Class: RuboCop::Cop::Sequra::PrometheusMetricLabels

Inherits:
Base
  • Object
show all
Extended by:
AutoCorrector
Defined in:
lib/rubocop/cop/sequra/prometheus_metric_labels.rb

Overview

Detects malformed label arguments on prometheus_exporter metric calls.

PrometheusExporter::Client::RemoteMetric takes labels as a positional hash, and the position differs per method:

increment(keys = nil, value = 1)
decrement(keys = nil, value = 1)
observe(value = 1, keys = nil)

None of them accept keyword arguments, so increment(labels: {...}) does not raise. Ruby folds the keyword into the positional keys hash and the exporter emits a series carrying a label literally named labels, whose value is the stringified inner hash. The result is valid Prometheus exposition text, so the scrape succeeds and the target stays healthy — sum by (outcome) simply never matches.

Nothing downstream can catch it: the exporter keys its series by the raw hash, labels_text interpolates label names without validating them, and opts: { labels: [...] } at registration is discarded outright for counters and gauges. There is no label schema anywhere, so a wrong label name is indistinguishable from a new one, and the mistake only surfaces when a query silently returns nothing.

Examples:

# bad - folded into a label named "labels"
counter.increment(labels: { outcome: :bound })

# good
counter.increment({ outcome: :bound })

# bad - observe takes the value first
histogram.observe({ outcome: :ok }, duration)

# good
histogram.observe(duration, { outcome: :ok })

# bad - increment takes the labels first
counter.increment(1, { outcome: :bound })

# good
counter.increment({ outcome: :bound }, 1)

# bad - freezes the mistake into a passing spec
expect(counter).to receive(:increment).with(labels: { outcome: :bound })

# good
expect(counter).to receive(:increment).with({ outcome: :bound })

Constant Summary collapse

MSG_LABELS_KEYWORD = 
"Pass Prometheus labels positionally, not as a `labels:` keyword. " \
"`increment`/`observe`/`decrement` take no keyword arguments, so this " \
"becomes a single series labelled `labels=\"{...}\"` and by-label " \
"queries never match. Use `increment({ outcome: :bound })`.".freeze
MSG_VALUE_FIRST = 
"`observe` takes the value first and the labels second: " \
"`observe(value, { outcome: :ok })`. A labels hash in the first position " \
"is recorded as the observed value.".freeze
MSG_LABELS_FIRST = 
"`%<method>s` takes the labels first and the value second: " \
"`%<method>s({ outcome: :ok }, 1)`. This is the reverse of `observe`.".freeze
MSG_STUBBED_LABELS = 
"Stubbing `increment` with a `labels:` keyword asserts a label shape " \
"prometheus_exporter never produces, so the spec stays green while " \
"the metric is broken. Match the positional hash instead: " \
"`.with({ outcome: :bound })`.".freeze
LABELS_FIRST_METHODS = 
[:increment, :decrement].freeze
VALUE_FIRST_METHODS = 
[:observe].freeze
METRIC_METHODS = 
(LABELS_FIRST_METHODS + VALUE_FIRST_METHODS).freeze

Instance Method Summary collapse

Instance Method Details

#on_send(node) ⇒ Object 



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# File 'lib/rubocop/cop/sequra/prometheus_metric_labels.rb', line 84

def on_send(node)
  check_stubbed_labels_keyword(node)

  return unless node.receiver && METRIC_METHODS.include?(node.method_name)

  # A `labels:` key is the more specific diagnosis, so when it is
  # present it wins and the positional checks stay quiet.
  return if check_labels_keyword(node)

  check_argument_order(node)
end