src/main/java/com/google/devtools/build/lib/sandbox/SandboxStash.java

// Copyright 2023 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.sandbox;

import com.google.common.flogger.GoogleLogger;
import com.google.devtools.build.lib.vfs.Path;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Collection;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import javax.annotation.Nullable;

/**
 * Singleton class for the `--reuse_sandbox_directories` flag: Controls a "stash" of old sandbox
 * directories. When a sandboxed runner needs its directory tree, it first tries to grab a stash by
 * just moving it. They are separated by mnemonic because that makes them much more likely to be
 * able to reuse things common for that mnemonic, e.g. standard libraries.
 */
public class SandboxStash {
  private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();

  /** An incrementing count of stashes to avoid filename clashes. */
  static AtomicInteger stash = new AtomicInteger(0);

  /** If true, we have already warned about an error causing us to turn off reuse. */
  private final AtomicBoolean warnedAboutTurningOffReuse = new AtomicBoolean();

  /**
   * Whether to attempt to reuse previously-created sandboxes. Not final because we may turn it off
   * in case of errors.
   */
  static boolean reuseSandboxDirectories;

  private static SandboxStash instance;
  private final String workspaceName;
  private final Path sandboxBase;

  public SandboxStash(String workspaceName, Path sandboxBase) {
    this.workspaceName = workspaceName;
    this.sandboxBase = sandboxBase;
  }

  static boolean takeStashedSandbox(Path sandboxPath, String mnemonic) {
    if (instance == null) {
      return false;
    }
    return instance.takeStashedSandboxInternal(sandboxPath, mnemonic);
  }

  private boolean takeStashedSandboxInternal(Path sandboxPath, String mnemonic) {
    try {
      Path sandboxes = getSandboxStashDir(mnemonic);
      if (sandboxes == null) {
        return false;
      }
      Collection<Path> stashes = sandboxes.getDirectoryEntries();
      // We have to remove the sandbox root to move a stash there, but it is currently empty
      // and we reinstate it if we don't get a sandbox.
      sandboxPath.deleteTree();
      for (Path stash : stashes) {
        try {
          stash.renameTo(sandboxPath);
          return true;
        } catch (FileNotFoundException e) {
          // Try the next one, somebody else took this one.
        } catch (IOException e) {
          turnOffReuse("Error renaming sandbox stash %s to %s: %s\n", stash, sandboxPath, e);
          return false;
        }
      }
      return false;
    } catch (IOException e) {
      turnOffReuse("Failed to prepare for reusing stashed sandbox for %s: %s", sandboxPath, e);
      return false;
    }
  }

  /** Atomically moves the sandboxPath directory aside for later reuse. */
  static boolean stashSandbox(Path path, String mnemonic) {
    if (instance == null) {
      return false;
    }
    return instance.stashSandboxInternal(path, mnemonic);
  }

  private boolean stashSandboxInternal(Path path, String mnemonic) {
    Path sandboxes = getSandboxStashDir(mnemonic);
    if (sandboxes == null) {
      return false;
    }
    String stashName;
    synchronized (stash) {
      stashName = Integer.toString(stash.incrementAndGet());
    }
    Path stashPath = sandboxes.getChild(stashName);
    if (!path.exists()) {
      return false;
    }
    try {
      path.renameTo(stashPath);
    } catch (IOException e) {
      // Since stash names are unique, this IOException indicates some other problem with stashing,
      // so we turn it off.
      turnOffReuse("Error stashing sandbox at %s: %s", stashPath, e);
      return false;
    }
    return true;
  }

  /**
   * Returns the sandbox stashing directory appropriate for this mnemonic. In order to maximize
   * reuse, we keep stashed sandboxes separated by mnemonic. May return null if there are errors, in
   * which case sandbox reuse also gets turned off.
   */
  @Nullable
  private Path getSandboxStashDir(String mnemonic) {
    Path stashDir = getStashBase();
    try {
      stashDir.createDirectory();
      if (!maybeClearExistingStash(stashDir)) {
        return null;
      }
    } catch (IOException e) {
      turnOffReuse(
          "Error creating sandbox stash dir %s, disabling sandbox reuse: %s\n",
          stashDir, e.getMessage());
      return null;
    }
    Path mnemonicStashDir = stashDir.getChild(mnemonic);
    try {
      mnemonicStashDir.createDirectory();
      return mnemonicStashDir;
    } catch (IOException e) {
      turnOffReuse("Error creating mnemonic stash dir %s: %s\n", mnemonicStashDir, e.getMessage());
      return null;
    }
  }

  private Path getStashBase() {
    return this.sandboxBase.getChild("sandbox_stash");
  }

  /**
   * Clears away existing stash if this is the first access to the stash in this Blaze server
   * instance.
   *
   * @param stashPath Path of the stashes.
   * @return True unless there was an error deleting sandbox stashes.
   */
  private boolean maybeClearExistingStash(Path stashPath) {
    synchronized (stash) {
      if (stash.getAndIncrement() == 0) {
        try {
          for (Path directoryEntry : stashPath.getDirectoryEntries()) {
            directoryEntry.deleteTree();
          }
        } catch (IOException e) {
          turnOffReuse("Unable to clear old sandbox stash %s: %s\n", stashPath, e.getMessage());
          return false;
        }
      }
    }
    return true;
  }

  private void turnOffReuse(String fmt, Object... args) {
    reuseSandboxDirectories = false;
    if (warnedAboutTurningOffReuse.compareAndSet(false, true)) {
      logger.atWarning().logVarargs("Turning off sandbox reuse: " + fmt, args);
    }
  }

  public static void initialize(String workspaceName, Path sandboxBase, SandboxOptions options) {
    if (options.reuseSandboxDirectories) {
      if (instance == null) {
        instance = new SandboxStash(workspaceName, sandboxBase);
      } else if (!Objects.equals(workspaceName, instance.workspaceName)) {
        Path stashBase = instance.getStashBase();
        try {
          for (Path directoryEntry : stashBase.getDirectoryEntries()) {
            directoryEntry.deleteTree();
          }
        } catch (IOException e) {
          instance.turnOffReuse(
              "Unable to clear old sandbox stash %s: %s\n", stashBase, e.getMessage());
        }
        instance = new SandboxStash(workspaceName, sandboxBase);
      }
    } else {
      instance = null;
    }
  }
}