vigyasharma commented on code in PR #15378:
URL: https://github.com/apache/lucene/pull/15378#discussion_r2483795145
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lucene/core/src/java/org/apache/lucene/index/MergePolicy.java:
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@@ -939,4 +940,93 @@ static final class MergeReader {
this.hardLiveDocs = hardLiveDocs;
}
}
+
+ /**
+ * Observer for merge operations returned by {@link
IndexWriter#forceMergeDeletes(boolean)}.
+ * Provides methods to query merge status and wait for completion.
+ *
+ * <p>When no merges are needed, {@link #numMerges()} returns 0. In this
case, {@link #await()}
+ * returns {@code true} immediately since there is nothing to wait for.
+ *
+ * @lucene.experimental
+ */
+ public static final class MergeObserver {
+ private final MergePolicy.MergeSpecification spec;
+
+ MergeObserver(MergePolicy.MergeSpecification spec) {
+ this.spec = spec;
+ }
+
+ /**
+ * Returns the number of merges in this specification.
+ *
+ * @return number of merges, or 0 if no merges were scheduled
+ */
+ public int numMerges() {
+ return spec == null ? 0 : spec.merges.size();
+ }
+
+ /**
+ * Returns the number of completed merges in this specification. Useful
for tracking merge
+ * progress: {@code numCompletedMerges() / numMerges()}.
+ *
+ * @return number of completed merges
+ */
+ public int numCompletedMerges() {
+ if (spec == null) {
+ return 0;
+ }
+ int completed = 0;
+ for (OneMerge merge : spec.merges) {
+ if (merge.mergeCompleted.isDone()) {
+ completed++;
+ }
+ }
+ return completed;
+ }
+
+ /**
+ * Waits for all merges in this specification to complete. Returns
immediately if no merges were
+ * scheduled.
+ *
+ * @return {@code true} if all merges completed successfully or no merges
were needed, {@code
+ * false} on error
+ */
+ public boolean await() {
+ return spec == null || spec.await();
+ }
+
+ /**
+ * Waits for all merges in this specification to complete, with timeout.
Returns immediately if
+ * no merges were scheduled.
+ *
+ * @param timeout maximum time to wait
+ * @param unit time unit for timeout
+ * @return {@code true} if all merges completed within timeout or no
merges were needed, {@code
+ * false} on timeout or error
+ */
+ public boolean await(long timeout, TimeUnit unit) {
+ return spec == null || spec.await(timeout, unit);
+ }
+
+ /**
+ * Returns a {@link CompletableFuture} that completes when all merges
finish. Returns an
+ * already-completed future if no merges were scheduled.
+ *
+ * @return future that completes when merges finish
+ */
+ public CompletableFuture<Void> awaitAsync() {
+ return spec == null
+ ? CompletableFuture.completedFuture(null)
+ : spec.getMergeCompletedFutures();
+ }
+
+ @Override
+ public String toString() {
+ if (spec == null) {
+ return "MergeObserver: no merges";
+ }
+ return String.format(Locale.ROOT, "MergeObserver: %d merges",
numMerges());
Review Comment:
We could include `spec.toString()` here. It has some useful info about the
segments in each merge.
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