Support --unix_digest_hash_attribute_name in repository cache

We extend `RepositoryCache` and `DownloadManager` to support pulling checksums from extended attributes. In some example builds, I observe a 5x speedup in repository fetching: https://github.com/sushain97/bazel-faster-fetch-repro.

I would like to make `xattrProvider` not-nullable in any context. However, it's not immediately clear how I could thread access to one in the `BazelPackageLoader` context: https://github.com/sushain97/bazel/blob/1518e04579e6a1c966eb8fbf6d65603e0a1d41d5/src/main/java/com/google/devtools/build/lib/skyframe/packages/BazelPackageLoader.java#L108-L110. Open to any suggestions.

Closes #18221.

PiperOrigin-RevId: 529730007
Change-Id: Ic06b4585988a6b5c09f61cee68daf1dcdf3e46fc

Author: sushain97

src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepositoryCache.java

@@ -21,6 +21,7 @@
 import com.google.common.hash.HashFunction;
 import com.google.common.hash.Hasher;
 import com.google.common.hash.Hashing;
+import com.google.common.io.BaseEncoding;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
 import com.google.devtools.build.lib.vfs.Path;
 import java.io.IOException;
@@ -310,6 +311,15 @@ public static void assertFileChecksum(String expectedChecksum, Path filePath, Ke
    */
   public static String getChecksum(KeyType keyType, Path path)
       throws IOException, InterruptedException {
+    // Attempt to use the fast digest if the hash function of the filesystem
+    // matches `keyType` and it's available.
+    if (path.getFileSystem().getDigestFunction().getHashFunction().equals(keyType.hashFunction)) {
+      byte[] digest = path.getFastDigest();
+      if (digest != null) {
+        return BaseEncoding.base16().lowerCase().encode(digest);
+      }
+    }
+
     Hasher hasher = keyType.newHasher();
     byte[] byteBuffer = new byte[BUFFER_SIZE];
     try (InputStream stream = path.getInputStream()) {

src/test/java/com/google/devtools/build/lib/bazel/repository/cache/BUILD

@@ -20,7 +20,9 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/bazel/repository/cache",
         "//src/main/java/com/google/devtools/build/lib/vfs",
         "//src/test/java/com/google/devtools/build/lib/testutil",
+        "//third_party:guava",
         "//third_party:junit4",
+        "//third_party:mockito",
         "//third_party:truth",
     ],
 )

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

@@ -15,7 +15,10 @@
 package com.google.devtools.build.lib.bazel.repository.cache;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.spy;
 
+import com.google.common.io.BaseEncoding;
 import com.google.devtools.build.lib.bazel.repository.cache.RepositoryCache.KeyType;
 import com.google.devtools.build.lib.testutil.Scratch;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
@@ -171,6 +174,18 @@ public void testGetChecksum() throws Exception {
     assertThat(actualChecksum).isEqualTo(downloadedFileSha256);
   }
 
+  @Test
+  public void testGetChecksumWithFastDigest() throws Exception {
+    String fastDigestChecksum = "cfe5ed57e6e323555b379c660aa8d35b70c2f8f07cf03ad6747266495ac13be0";
+    downloadedFile = spy(downloadedFile);
+    doReturn(BaseEncoding.base16().lowerCase().decode(fastDigestChecksum))
+        .when(downloadedFile)
+        .getFastDigest();
+
+    String actualChecksum = RepositoryCache.getChecksum(KeyType.SHA256, downloadedFile);
+    assertThat(actualChecksum).isEqualTo(fastDigestChecksum);
+  }
+
   @Test
   public void testAssertFileChecksumPass() throws Exception {
     RepositoryCache.assertFileChecksum(downloadedFileSha256, downloadedFile, KeyType.SHA256);