Remote: Fix a race when reporting action progresses.

Fixes bazelbuild#13713.

I will create checkpick request for 4.2.0 once merged into main branch.

Closes bazelbuild#13719.

PiperOrigin-RevId: 386213448

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

@@ -62,12 +62,14 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.PriorityQueue;
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
 import javax.annotation.concurrent.ThreadSafe;
 
 /** Tracks state for the UI. */
@@ -228,6 +230,7 @@ private ProgressState(String id, long nanoStartTime) {
       }
     }
 
+    @GuardedBy("this")
     private final LinkedHashMap<String, ProgressState> runningProgresses = new LinkedHashMap<>();
 
     /** Starts tracking the state of an action. */
@@ -333,6 +336,13 @@ synchronized void onProgressEvent(ActionProgressEvent event, long nanoChangeTime
       state.latestEvent = event;
     }
 
+    synchronized Optional<ProgressState> firstProgress() {
+      if (runningProgresses.isEmpty()) {
+        return Optional.empty();
+      }
+      return Optional.of(runningProgresses.entrySet().iterator().next().getValue());
+    }
+
     /** Generates a human-readable description of this action's state. */
     synchronized String describe() {
       if (runningStrategiesBitmap != 0) {
@@ -705,12 +715,12 @@ private String describeTestGroup(
   }
 
   private String describeActionProgress(ActionState action, int desiredWidth) {
-    if (action.runningProgresses.isEmpty()) {
+    Optional<ActionState.ProgressState> stateOpt = action.firstProgress();
+    if (!stateOpt.isPresent()) {
       return "";
     }
 
-    ActionState.ProgressState state =
-        action.runningProgresses.entrySet().iterator().next().getValue();
+    ActionState.ProgressState state = stateOpt.get();
     ActionProgressEvent event = state.latestEvent;
     String message = event.progress();
     if (message.isEmpty()) {