Avoid per-poll ConcurrentHashMap KeyIterator allocation in operation managers

[nicolaslopezbravo]

What

Guard the per-poll() iteration of each operation manager's in-flight operation set with isEmpty(), so the enhanced-for loop does not allocate a ConcurrentHashMap$KeyIterator when the set is empty.

Why

Each operation manager (GetManager, PutManager, DeleteManager, TtlUpdateManager, UndeleteManager, ReplicateBlobManager) stores its in-flight operations in a ConcurrentHashMap.newKeySet() and iterates it once per poll() with an enhanced-for loop. The for-each calls .iterator(), and ConcurrentHashMap.KeySetView.iterator() allocates a KeyIterator unconditionally — even for an empty set — before hasNext() returns false:

public Iterator<K> iterator() {
    Node<K,V>[] t;
    ConcurrentHashMap<K,V> m = map;
    int f = (t = m.table) == null ? 0 : t.length;
    return new KeyIterator<K,V>(t, f, 0, f, m);  // allocated even when empty
}

The RequestResponseHandler poll loop runs continuously across all six managers, so an idle or lightly-loaded router allocates a throwaway iterator per manager per poll. Heap analysis of a busy frontend showed tens of millions of these short-lived ConcurrentHashMap$KeyIterator objects as the dominant young-gen garbage, inflating GC frequency.

Guarding with !operations.isEmpty() skips the .iterator() call in the empty case; isEmpty() checks the map count and allocates nothing.

Behavior

No behavior change — an empty set's for-each is already a no-op, so the guard only removes the wasted allocation. When operations are present the loop runs exactly as before.

Testing Done

• ./gradlew :ambry-router:compileJava — BUILD SUCCESSFUL.
• ./gradlew :ambry-router:test --tests GetManagerTest --tests PutManagerTest --tests DeleteManagerTest --tests TtlUpdateManagerTest --tests UndeleteManagerTest — 201 tests, 201 passed, 0 failed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.37288% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.85%. Comparing base (52ba813) to head (c7b5b95).
⚠️ Report is 397 commits behind head on master.

Files with missing lines	Patch %	Lines
.../com/github/ambry/router/ReplicateBlobManager.java	0.00%	10 Missing and 1 partial ⚠️
...in/java/com/github/ambry/router/DeleteManager.java	45.45%	5 Missing and 1 partial ⚠️
...java/com/github/ambry/router/TtlUpdateManager.java	45.45%	5 Missing and 1 partial ⚠️
.../java/com/github/ambry/router/UndeleteManager.java	45.45%	5 Missing and 1 partial ⚠️
.../main/java/com/github/ambry/router/PutManager.java	62.50%	2 Missing and 1 partial ⚠️
.../main/java/com/github/ambry/router/GetManager.java	71.42%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3271       +/-   ##
=============================================
- Coverage     64.24%   50.85%   -13.39%     
+ Complexity    10398     8680     -1718     
=============================================
  Files           840      937       +97     
  Lines         71755    80235     +8480     
  Branches       8611     9645     +1034     
=============================================
- Hits          46099    40804     -5295     
- Misses        23004    36038    +13034     
- Partials       2652     3393      +741     

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