sandbox: Switch Darwin sandbox to the SymlinkedExecRoot strategy.

Hardlinks are problematic due to not working across filesystem
boundaries and causing Bazel to do lots of I/O because it has to create
a hardlink and a symlink for each input file.

This improves performance of Bazel building itself by 10% on my system.

Change-Id: I8acb77053de875160a046e38624735ed18375bed
PiperOrigin-RevId: 155493583

2 files changed, 3 insertions, 172 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/sandbox/DarwinSandboxedStrategy.java b/src/main/java/com/google/devtools/build/lib/sandbox/DarwinSandboxedStrategy.java
index 6bd782a047..31fa4562e4 100644
--- a/src/main/java/com/google/devtools/build/lib/sandbox/DarwinSandboxedStrategy.java
+++ b/src/main/java/com/google/devtools/build/lib/sandbox/DarwinSandboxedStrategy.java
@@ -186,10 +186,9 @@ public class DarwinSandboxedStrategy extends SandboxStrategy {
     HashSet<Path> writableDirs = new HashSet<>(alwaysWritableDirs);
     writableDirs.addAll(getWritableDirs(sandboxExecRoot, spawnEnvironment));
 
-    HardlinkedExecRoot hardlinkedExecRoot =
-        new HardlinkedExecRoot(execRoot, sandboxPath, sandboxExecRoot);
+    SymlinkedExecRoot symlinkedExecRoot = new SymlinkedExecRoot(sandboxExecRoot);
     ImmutableSet<PathFragment> outputs = SandboxHelpers.getOutputFiles(spawn);
-    hardlinkedExecRoot.createFileSystem(
+    symlinkedExecRoot.createFileSystem(
         getMounts(spawn, actionExecutionContext), outputs, writableDirs);
 
     DarwinSandboxRunner runner =
@@ -199,7 +198,7 @@ public class DarwinSandboxedStrategy extends SandboxStrategy {
           spawn,
           actionExecutionContext,
           spawnEnvironment,
-          hardlinkedExecRoot,
+          symlinkedExecRoot,
           outputs,
           runner,
           writeOutputFiles);
diff --git a/src/main/java/com/google/devtools/build/lib/sandbox/HardlinkedExecRoot.java b/src/main/java/com/google/devtools/build/lib/sandbox/HardlinkedExecRoot.java
deleted file mode 100644
index 4f5c8079e8..0000000000
--- a/src/main/java/com/google/devtools/build/lib/sandbox/HardlinkedExecRoot.java
+++ /dev/null
@@ -1,168 +0,0 @@
-// Copyright 2016 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.collect.ImmutableMap;
-import com.google.common.io.Files;
-import com.google.devtools.build.lib.vfs.FileSystemUtils;
-import com.google.devtools.build.lib.vfs.Path;
-import com.google.devtools.build.lib.vfs.PathFragment;
-import java.io.IOException;
-import java.util.Collection;
-import java.util.HashSet;
-import java.util.Map;
-import java.util.Set;
-
-/**
- * Creates an execRoot for a Spawn that contains input files as symlinks to hardlinks of the
- * original input files.
- */
-public class HardlinkedExecRoot implements SandboxExecRoot {
-
-  private final Path execRoot;
-  private final Path sandboxPath;
-  private final Path sandboxExecRoot;
-
-  public HardlinkedExecRoot(Path execRoot, Path sandboxPath, Path sandboxExecRoot) {
-    this.execRoot = execRoot;
-    this.sandboxPath = sandboxPath;
-    this.sandboxExecRoot = sandboxExecRoot;
-  }
-
-  @Override
-  public void createFileSystem(
-      Map<PathFragment, Path> inputs, Collection<PathFragment> outputs, Set<Path> writableDirs)
-      throws IOException {
-    Set<Path> createdDirs = new HashSet<>();
-    FileSystemUtils.createDirectoryAndParentsWithCache(createdDirs, sandboxExecRoot);
-    createDirectoriesForOutputs(outputs, createdDirs);
-
-    // Create all needed directories.
-    for (Path createDir : writableDirs) {
-      FileSystemUtils.createDirectoryAndParentsWithCache(createdDirs, createDir);
-    }
-
-    // Link all the inputs.
-    linkInputs(inputs, createdDirs);
-  }
-
-  private void createDirectoriesForOutputs(Collection<PathFragment> outputs, Set<Path> createdDirs)
-      throws IOException {
-    // Prepare the output directories in the sandbox.
-    for (PathFragment output : outputs) {
-      FileSystemUtils.createDirectoryAndParentsWithCache(
-          createdDirs, sandboxExecRoot.getRelative(output.getParentDirectory()));
-    }
-  }
-
-  /**
-   * Make all specified inputs available in the sandbox.
-   *
-   * <p>We want the sandboxed process to have access only to these input files and not anything else
-   * from the workspace. Furthermore, the process should not be able to modify these input files. We
-   * achieve this by hardlinking all input files into a temporary "inputs" directory, then
-   * symlinking them into their correct place inside the sandbox.
-   *
-   * <p>The hardlinks / symlinks combination (as opposed to simply directly hardlinking to the final
-   * destination) is necessary, because we build a solib symlink tree for shared libraries where the
-   * original file and the created symlink have two different file names (libblaze_util.so vs.
-   * src_Stest_Scpp_Sblaze_Uutil_Utest.so) and our cc_wrapper.sh needs to be able to figure out both
-   * names (by following solib symlinks back) to modify the paths to the shared libraries in
-   * cc_binaries.
-   */
-  private void linkInputs(Map<PathFragment, Path> inputs, Set<Path> createdDirs)
-      throws IOException {
-    // Create directory for input files.
-    Path inputsDir = sandboxPath.getRelative("inputs");
-    if (!inputsDir.exists()) {
-      inputsDir.createDirectory();
-    }
-
-    for (ImmutableMap.Entry<PathFragment, Path> entry : inputs.entrySet()) {
-      Path targetName = sandboxExecRoot.getRelative(entry.getKey());
-      FileSystemUtils.createDirectoryAndParentsWithCache(
-          createdDirs, targetName.getParentDirectory());
-
-      // The target is supposed to be an empty file.
-      if (entry.getValue() == null) {
-        FileSystemUtils.createEmptyFile(targetName);
-        continue;
-      }
-
-      // Hardlink, resolve symlink here instead in finalizeLinks.
-      Path target = entry.getValue().resolveSymbolicLinks();
-      Path hardlinkName =
-          target.startsWith(execRoot)
-              ? inputsDir.getRelative(target.relativeTo(execRoot))
-              : inputsDir.getRelative(entry.getKey());
-      try {
-        createHardLink(hardlinkName, target);
-      } catch (IOException e) {
-        // Creating a hardlink might fail when the input file and the sandbox directory are not on
-        // the same filesystem / device. Then we use symlink instead.
-        hardlinkName.createSymbolicLink(target);
-      }
-
-      // symlink
-      targetName.createSymbolicLink(hardlinkName);
-    }
-  }
-
-  private void createHardLink(Path target, Path source) throws IOException {
-    java.nio.file.Path targetNio = java.nio.file.Paths.get(target.toString());
-    java.nio.file.Path sourceNio = java.nio.file.Paths.get(source.toString());
-
-    if (!source.exists() || target.exists()) {
-      return;
-    }
-    // Regular file
-    if (source.isFile()) {
-      Path parentDir = target.getParentDirectory();
-      if (!parentDir.exists()) {
-        FileSystemUtils.createDirectoryAndParents(parentDir);
-      }
-      java.nio.file.Files.createLink(targetNio, sourceNio);
-      // Directory
-    } else if (source.isDirectory()) {
-      // Eagerly create target directory in case source is an empty directory.
-      FileSystemUtils.createDirectoryAndParents(target);
-
-      Collection<Path> subpaths = source.getDirectoryEntries();
-      for (Path sourceSubpath : subpaths) {
-        Path targetSubpath = target.getRelative(sourceSubpath.relativeTo(source));
-        createHardLink(targetSubpath, sourceSubpath);
-      }
-    }
-  }
-
-  @Override
-  public void copyOutputs(Path execRoot, Collection<PathFragment> outputs) throws IOException {
-    for (PathFragment output : outputs) {
-      Path source = sandboxExecRoot.getRelative(output);
-      Path target = execRoot.getRelative(output);
-      if (source.isFile() || source.isSymbolicLink()) {
-        Files.move(source.getPathFile(), target.getPathFile());
-      } else if (source.isDirectory()) {
-        try {
-          source.renameTo(target);
-        } catch (IOException e) {
-          // Failed to move directory directly, thus move it recursively.
-          target.createDirectory();
-          FileSystemUtils.moveTreesBelow(source, target);
-        }
-      }
-    }
-  }
-}