pg_creating_extension() to inspect if an extension is being created.

This allows for much better control of what data is subject to
`pg_extension_config_dump` as such data provenance can be established
using this function.

Author: yrashk

src/backend/commands/extension.c

@@ -3906,3 +3906,13 @@ find_in_paths(const char *basename, List *paths)
 
 	return NULL;
 }
+
+Datum
+pg_creating_extension(PG_FUNCTION_ARGS)
+{
+	if (creating_extension)
+	{
+		PG_RETURN_OID(CurrentExtensionObject);
+	}
+	PG_RETURN_NULL();
+}

src/include/catalog/pg_proc.dat

@@ -10727,6 +10727,11 @@
   proname => 'pg_extension_config_dump', provolatile => 'v', proparallel => 'u',
   prorettype => 'void', proargtypes => 'regclass text',
   prosrc => 'pg_extension_config_dump' },
+{ oid => '3814',
+  descr => 'determine if an extension is currently being created',
+  proname => 'pg_creating_extension', provolatile => 'v', proparallel => 'u',
+  prorettype => 'oid', proargtypes => '',
+  prosrc => 'pg_creating_extension' },
 
 # SQL-spec window functions
 { oid => '3100', descr => 'row number within partition',