HTTPX instrumentation crashes with `NoMethodError: undefined method 'add_request_headers' for nil` when `start_external_request_segment` returns nil

[thebravoman]

Versions:

• newrelic_rpm 9.23.0 (also present in 10.2.0 — code is identical)
• httpx 1.7.5
• Ruby 3.4.4
• Passenger (multi-threaded)

Description:

nr_start_segment in lib/new_relic/agent/instrumentation/httpx/instrumentation.rb calls start_external_request_segment which can return nil when the internal rescue => exception catches an error. The return value is then used on line 26 without a nil check:

def nr_start_segment(request)
  return unless NewRelic::Agent::Tracer.state.is_execution_traced?

  wrapped_request = NewRelic::Agent::HTTPClients::HTTPXHTTPRequest.new(request)
  segment = NewRelic::Agent::Tracer.start_external_request_segment(
    library: wrapped_request.type,
    uri: wrapped_request.uri,
    procedure: wrapped_request.method
  )
  segment.add_request_headers(wrapped_request)  # <-- crashes when segment is nil
  # ...
end

start_external_request_segment in tracer.rb has:

def start_external_request_segment(library:, uri:, procedure:, start_time: nil, parent: nil)
  segment = Transaction::ExternalRequestSegment.new(library, uri, procedure, start_time)
  start_and_add_segment(segment, parent)
rescue ArgumentError
  raise
rescue => exception
  log_error('start_external_request_segment', exception)
  # implicitly returns nil
end

When any exception occurs during segment creation (e.g. URI parsing edge case, threading issue), the rescue swallows it, logs it, and returns nil. The HTTPX instrumentation doesn't guard against this.

Error:

NoMethodError: undefined method 'add_request_headers' for nil
  newrelic_rpm-9.23.0/lib/new_relic/agent/instrumentation/httpx/instrumentation.rb:26:in 'nr_start_segment'
  newrelic_rpm-9.23.0/lib/new_relic/agent/instrumentation/httpx/instrumentation.rb:13:in 'block in send_requests_with_tracing'
  newrelic_rpm-9.23.0/lib/new_relic/agent/instrumentation/httpx/instrumentation.rb:13:in 'Array#each'
  newrelic_rpm-9.23.0/lib/new_relic/agent/instrumentation/httpx/instrumentation.rb:13:in 'send_requests_with_tracing'
  newrelic_rpm-9.23.0/lib/new_relic/agent/instrumentation/httpx/prepend.rb:11:in 'send_requests'

Context: This occurs in a multi-threaded Passenger environment making concurrent HTTPX requests. The actual HTTP requests succeed (fired pixels are stored), but New Relic's instrumentation crashes during tracing setup.

Suggested fix:

Add a nil guard in nr_start_segment:

def nr_start_segment(request)
  return unless NewRelic::Agent::Tracer.state.is_execution_traced?

  wrapped_request = NewRelic::Agent::HTTPClients::HTTPXHTTPRequest.new(request)
  segment = NewRelic::Agent::Tracer.start_external_request_segment(
    library: wrapped_request.type,
    uri: wrapped_request.uri,
    procedure: wrapped_request.method
  )
  return unless segment  # <-- guard against nil

  segment.add_request_headers(wrapped_request)
  request.on(:response) { nr_finish_segment.call(request, segment) }
end

Workaround: Disable HTTPX instrumentation in newrelic.yml:

instrumentation:
  httpx: disabled

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-545596

[kaylareopelle]

Hi @thebravoman, thanks for bringing this to our attention! I really appreciate your thorough writeup and suggestions for a solution and a workaround.

I agree that the nil guard is the best way to fix this issue for the HTTPX instrumentation. I opened up #3510 with that fix. My team will take a look at the PR today and we'll have more for you soon.