[GitHub] [spark] allisonwang-db commented on a diff in pull request #40557: [SPARK-42928][SQL] Make resolvePersistentFunction synchronized

["allisonwang-db (via GitHub)" <gi...@apache.org>]

allisonwang-db commented on code in PR #40557:
URL: https://github.com/apache/spark/pull/40557#discussion_r1148912737


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/SessionCatalog.scala:
##########
@@ -1719,7 +1719,8 @@ class SessionCatalog(
       name: FunctionIdentifier,
       arguments: Seq[Expression],
       registry: FunctionRegistryBase[T],
-      createFunctionBuilder: CatalogFunction => FunctionRegistryBase[T]#FunctionBuilder): T = {
+      createFunctionBuilder: CatalogFunction => FunctionRegistryBase[T]#FunctionBuilder
+  ): T = synchronized {

Review Comment:
   Let's say we have two threads. If a persistent function has not been loaded into the function registry, the `if` condition here can return false for both threads.
   ```
   if (registry.functionExists(qualifiedIdent)) {
     // This function has been already loaded into the function registry.
     registry.lookupFunction(qualifiedIdent, arguments)
   } 
   ```
   Then, if one thread successfully registers the persistent function in 
   ```
   registerFunction(
       funcMetadata,
       overrideIfExists = false,
       registry = registry,
       functionBuilder = createFunctionBuilder(funcMetadata))
   ```
   Then another thread can throw exceptions when trying to register this function again, due to `overrideIfExists = false`
   



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