Track & log when there is insufficient disk space available to execute merges

docs/changelog/131711.yaml

@@ -0,0 +1,5 @@
+pr: 131711
+summary: Track & log when there is insufficient disk space available to execute merges
+area: Engine
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java

@@ -35,6 +35,7 @@
 import java.util.IdentityHashMap;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.PriorityQueue;
 import java.util.Set;
@@ -552,6 +553,7 @@ private static ByteSizeValue getFreeBytesThreshold(
     }
 
     static class MergeTaskPriorityBlockingQueue extends PriorityBlockingQueueWithBudget<MergeTask> {
+        private static final Logger LOGGER = LogManager.getLogger(MergeTaskPriorityBlockingQueue.class);
         MergeTaskPriorityBlockingQueue() {
             // by default, start with 0 budget (so takes on this queue will always block until the first {@link #updateBudget} is invoked)
             // use the estimated *remaining* merge size as the budget function so that the disk space budget of elements is updated
@@ -567,6 +569,40 @@ long getAvailableBudget() {
         MergeTask peekQueue() {
             return enqueuedByBudget.peek().v1();
         }
+
+        @Override
+        void postBudgetUpdate() {
+            assert super.lock.isHeldByCurrentThread();
+            Tuple<MergeTask, Long> head = enqueuedByBudget.peek();
+            if (head != null && head.v2() > availableBudget) {
+                LOGGER.warn("There are merge tasks enqueued but there's insufficient disk space available to execute them");
+                if (LOGGER.isDebugEnabled()) {
+                    if (unreleasedBudgetPerElement.isEmpty()) {
+                        LOGGER.debug(
+                            String.format(
+                                Locale.ROOT,
+                                "There are no merge tasks currently running, "
+                                    + "but there are [%d] enqueued ones that are blocked because of insufficient disk space",
+                                enqueuedByBudget.size()
+                            )
+                        );
+                    } else {
+                        StringBuilder messageBuilder = new StringBuilder();
+                        messageBuilder.append("The following merge tasks are currently running [");
+                        for (var runningMergeTask : super.unreleasedBudgetPerElement.entrySet()) {
+                            messageBuilder.append(runningMergeTask.getKey().element().toString());
+                            messageBuilder.append(" with disk space budgets in bytes " + runningMergeTask.getValue() + " , ");
+                        }
+                        messageBuilder.delete(messageBuilder.length() - 3, messageBuilder.length());
+                        messageBuilder.append(
+                            "], and there are ["
+                                + enqueuedByBudget.size()
+                                + "] additional enqueued ones that are blocked because of insufficient disk space"
+                        );
+                    }
+                }
+            }
+        }
     }
 
     /**
@@ -576,7 +612,7 @@ MergeTask peekQueue() {
     static class PriorityBlockingQueueWithBudget<E> {
         private final ToLongFunction<? super E> budgetFunction;
         protected final PriorityQueue<Tuple<E, Long>> enqueuedByBudget;
-        private final IdentityHashMap<ElementWithReleasableBudget, Long> unreleasedBudgetPerElement;
+        protected final IdentityHashMap<ElementWithReleasableBudget, Budgets> unreleasedBudgetPerElement;
         private final ReentrantLock lock;
         private final Condition elementAvailable;
         protected long availableBudget;
@@ -637,15 +673,18 @@ void updateBudget(long availableBudget) {
                 // updates the budget of enqueued elements (and possibly reorders the priority queue)
                 updateBudgetOfEnqueuedElementsAndReorderQueue();
                 // update the budget of dequeued, but still in-use elements (these are the elements that are consuming budget)
-                unreleasedBudgetPerElement.replaceAll((e, v) -> budgetFunction.applyAsLong(e.element()));
+                unreleasedBudgetPerElement.replaceAll((e, v) -> v.updateBudgetEstimation(budgetFunction.applyAsLong(e.element())));
                 // the available budget is decreased by the budget of still in-use elements (dequeued elements that are still in-use)
-                this.availableBudget -= unreleasedBudgetPerElement.values().stream().mapToLong(i -> i).sum();
+                this.availableBudget -= unreleasedBudgetPerElement.values().stream().mapToLong(i -> i.latestBudgetEstimationForElement).sum();
                 elementAvailable.signalAll();
+                postBudgetUpdate();
             } finally {
                 lock.unlock();
             }
         }
 
+        void postBudgetUpdate() {};
+
         private void updateBudgetOfEnqueuedElementsAndReorderQueue() {
             assert this.lock.isHeldByCurrentThread();
             int queueSizeBefore = enqueuedByBudget.size();
@@ -686,7 +725,7 @@ private ElementWithReleasableBudget newElementWithReleasableBudget(E element, lo
             ElementWithReleasableBudget elementWithReleasableBudget = new ElementWithReleasableBudget(element);
             assert this.lock.isHeldByCurrentThread();
             // the taken element holds up some budget
-            var prev = this.unreleasedBudgetPerElement.put(elementWithReleasableBudget, budget);
+            var prev = this.unreleasedBudgetPerElement.put(elementWithReleasableBudget, new Budgets(budget, budget, this.availableBudget));
             assert prev == null;
             this.availableBudget -= budget;
             assert this.availableBudget >= 0L;
@@ -736,6 +775,16 @@ E element() {
                 return element;
             }
         }
+
+        record Budgets(long initialBudgetEstimationForElement, long latestBudgetEstimationForElement, long initialTotalAvailableBudget) {
+            Budgets updateBudgetEstimation(long latestBudgetEstimationForElement) {
+                return new Budgets(
+                    this.initialBudgetEstimationForElement,
+                    latestBudgetEstimationForElement,
+                    this.initialTotalAvailableBudget
+                );
+            }
+        }
     }
 
     private static long newTargetIORateBytesPerSec(