UnixFileSystem: read cached hashes from extended attributes

There are certain workloads where Bazel's running time gets dominated by
checksum computation. Examples include:

- People adding local_repository()s to their project that point to
  networked file shares.
- The use of repositories that contain very large input files.

When using remote execution, we need to compute digests to be able to
place such files in input roots. In many cases, a centralized CAS will
already contain these files. It would be nice if Bazel could efficiently
check for existence of such objects without needing to scan the file
locally.

This change extends UnixFileSystem to call getxattr() on an attribute
prior to falling back to reading file contents. The name of the extended
attribute that is used is configurable through a command line flag.

Using extended attributes to store this information also seems to be a
fairly common approach. Apparently it is also used within Google itself:

https://groups.google.com/g/bazel-discuss/c/6VmjSOLySnY/m/v2dpwt8jBgAJ

So far no code has been added to let Bazel write these attributes to
disk. The main goal so far is to speed up access to read-only corpora,
where the maintainers have spent the effort adding these attributes.

src/main/cpp/blaze.cc

@@ -485,6 +485,9 @@ static vector<string> GetServerExeArgs(const blaze_util::Path &jvm_path,
     // being "null" to set the programmatic default in the server.
     result.push_back("--digest_function=" + startup_options.digest_function);
   }
+  if (!startup_options.unix_digest_hash_attribute_name.empty()) {
+    result.push_back("--unix_digest_hash_attribute_name=" + startup_options.unix_digest_hash_attribute_name);
+  }
   if (startup_options.idle_server_tasks) {
     result.push_back("--idle_server_tasks");
   } else {

src/main/cpp/startup_options.cc

@@ -150,6 +150,7 @@ StartupOptions::StartupOptions(const string &product_name,
   RegisterUnaryStartupFlag("command_port");
   RegisterUnaryStartupFlag("connect_timeout_secs");
   RegisterUnaryStartupFlag("digest_function");
+  RegisterUnaryStartupFlag("unix_digest_hash_attribute_name");
   RegisterUnaryStartupFlag("server_javabase");
   RegisterUnaryStartupFlag("host_jvm_args");
   RegisterUnaryStartupFlag("host_jvm_profile");
@@ -359,6 +360,10 @@ blaze_exit_code::ExitCode StartupOptions::ProcessArg(
              NULL) {
     digest_function = value;
     option_sources["digest_function"] = rcfile;
+  } else if ((value = GetUnaryOption(arg, next_arg, "--unix_digest_hash_attribute_name")) !=
+             NULL) {
+    unix_digest_hash_attribute_name = value;
+    option_sources["unix_digest_hash_attribute_name"] = rcfile;
   } else if ((value = GetUnaryOption(arg, next_arg, "--command_port")) !=
              NULL) {
     if (!blaze_util::safe_strto32(value, &command_port) ||

src/main/cpp/startup_options.h

@@ -254,6 +254,8 @@ class StartupOptions {
   // The hash function to use when computing file digests.
   std::string digest_function;
 
+  std::string unix_digest_hash_attribute_name;
+
   bool idle_server_tasks;
 
   // The startup options as received from the user and rc files, tagged with

src/main/java/com/google/devtools/build/lib/runtime/BazelFileSystemModule.java

@@ -90,6 +90,7 @@ public ModuleFileSystem getFileSystem(
       throws DefaultHashFunctionNotSetException {
     BlazeServerStartupOptions options = startupOptions.getOptions(BlazeServerStartupOptions.class);
     boolean enableSymLinks = options != null && options.enableWindowsSymlinks;
+    String unixDigestHashAttributeName = options != null ? options.unixDigestHashAttributeName : "";
     if ("0".equals(System.getProperty("io.bazel.EnableJni"))) {
       // Ignore UnixFileSystem, to be used for bootstrapping.
       return ModuleFileSystem.create(
@@ -101,6 +102,6 @@ public ModuleFileSystem getFileSystem(
     return ModuleFileSystem.create(
         OS.getCurrent() == OS.WINDOWS
             ? new WindowsFileSystem(enableSymLinks)
-            : new UnixFileSystem());
+            : new UnixFileSystem(unixDigestHashAttributeName));
   }
 }

src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java

@@ -1130,7 +1130,7 @@ private static FileSystem defaultFileSystemImplementation(
     // The JNI-based UnixFileSystem is faster, but on Windows it is not available.
     return OS.getCurrent() == OS.WINDOWS
         ? new WindowsFileSystem(startupOptions.enableWindowsSymlinks)
-        : new UnixFileSystem();
+        : new UnixFileSystem(startupOptions.unixDigestHashAttributeName);
   }
 
   private static SubprocessFactory subprocessFactoryImplementation() {

src/main/java/com/google/devtools/build/lib/runtime/BlazeServerStartupOptions.java

@@ -482,4 +482,17 @@ public String getTypeDescription() {
               + "Requires Windows developer mode to be enabled and Windows 10 version 1703 or "
               + "greater.")
   public boolean enableWindowsSymlinks;
+
+  @Option(
+      name = "unix_digest_hash_attribute_name",
+      defaultValue = "",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {OptionEffectTag.CHANGES_INPUTS, OptionEffectTag.LOSES_INCREMENTAL_STATE},
+      help =
+          "The name of an extended attribute that can be placed on files to store a precomputed "
+              + "copy of the file's hash, corresponding with --digest_function. This option "
+              + "can be used to reduce disk I/O and CPU load caused by hash computation. This "
+              + "extended attribute is checked on all source files and output files, meaning "
+              + "that it causes a significant number of invocations of the getxattr() system call.")
+  public String unixDigestHashAttributeName;
 }

src/main/java/com/google/devtools/build/lib/unix/UnixFileSystem.java

@@ -46,10 +46,15 @@
 @ThreadSafe
 public class UnixFileSystem extends AbstractFileSystemWithCustomStat {
 
-  public UnixFileSystem() throws DefaultHashFunctionNotSetException {}
+  private final String hashAttributeName;
 
-  public UnixFileSystem(DigestHashFunction hashFunction) {
+  public UnixFileSystem(String hashAttributeName) throws DefaultHashFunctionNotSetException {
+    this.hashAttributeName = hashAttributeName;
+  }
+
+  public UnixFileSystem(DigestHashFunction hashFunction, String hashAttributeName) {
     super(hashFunction);
+    this.hashAttributeName = hashAttributeName;
   }
 
   /**
@@ -407,6 +412,13 @@ public byte[] getxattr(Path path, String name, boolean followSymlinks) throws IO
     }
   }
 
+  @Override
+  protected byte[] getFastDigest(Path path) throws IOException {
+    // Attempt to obtain the digest from an extended attribute attached to the file. This prevents
+    // the checksum from being recomputed unnecessarily.
+    return hashAttributeName.isEmpty() ? null : getxattr(path, hashAttributeName, true);
+  }
+
   @Override
   protected byte[] getDigest(Path path) throws IOException {
     String name = path.toString();

src/main/java/com/google/devtools/build/lib/vfs/Path.java

@@ -766,7 +766,9 @@ public byte[] getFastDigest() throws IOException {
   }
 
   /**
-   * Returns the digest of the file denoted by the current path, following symbolic links.
+   * Returns the digest of the file denoted by the current path, following symbolic links. This
+   * function should not be called directly. Use
+   * {@link com.google.devtools.build.lib.actions.cache.DigestUtils} instead.
    *
    * @return a new byte array containing the file's digest
    * @throws IOException if the digest could not be computed for any reason

src/test/java/com/google/devtools/build/lib/BUILD

@@ -197,12 +197,15 @@ java_test(
     test_class = "com.google.devtools.build.lib.AllTests",
     deps = [
         ":AllTests",
+        "//src/main/java/com/google/devtools/build/lib/actions",
+        "//src/main/java/com/google/devtools/build/lib/actions:file_metadata",
         "//src/main/java/com/google/devtools/build/lib/unix",
         "//src/main/java/com/google/devtools/build/lib/util",
         "//src/main/java/com/google/devtools/build/lib/util:os",
         "//src/main/java/com/google/devtools/build/lib/util:string",
         "//src/main/java/com/google/devtools/build/lib/vfs",
         "//src/main/java/com/google/devtools/build/lib/vfs:pathfragment",
+        "//src/test/java/com/google/devtools/build/lib/vfs:testutil",
         "//src/test/java/com/google/devtools/build/lib/events:testutil",
         "//src/test/java/com/google/devtools/build/lib/testutil",
         "//src/test/java/com/google/devtools/build/lib/testutil:JunitUtils",

src/test/java/com/google/devtools/build/lib/bazel/repository/TestArchiveDescriptor.java

@@ -69,7 +69,7 @@ DecompressorDescriptor.Builder createDescriptorBuilder() throws IOException {
     FileSystem testFS =
         OS.getCurrent() == OS.WINDOWS
             ? new JavaIoFileSystem(DigestHashFunction.getDefaultUnchecked())
-            : new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+            : new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
 
     // do not rely on TestConstants.JAVATESTS_ROOT end with slash, but ensure separators
     // are not duplicated

src/test/java/com/google/devtools/build/lib/buildeventstream/LastBuildEventTest.java

@@ -33,7 +33,7 @@ public class LastBuildEventTest {
 
   @Test
   public void testForwardsReferencedLocalFilesCall() {
-    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
     LocalFile localFile = new LocalFile(fs.getPath("/some/file"), LocalFileType.FAILED_TEST_OUTPUT);
     LastBuildEvent event = new LastBuildEvent(new BuildEvent() {
       @Override

src/test/java/com/google/devtools/build/lib/buildtool/InconsistentFilesystemTest.java

@@ -38,7 +38,7 @@ protected boolean realFileSystem() {
 
   @Override
   protected FileSystem createFileSystem() {
-    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked()) {
+    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "") {
       boolean threwException = false;
 
       @Override

src/test/java/com/google/devtools/build/lib/buildtool/IoHookTestCase.java

@@ -63,7 +63,7 @@ protected void setListener(FileListener listener) {
   @Override
   protected FileSystem createFileSystem() {
     setListener(DUMMY_LISTENER);
-    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked()) {
+    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "") {
       @Override
       protected void chmod(Path path, int chmod) throws IOException {
         listener.get().handle(PathOp.CHMOD, path);

src/test/java/com/google/devtools/build/lib/buildtool/ProgressReportingTest.java

@@ -64,7 +64,7 @@ protected boolean realFileSystem() {
 
   @Override
   protected FileSystem createFileSystem() {
-    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked()) {
+    return new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "") {
 
       private void recordAccess(PathOp op, Path path) {
         if (receiver != null) {

src/test/java/com/google/devtools/build/lib/exec/local/LocalSpawnRunnerTest.java

@@ -882,7 +882,7 @@ public void hasExecutionStatistics_whenOptionIsEnabled() throws Exception {
     // TODO(b/62588075) Currently no process-wrapper or execution statistics support in Windows.
     assumeTrue(OS.getCurrent() != OS.WINDOWS);
 
-    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
 
     LocalExecutionOptions options = Options.getDefaults(LocalExecutionOptions.class);
     options.collectLocalExecutionStatistics = true;
@@ -956,7 +956,7 @@ public void hasNoExecutionStatistics_whenOptionIsDisabled() throws Exception {
     // TODO(b/62588075) Currently no process-wrapper or execution statistics support in Windows.
     assumeTrue(OS.getCurrent() != OS.WINDOWS);
 
-    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    FileSystem fs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
 
     LocalExecutionOptions options = Options.getDefaults(LocalExecutionOptions.class);
     options.collectLocalExecutionStatistics = false;

src/test/java/com/google/devtools/build/lib/shell/CommandUsingLinuxSandboxTest.java

@@ -43,7 +43,7 @@ public final class CommandUsingLinuxSandboxTest {
 
   @Before
   public final void createFileSystem() throws Exception {
-    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
     runfilesDir = testFS.getPath(BlazeTestUtils.runfilesDir());
   }
 

src/test/java/com/google/devtools/build/lib/shell/CommandUsingProcessWrapperTest.java

@@ -40,7 +40,7 @@ public final class CommandUsingProcessWrapperTest {
 
   @Before
   public final void createFileSystem() throws Exception {
-    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
   }
 
   private ProcessWrapper getProcessWrapper() {

src/test/java/com/google/devtools/build/lib/unix/NativePosixFilesTest.java

@@ -42,7 +42,7 @@ public class NativePosixFilesTest {
 
   @Before
   public final void createFileSystem() throws Exception  {
-    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    testFS = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
     workingDir = testFS.getPath(new File(TestUtils.tmpDir()).getCanonicalPath());
     testFile = workingDir.getRelative("test");
   }

src/test/java/com/google/devtools/build/lib/unix/UnixDigestHashAttributeNameTest.java

@@ -0,0 +1,62 @@
+// Copyright 2014 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.unix;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.devtools.build.lib.actions.cache.DigestUtils;
+import com.google.devtools.build.lib.vfs.DigestHashFunction;
+import com.google.devtools.build.lib.vfs.FileSystem;
+import com.google.devtools.build.lib.vfs.FileSystemTest;
+import com.google.devtools.build.lib.vfs.Path;
+import java.io.IOException;
+import org.junit.Test;
+
+/** Test for {@link com.google.devtools.build.lib.unix.UnixFileSystem#getFastDigest()}. */
+public class UnixDigestHashAttributeNameTest extends FileSystemTest {
+  private static final byte FAKE_DIGEST[] = {
+      0x18, 0x5f, 0x3d, 0x33, 0x22, 0x71, 0x7e, 0x25,
+      0x55, 0x61, 0x26, 0x0c, 0x03, 0x6b, 0x2e, 0x26,
+      0x43, 0x06, 0x7c, 0x30, 0x4e, 0x3a, 0x51, 0x20,
+      0x07, 0x71, 0x76, 0x48, 0x26, 0x38, 0x19, 0x69,
+  };
+
+  @Override
+  protected FileSystem getFreshFileSystem(DigestHashFunction digestHashFunction) {
+    return new FakeAttributeFileSystem(digestHashFunction);
+  }
+
+  @Test
+  public void testFoo() throws Exception {
+    // Instead of actually trying to access this file, a call to getxattr() should be made. We
+    // intercept this call and return a fake extended attribute value, thereby causing the checksum
+    // computation to be skipped entirely.
+    assertThat(DigestUtils.getDigestWithManualFallback(absolutize("myfile"), 123))
+        .isEqualTo(FAKE_DIGEST);
+  }
+
+  private class FakeAttributeFileSystem extends UnixFileSystem {
+    public FakeAttributeFileSystem(DigestHashFunction hashFunction) {
+      super(hashFunction, "user.checksum.sha256");
+    }
+
+    @Override
+    public byte[] getxattr(Path path, String name, boolean followSymlinks) throws IOException {
+      assertThat(path).isEqualTo(absolutize("myfile"));
+      assertThat(name).isEqualTo("user.checksum.sha256");
+      assertThat(followSymlinks).isEqualTo(true);
+      return FAKE_DIGEST;
+    }
+  }
+}

src/test/java/com/google/devtools/build/lib/unix/UnixFileSystemTest.java

@@ -30,7 +30,7 @@ public class UnixFileSystemTest extends SymlinkAwareFileSystemTest {
 
   @Override
   protected FileSystem getFreshFileSystem(DigestHashFunction digestHashFunction) {
-    return new UnixFileSystem(digestHashFunction);
+    return new UnixFileSystem(digestHashFunction, "");
   }
 
   @Override

src/test/java/com/google/devtools/build/lib/unix/UnixPathEqualityTest.java

@@ -37,8 +37,8 @@ public class UnixPathEqualityTest {
 
   @Before
   public final void initializeFileSystem() throws Exception  {
-    unixFs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
-    otherUnixFs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked());
+    unixFs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
+    otherUnixFs = new UnixFileSystem(DigestHashFunction.getDefaultUnchecked(), "");
     assertThat(unixFs != otherUnixFs).isTrue();
   }
 

src/test/java/com/google/devtools/build/lib/vfs/util/FileSystems.java

@@ -34,8 +34,8 @@ public static FileSystem getNativeFileSystem() {
     try {
       return Class.forName(TestConstants.TEST_REAL_UNIX_FILE_SYSTEM)
           .asSubclass(FileSystem.class)
-          .getDeclaredConstructor(DigestHashFunction.class)
-          .newInstance(DigestHashFunction.getDefaultUnchecked());
+          .getDeclaredConstructor(DigestHashFunction.class, String.class)
+          .newInstance(DigestHashFunction.getDefaultUnchecked(), "");
     } catch (Exception e) {
       throw new IllegalStateException(e);
     }

src/test/shell/bazel/BUILD

@@ -1319,3 +1319,11 @@ sh_test(
     data = [":test-deps"],
     deps = ["@bazel_tools//tools/bash/runfiles"],
 )
+
+sh_test(
+    name = "unix_digest_hash_attribute_name_test",
+    srcs = ["unix_digest_hash_attribute_name_test.sh"],
+    data = [":test-deps"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+    tags = ["no_windows"],
+)

src/test/shell/bazel/remote/BUILD

@@ -88,3 +88,16 @@ sh_test(
         "@bazel_tools//tools/bash/runfiles",
     ],
 )
+
+sh_test(
+    name = "remote_execution_with_xattr_test",
+    size = "large",
+    timeout = "eternal",
+    srcs = ["remote_execution_with_xattr_test.sh"],
+    data = [
+        ":remote_utils",
+        "//src/test/shell/bazel:test-deps",
+        "//src/tools/remote:worker",
+        "@bazel_tools//tools/bash/runfiles",
+    ],
+)