Log deprecation warn if memory buffer type not defined (#16498)

On 8.x series log a deprecation log if the user didn't explicitly specify a selection for pipeline.buffer.type. Before this change the default was silently set to direct, after this change if not explicitly defined, the default is still direct but log a deprecation log.

Co-authored-by: Ry Biesemeyer <[email protected]>

config/logstash.yml

@@ -332,7 +332,7 @@
 #
 # Determine where to allocate memory buffers, for plugins that leverage them.
 # Default to direct, optionally can be switched to heap to select Java heap space.
-# pipeline.buffer.type: direct
+# pipeline.buffer.type: heap
 #
 # ------------ X-Pack Settings (not applicable for OSS build)--------------
 #

logstash-core/lib/logstash/environment.rb

@@ -111,7 +111,7 @@ module Environment
             Setting::String.new("keystore.classname", "org.logstash.secret.store.backend.JavaKeyStore"),
             Setting::String.new("keystore.file", ::File.join(::File.join(LogStash::Environment::LOGSTASH_HOME, "config"), "logstash.keystore"), false), # will be populated on
     Setting::NullableString.new("monitoring.cluster_uuid"),
-            Setting::String.new("pipeline.buffer.type", "direct", true, ["direct", "heap"])
+            Setting::String.new("pipeline.buffer.type", nil, false, ["direct", "heap"])
   # post_process
   ].each {|setting| SETTINGS.register(setting) }
 

logstash-core/lib/logstash/runner.rb

@@ -312,9 +312,17 @@ def execute
     if setting("config.debug") && !logger.debug?
       logger.warn("--config.debug was specified, but log.level was not set to \'debug\'! No config info will be logged.")
     end
-    if setting("pipeline.buffer.type") != nil
-      configure_pipeline_buffer_type
+    if setting("pipeline.buffer.type") == nil
+      deprecation_logger.deprecated(
+        "'pipeline.buffer.type' setting is not explicitly defined."\
+        "Before moving to 9.x set it to 'heap' and tune heap size upward, or set it to 'direct' to maintain existing behavior."
+      )
+
+      # set to direct to keep backward ecs_compatibility
+      buffer_type_setting = @settings.get_setting("pipeline.buffer.type")
+      buffer_type_setting.set("direct")
     end
+    configure_pipeline_buffer_type
 
     while (msg = LogStash::DeprecationMessage.instance.shift)
       deprecation_logger.deprecated msg