Avoid some uses of ConcurrentLinkedQueue.size() in SharedBlobCacheService (elastic#128119)

SharedBlobCacheService keeps track of the free regions in a
ConcurrentLinkedQueue. We use its "size()" method in three places
outside of tests but unfortunately this is not a constant time operation
because of the asynchronous nature of this queue. This change removes
two of the uses where we only check if the queue is empty by calling the
"isEmpty()" method instead.

Author: cbuescher

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java

@@ -500,6 +500,7 @@ public boolean maybeFetchFullEntry(
         ActionListener<Void> listener
     ) {
         int finalRegion = getEndingRegion(length);
+        // TODO freeRegionCount uses freeRegions.size() which is is NOT a constant-time operation. Can we do better?
         if (freeRegionCount() < finalRegion) {
             // Not enough room to download a full file without evicting existing data, so abort
             listener.onResponse(null);
@@ -571,7 +572,7 @@ public void maybeFetchRegion(
         final Executor fetchExecutor,
         final ActionListener<Boolean> listener
     ) {
-        if (freeRegionCount() < 1 && maybeEvictLeastUsed() == false) {
+        if (freeRegions.isEmpty() && maybeEvictLeastUsed() == false) {
             // no free page available and no old enough unused region to be evicted
             logger.info("No free regions, skipping loading region [{}]", region);
             listener.onResponse(false);
@@ -619,7 +620,7 @@ public void maybeFetchRange(
         final Executor fetchExecutor,
         final ActionListener<Boolean> listener
     ) {
-        if (freeRegionCount() < 1 && maybeEvictLeastUsed() == false) {
+        if (freeRegions.isEmpty() && maybeEvictLeastUsed() == false) {
             // no free page available and no old enough unused region to be evicted
             logger.info("No free regions, skipping loading region [{}]", region);
             listener.onResponse(false);
@@ -671,7 +672,11 @@ private static void throwAlreadyClosed(String message) {
         throw new AlreadyClosedException(message);
     }
 
-    // used by tests
+    /**
+     * NOTE: Method is package private mostly to allow checking the number of fee regions in tests.
+     * However, it is also used by {@link SharedBlobCacheService#maybeFetchFullEntry} but we should try
+     * to move away from that because calling "size" on a ConcurrentLinkedQueue is not a constant time operation.
+     */
     int freeRegionCount() {
         return freeRegions.size();
     }