Do not tickle TimestampGranularityMonitor for CONSTANT_METADATA artifacts.

"Constant metadata" artifacts represent real files whose changes should be
ignored by the build system.  However, these artifacts were triggering the
timestamp granularity checks in TimestampGranularityMonitor because the fact
that they were "constant metadata" was not respected.  Avoid this so that
their regeneration does not cause the build to unnecessarily stall.

One of these artifacts is the volatile workspace status file, which is
unconditionally updated on each build.  Before this fix, "blaze build" would
get stuck for up to a second waiting for file system timestamps to catch up.
With this fix, the artifact is ignored and the wait is gone.  This problem
is magnified on macOS where the default HFS+ file system only has
second-level granularity.  (This also affects Linux, but because current
Linux file systems have milli/nanosecond-level granularity, the wait imposed
by TimestampGranularityMonitor is minimal and thus not generally noticeable.)

--
MOS_MIGRATED_REVID=137867586

1 files changed, 22 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
index 28f676342c..971d84df61 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
@@ -108,6 +108,11 @@ public class ActionMetadataHandler implements MetadataHandler {
   private final Set<Artifact> injectedFiles = Sets.newConcurrentHashSet();
 
   private final ImmutableSet<Artifact> outputs;
+
+  /**
+   * The timestamp granularity monitor for this build.
+   * Use {@link #getTimestampGranularityMonitor(Artifact)} to fetch this member.
+   */
   private final TimestampGranularityMonitor tsgm;
 
   @VisibleForTesting
@@ -128,6 +133,19 @@ public class ActionMetadataHandler implements MetadataHandler {
     }
   }
 
+  /**
+   * Gets the {@link TimestampGranularityMonitor} to use for a given artifact.
+   *
+   * <p>If the artifact is of type "constant metadata", this returns null so that changes to such
+   * artifacts do not tickle the timestamp granularity monitor, delaying the build for no reason.
+   *
+   * @param artifact the artifact for which to fetch the timestamp granularity monitor
+   * @return the timestamp granularity monitor to use, which may be null
+   */
+  private TimestampGranularityMonitor getTimestampGranularityMonitor(Artifact artifact) {
+    return artifact.isConstantMetadata() ? null : tsgm;
+  }
+
   private static Metadata metadataFromValue(FileArtifactValue value) throws FileNotFoundException {
     if (value == FileArtifactValue.MISSING_FILE_MARKER
         || value == FileArtifactValue.OMITTED_FILE_MARKER) {
@@ -422,7 +440,8 @@ public class ActionMetadataHandler implements MetadataHandler {
         // from the filesystem, this FileValue will not compare equal to another one created for the
         // same file, because the other one will be missing its digest.
         fileValue = fileValueFromArtifact(artifact,
-            FileStatusWithDigestAdapter.adapt(statNoFollow), tsgm);
+            FileStatusWithDigestAdapter.adapt(statNoFollow),
+            getTimestampGranularityMonitor(artifact));
         // Ensure the digest supplied matches the actual digest if it exists.
         byte[] fileDigest = fileValue.getDigest();
         if (fileDigest != null && !Arrays.equals(digest, fileDigest)) {
@@ -533,7 +552,8 @@ public class ActionMetadataHandler implements MetadataHandler {
   /** Constructs a new FileValue, saves it, and checks inconsistent data. */
   FileValue constructFileValue(Artifact artifact, @Nullable FileStatusWithDigest statNoFollow)
       throws IOException {
-    FileValue value = fileValueFromArtifact(artifact, statNoFollow, tsgm);
+    FileValue value = fileValueFromArtifact(artifact, statNoFollow,
+        getTimestampGranularityMonitor(artifact));
     FileValue oldFsValue = outputArtifactData.putIfAbsent(artifact, value);
     checkInconsistentData(artifact, oldFsValue, null);
     return value;