Enabling message_containers feature flag with log.exchange.level = debug leads to lock and memory exhaustion

[johanrhodin]

Background

The enabling of the message_containers feature flag contains a deadlock in the (rare) case of when the log exchange is set to debug level.

Reproduction

Follow these steps to reproduce the issue:

• Install a late 3.12.x release, such as 3.12.14
• Update to 3.13.4
• Set the log level for the log exchange to debug:

{rabbit, [
    {log, [
      {exchange, [{enabled, true}, {level, debug}, {formatter, {rabbit_logger_text_fmt, #{single_line => true}}}]},
      {file,     [{level, debug}]} %% not needed, but nice to have
    ]}
  ]}

• Enable the feature flag:
  sudo rabbitmqctl enable_feature_flag message_containers
• Watch in observer how all memory is consumed

Logs

See attached log file
rabbit_logger_exchange_h.txt

[gomoripeti]

Tracing the function which does the debug logging of "Feature flags: acquiring lock" it is visible there is an infinite loop in the stacktrace. rabbit_logger_exchange_h:do_log/2 and rabbit_feature_flags:is_enabled/2 mutually call each other:

rabbit_ff_registry_factory:acquire_state_change_lock()
 [{rabbit_feature_flags,is_enabled,2,
                       [{file,"rabbit_feature_flags.erl"},{line,584}]},
 {mc_amqpl,message,4,[{file,"mc_amqpl.erl"},{line,464}]},
 {rabbit_logger_exchange_h,do_log,2,
                           [{file,"rabbit_logger_exchange_h.erl"},{line,58}]},
 {logger_backend,call_handlers,3,[{file,"logger_backend.erl"},{line,51}]},
 {rabbit_ff_registry_factory,acquire_state_change_lock,0,
                             [{file,"rabbit_ff_registry_factory.erl"},
                              {line,37}]},
 {rabbit_feature_flags,is_enabled,2,
                       [{file,"rabbit_feature_flags.erl"},{line,584}]},
 {mc_amqpl,message,4,[{file,"mc_amqpl.erl"},{line,464}]},
 {rabbit_logger_exchange_h,do_log,2,
                           [{file,"rabbit_logger_exchange_h.erl"},{line,58}]}]

[michaelklishin]

The only solution I see is to disable exchange logging before upgrading. Other logging mechanisms do not depend on feature flags, and feature flag changes do and will produce some log entries.

I don't see how the exchange logging path can determine whether it should check for the flag.