw new InterruptedException();
	}
	ImmutableList<SpawnResult> spawnResults =
	runRemotely(
	spawn,
	context,
	() ->
	stopBranch(localBranch, localDone, "remote", strategyThatCancelled));
	delayLocalExecution.set(true);
	return spawnResults;
	} finally {
	remoteDone.release();
	}
	}
	}));
	remoteBranch.addListener(
	() -> {
	if (remoteCanReportDone.compareAndSet(false, true)) {
	remoteDone.release();
	}
	if (!remoteBranch.isCancelled()) {
	localBranch.cancel(true);
	}
	},
	MoreExecutors.directExecutor());

	try {
	return waitBranches(localBranch, remoteBranch);
	} finally {
	checkState(localBranch.isDone());
	checkState(remoteBranch.isDone());
	}
	}

	@Override
	public boolean canExec(Spawn spawn, ActionContext.ActionContextRegistry actionContextRegistry) {
	DynamicStrategyRegistry dynamicStrategyRegistry =
	actionContextRegistry.getContext(DynamicStrategyRegistry.class);
	for (SandboxedSpawnStrategy strategy :
	dynamicStrategyRegistry.getDynamicSpawnActionContexts(
	spawn, DynamicStrategyRegistry.DynamicMode.LOCAL)) {
	if (strategy.canExec(spawn, actionContextRegistry)) {
	return true;
	}
	}
	for (SandboxedSpawnStrategy strategy :
	dynamicStrategyRegistry.getDynamicSpawnActionContexts(
	spawn, DynamicStrategyRegistry.DynamicMode.REMOTE)) {
	if (strategy.canExec(spawn, actionContextRegistry)) {
	return true;
	}
	}
	return false;
	}

	@Override
	public void usedContext(ActionContext.ActionContextRegistry actionContextRegistry) {
	actionContextRegistry
	.getContext(DynamicStrategyRegistry.class)
	.notifyUsedDynamic(actionContextRegistry);
	}

	private static FileOutErr getSuffixedFileOutErr(FileOutErr fileOutErr, String suffix) {
	Path outDir = checkNotNull(fileOutErr.getOutputPath().getParentDirectory());
	String outBaseName = fileOutErr.getOutputPath().getBaseName();
	Path errDir = checkNotNull(fileOutErr.getErrorPath().getParentDirectory());
	String errBaseName = fileOutErr.getErrorPath().getBaseName();
	return new FileOutErr(
	outDir.getChild(outBaseName + suffix), errDir.getChild(errBaseName + suffix));
	}

	private static ImmutableList<SpawnResult> runLocally(
	Spawn spawn,
	ActionExecutionContext actionExecutionContext,
	@Nullable SandboxedSpawnStrategy.StopConcurrentSpawns stopConcurrentSpawns)
	throws ExecException, InterruptedException {
	DynamicStrategyRegistry dynamicStrategyRegistry =
	actionExecutionContext.getContext(DynamicStrategyRegistry.class);

	for (SandboxedSpawnStrategy strategy :
	dynamicStrategyRegistry.getDynamicSpawnActionContexts(
	spawn, DynamicStrategyRegistry.DynamicMode.LOCAL)) {
	if (strategy.canExec(spawn, actionExecutionContext)) {
	return strategy.exec(spawn, actionExecutionContext, stopConcurrentSpawns);
	}
	}
	throw new RuntimeException(
	"executorCreated not yet called or no default dynamic_local_strategy set");
	}

	private static ImmutableList<SpawnResult> runRemotely(
	Spawn spawn,
	ActionExecutionContext actionExecutionContext,
	@Nullable SandboxedSpawnStrategy.StopConcurrentSpawns stopConcurrentSpawns)
	throws ExecException, InterruptedException {
	DynamicStrategyRegistry dynamicStrategyRegistry =
	actionExecutionContext.getContext(DynamicStrategyRegistry.class);

	for (SandboxedSpawnStrategy strategy :
	dynamicStrategyRegistry.getDynamicSpawnActionContexts(
	spawn, DynamicStrategyRegistry.DynamicMode.REMOTE)) {
	if (strategy.canExec(spawn, actionExecutionContext)) {
	return strategy.exec(spawn, actionExecutionContext, stopConcurrentSpawns);
	}
	}
	throw new RuntimeException(
	"executorCreated not yet called or no default dynamic_remote_strategy set");
	}

	/**
	* Wraps the execution of a function that is supposed to execute a spawn via a strategy and only
	* updates the stdout/stderr files if this spawn succeeds.
	*/
	private abstract static class Branch implements Callable<ImmutableList<SpawnResult>> {
	private final String name;
	private final ActionExecutionContext context;

	/**
	* Creates a new branch of dynamic execution.
	*
	* @param name a name to describe what this branch represents (e.g. {@code remote}). Used to
	* qualify temporary files.
	* @param context the action execution context given to the dynamic strategy, used to obtain the
	* final location of the stdout/stderr
	*/
	Branch(String name, ActionExecutionContext context) {
	this.name = name;
	this.context = context;
	}

	/**
	* Moves a set of stdout/stderr files over another one. Errors during the move are logged and
	* swallowed.
	*
	* @param from the source location
	* @param to the target location
	*/
	private static void moveFileOutErr(FileOutErr from, FileOutErr to) {
	try {
	if (from.getOutputPath().exists()) {
	Files.move(from.getOutputPath().getPathFile(), to.getOutputPath().getPathFile());
	}
	if (from.getErrorPath().exists()) {
	Files.move(from.getErrorPath().getPathFile(), to.getErrorPath().getPathFile());
	}
	} catch (IOException e) {
	logger.atWarning().withCause(e).log("Could not move action logs from execution");
	}
	}

	/**
	* Hook to execute a spawn using an arbitrary strategy.
	*
	* @param context the action execution context where the spawn can write its stdout/stderr. The
	* location of these files is specific to this branch.
	* @return the spawn results if execution was successful
	* @throws InterruptedException if the branch was cancelled or an interrupt was caught
	* @throws ExecException if the spawn execution fails
	*/
	abstract ImmutableList<SpawnResult> callImpl(ActionExecutionContext context)
	throws InterruptedException, ExecException;

	/**
	* Executes the {@link #callImpl} hook and handles stdout/stderr.
	*
	* @return the spawn results if execution was successful
	* @throws InterruptedException if the branch was cancelled or an interrupt was caught
	* @throws ExecException if the spawn execution fails
	*/
	@Override
	public final ImmutableList<SpawnResult> call() throws InterruptedException, ExecException {
	FileOutErr fileOutErr = getSuffixedFileOutErr(context.getFileOutErr(), "." + name);

	ImmutableList<SpawnResult> results = null;
	ExecException exception = null;
	try {
	results = callImpl(context.withFileOutErr(fileOutErr));
	} catch (ExecException e) {
	exception = e;
	} finally {
	try {
	fileOutErr.close();
	} catch (IOException ignored) {
	// Nothing we can do here.
	}
	}

	moveFileOutErr(fileOutErr, context.getFileOutErr());

	if (exception != null) {
	throw exception;
	} else {
	checkNotNull(results);
	return results;
	}
	}
	}
	}