Remove DigestUtils.getDigestInExclusiveMode() now that SsdModule has … (

bazelbuild#14242)

…been removed

There used to be a cli flag --experimental_multi_threaded_digest which defaulted to true
and therefore would not compute the file digest in exclusive mode unless overridden to false.
When that flag was removed this code was accidentally left behind which instead causes bazel to always
compute file digests in exclusive mode for all files larger than MULTI_THREADED_DIGEST_MAX_FILE_SIZE.

This causes bazel to take 5-6x longer to 'check cached actions' in my org's builds,
specifically increasing the time from 30 secs to 2 mins 30 secs for fully cached no-op builds.

Fixes bazelbuild#14034

Closes bazelbuild#14231.

PiperOrigin-RevId: 407790990
(cherry picked from commit 77a002c)

Co-authored-by: Brandon Jacklyn <brandonjacklyn@gmail.com>

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

@@ -16,11 +16,8 @@
 import com.github.benmanes.caffeine.cache.Cache;
 import com.github.benmanes.caffeine.cache.Caffeine;
 import com.github.benmanes.caffeine.cache.stats.CacheStats;
-import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import com.google.common.primitives.Longs;
-import com.google.devtools.build.lib.profiler.Profiler;
-import com.google.devtools.build.lib.profiler.ProfilerTask;
 import java.io.IOException;
 
 /**
@@ -41,17 +38,9 @@
  * fail.
  */
 public class DigestUtils {
-  // Object to synchronize on when serializing large file reads.
-  private static final Object DIGEST_LOCK = new Object();
-
   // Typical size for a digest byte array.
   public static final int ESTIMATED_SIZE = 32;
 
-  // Files of this size or less are assumed to be readable in one seek.
-  // (This is the default readahead window on Linux.)
-  @VisibleForTesting // the unittest is in a different package!
-  public static final int MULTI_THREADED_DIGEST_MAX_FILE_SIZE = 128 * 1024;
-
   /**
    * Keys used to cache the values of the digests for files where we don't have fast digests.
    *
@@ -126,19 +115,6 @@ public int hashCode() {
   /** Private constructor to prevent instantiation of utility class. */
   private DigestUtils() {}
 
-  /**
-   * Obtain file's digset using synchronized method, ensuring that system is not overloaded in case
-   * when multiple threads are requesting digest calculations and underlying file system cannot
-   * provide it via extended attribute.
-   */
-  private static byte[] getDigestInExclusiveMode(Path path) throws IOException {
-    long startTime = Profiler.nanoTimeMaybe();
-    synchronized (DIGEST_LOCK) {
-      Profiler.instance().logSimpleTask(startTime, ProfilerTask.WAIT, path.getPathString());
-      return path.getDigest();
-    }
-  }
-
   /**
    * Enables the caching of file digests based on file status data.
    *
@@ -221,16 +197,7 @@ public static byte[] manuallyComputeDigest(Path path, long fileSize) throws IOEx
       }
     }
 
-    // Compute digest from the file contents.
-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
-      // We'll have to read file content in order to calculate the digest.
-      // We avoid overlapping this process for multiple large files, as
-      // seeking back and forth between them will result in an overall loss of
-      // throughput.
-      digest = getDigestInExclusiveMode(path);
-    } else {
-      digest = path.getDigest();
-    }
+    digest = path.getDigest();
 
     Preconditions.checkNotNull(digest, "Missing digest for %s (size %s)", path, fileSize);
     if (cache != null) {

src/test/java/com/google/devtools/build/lib/vfs/DigestUtilsTest.java

@@ -20,7 +20,6 @@
 import com.google.devtools.build.lib.testutil.TestUtils;
 import com.google.devtools.build.lib.vfs.inmemoryfs.InMemoryFileSystem;
 import java.io.IOException;
-import java.util.Arrays;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -93,25 +92,6 @@ protected byte[] getFastDigest(PathFragment path) throws IOException {
      thread2.joinAndAssertState(TestUtils.WAIT_TIMEOUT_MILLISECONDS);
   }
 
-  /**
-   * Ensures that digest calculation is synchronized for files greater than
-   * {@link DigestUtils#MULTI_THREADED_DIGEST_MAX_FILE_SIZE} bytes if the digest is not
-   * available cheaply, so machines with rotating drives don't become unusable.
-   */
-  @Test
-  public void testCalculationConcurrency() throws Exception {
-    int small = DigestUtils.MULTI_THREADED_DIGEST_MAX_FILE_SIZE;
-    int large = DigestUtils.MULTI_THREADED_DIGEST_MAX_FILE_SIZE + 1;
-    for (DigestHashFunction hf :
-        Arrays.asList(DigestHashFunction.SHA256, DigestHashFunction.SHA1)) {
-      assertDigestCalculationConcurrency(true, true, small, small, hf);
-      assertDigestCalculationConcurrency(true, true, large, large, hf);
-      assertDigestCalculationConcurrency(true, false, small, small, hf);
-      assertDigestCalculationConcurrency(true, false, small, large, hf);
-      assertDigestCalculationConcurrency(false, false, large, large, hf);
-    }
-  }
-
   @Test
   public void testCache() throws Exception {
     AtomicInteger getFastDigestCounter = new AtomicInteger(0);