Tie-break by path, so that AC entries are garbage collected before CA…

…S entries with the same age.

PiperOrigin-RevId: 679138266
Change-Id: I6eadf1b872b4adaece69e56fd3977088634a348f

src/main/java/com/google/devtools/build/lib/remote/disk/DiskCacheGarbageCollector.java

@@ -14,9 +14,9 @@
 package com.google.devtools.build.lib.remote.disk;
 
 import static com.google.common.collect.ImmutableSet.toImmutableSet;
-import static java.util.Comparator.comparing;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ComparisonChain;
 import com.google.common.collect.ImmutableSet;
 import com.google.devtools.build.lib.concurrent.AbstractQueueVisitor;
 import com.google.devtools.build.lib.concurrent.ErrorClassifier;
@@ -29,6 +29,7 @@
 import java.time.Duration;
 import java.time.Instant;
 import java.util.ArrayList;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.ExecutorService;
@@ -62,13 +63,23 @@ private record Entry(String path, long size, long mtime) {}
    */
   public record CollectionPolicy(Optional<Long> maxSizeBytes, Optional<Duration> maxAge) {
 
+    // Sort older entries before newer ones, tie breaking by path. This causes AC entries to be
+    // sorted before CAS entries with the same age, making it less likely for garbage collection
+    // to break referential integrity in the event that mtime resolution is insufficient.
+    private static final Comparator<Entry> COMPARATOR =
+        (x, y) ->
+            ComparisonChain.start()
+                .compare(x.mtime(), y.mtime())
+                .compare(x.path(), y.path())
+                .result();
+
     /**
      * Returns the entries to be deleted.
      *
      * @param entries the full list of entries
      */
     List<Entry> getEntriesToDelete(List<Entry> entries) {
-      entries.sort(comparing(Entry::mtime));
+      entries.sort(COMPARATOR);
 
       long excessSizeBytes = getExcessSizeBytes(entries);
       long timeCutoff = getTimeCutoff();

src/test/java/com/google/devtools/build/lib/remote/disk/DiskCacheGarbageCollectorTest.java

@@ -79,6 +79,21 @@ public void sizePolicy_collectsOldest() throws Exception {
     assertFilesDoNotExist("ac/abc", "cas/def");
   }
 
+  @Test
+  public void sizePolicy_tieBreakByPath() throws Exception {
+    writeFiles(
+        Entry.of("ac/123", kbytes(1), daysAgo(1)),
+        Entry.of("cas/456", kbytes(1), daysAgo(1)),
+        Entry.of("ac/abc", kbytes(1), daysAgo(1)),
+        Entry.of("cas/def", kbytes(1), daysAgo(1)));
+
+    CollectionStats stats = runGarbageCollector(Optional.of(kbytes(2)), Optional.empty());
+
+    assertThat(stats).isEqualTo(new CollectionStats(4, kbytes(4), 2, kbytes(2)));
+    assertFilesExist("cas/456", "cas/def");
+    assertFilesDoNotExist("ac/123", "ac/abc");
+  }
+
   @Test
   public void agePolicy_noCollection() throws Exception {
     writeFiles(