salvatorecampagna commented on code in PR #15378:
URL: https://github.com/apache/lucene/pull/15378#discussion_r2482342022
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lucene/core/src/java/org/apache/lucene/index/MergePolicy.java:
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@@ -939,4 +939,96 @@ 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 #hasNewMerges()} returns {@code
false} and {@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 whether any new merges were scheduled.
+ *
+ * @return {@code true} if merges were scheduled, {@code false} if no
merges needed
+ */
+ public boolean hasNewMerges() {
+ return spec != null;
+ }
+
+ /**
+ * 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() {
+ return spec == null ? "MergeObserver: no merges" : spec.toString();
+ }
+
+ /**
+ * Returns the merge at the specified index. Caller must ensure {@link
#hasNewMerges()} returns
+ * {@code true} and index is within bounds.
+ *
+ * @param i merge index (0 to {@link #numMerges()} - 1)
+ * @return the merge at index i
+ * @throws IndexOutOfBoundsException if index is invalid or no merges exist
+ */
+ public MergePolicy.OneMerge getMerge(int i) {
Review Comment:
You're right on both points. My original intent was to provide detailed
observability, but as you noted, the caller can't meaningfully use index-based
access without already having the `MergeSpec`. And `OneMerge` is definitely
mutable, so exposing it breaks encapsulation.
The `umMerges()/numCompletedMerges()` approach you suggested is much better,
as it provides progress tracking without exposing internals.
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