Fix memory leak: ExecutionEngine recreated per query appending to global function registry (#5222)

* Fix memory leak: ExecutionEngine recreated per query appending to global function registry

The OpenSearchExecutionEngine was not scoped as @singleton in the Guice
module, causing a new instance to be created for every SQL/PPL request
via injector.getInstance(SQLService.class). Each construction called
registerOpenSearchFunctions(), which appended entries to the static
PPLFuncImpTable.INSTANCE.externalFunctionRegistry via compute(). After
N queries, the GEOIP function list contained N entries (each holding
GeoIpFunction, SqlIdentifier, CalciteFuncSignature, PPLTypeChecker,
etc.), leading to ~2GB heap retention after ~730K queries.

Fix:
1. Add @singleton to the ExecutionEngine provider in OpenSearchPluginModule
   so only one instance is created per Guice injector.
2. Change registerExternalOperator() from compute()/append to put()/replace
   so repeated registrations overwrite instead of accumulating.

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

* Apply spotless formatting

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

---------

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

core/src/main/java/org/opensearch/sql/expression/function/PPLFuncImpTable.java

@@ -423,14 +423,10 @@ public void registerExternalOperator(BuiltinFunctionName functionName, SqlOperat
             functionName.name(),
             operator instanceof SqlUserDefinedFunction);
     CalciteFuncSignature signature = new CalciteFuncSignature(functionName.getName(), typeChecker);
-    externalFunctionRegistry.compute(
+    externalFunctionRegistry.put(
         functionName,
-        (name, existingList) -> {
-          List<Pair<CalciteFuncSignature, FunctionImp>> list =
-              existingList == null ? new ArrayList<>() : new ArrayList<>(existingList);
-          list.add(Pair.of(signature, (builder, args) -> builder.makeCall(operator, args)));
-          return list;
-        });
+        List.of(
+            Pair.of(signature, (FunctionImp) (builder, args) -> builder.makeCall(operator, args))));
   }
 
   /**

plugin/src/main/java/org/opensearch/sql/plugin/config/OpenSearchPluginModule.java

@@ -58,6 +58,7 @@ public StorageEngine storageEngine(OpenSearchClient client, Settings settings) {
   }
 
   @Provides
+  @Singleton
   public ExecutionEngine executionEngine(
       OpenSearchClient client, ExecutionProtector protector, PlanSerializer planSerializer) {
     return new OpenSearchExecutionEngine(client, protector, planSerializer);