webhooker/internal/middleware/metrics_method_test.go

package middleware_test

import (
	"net/http"
	"net/http/httptest"
	"strings"
	"testing"

	"github.com/google/uuid"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
	dto "github.com/prometheus/client_model/go"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
	"sneak.berlin/go/webhooker/internal/middleware"
)

const (
	// metricsProbeMethods is how many distinct invented method tokens
	// each cardinality assertion drives. The measurement on the issue
	// took 300 tokens from 106 exposition lines to 7,631 — about 25
	// permanent lines per token, never reclaimed — so a probe of this
	// size puts a regression thousands of lines over the bound rather
	// than leaving it to a rounding argument.
	metricsProbeMethods = 300

	// probeMethodLen is how many characters each invented method
	// token carries, matching the 12 the issue measured with.
	probeMethodLen = 12

	// methodLabel is the label these tests are about.
	methodLabel = "method"
)

// realMethods is the positive control's domain: the methods chi's
// router can match a route for, every one of which a client
// legitimately sends and every one of which must keep a series of its
// own. Bounding the label by collapsing these into one bucket would
// destroy the metric it is meant to protect.
func realMethods() []string {
	return []string{
		http.MethodConnect, http.MethodDelete, http.MethodGet,
		http.MethodHead, http.MethodOptions, http.MethodPatch,
		http.MethodPost, http.MethodPut, http.MethodTrace,
	}
}

// methodProbePath returns the one receiver path a method probe
// targets. Holding the path fixed leaves the method as the only
// dimension varying, so any series growth a probe produces is the
// method label's and nothing else's.
func methodProbePath() string {
	return "/webhook/" + uuid.NewString()
}

// inventedMethods returns n distinct RFC 9110 method tokens that no
// router will ever match: uppercase hex from a fresh UUID, which is
// both the shape and the length an unauthenticated flood would send.
// net/http accepts any token as a method, so every one of these
// reaches the metrics pipeline exactly as a real method does.
func inventedMethods(n int) []string {
	methods := make([]string, 0, n)

	for range n {
		token := strings.ToUpper(
			strings.ReplaceAll(uuid.NewString(), "-", ""),
		)
		methods = append(methods, token[:probeMethodLen])
	}

	return methods
}

// driveMethods sends one request per supplied method to a single
// fixed path.
func driveMethods(
	t *testing.T,
	h http.Handler,
	path string,
	methods []string,
) map[int]int {
	t.Helper()

	probes := make([]probe, 0, len(methods))

	for _, m := range methods {
		probes = append(probes, probe{method: m, path: path})
	}

	return drive(t, h, probes)
}

// methodLabels returns the set of distinct `method` values across
// every gathered series that carries the label at all. The inflight
// gauge does not carry it, and so contributes nothing rather than an
// empty-string member.
func methodLabels(families []*dto.MetricFamily) map[string]struct{} {
	seen := make(map[string]struct{})

	for _, fam := range families {
		for _, m := range fam.GetMetric() {
			for _, pair := range m.GetLabel() {
				if pair.GetName() == methodLabel {
					seen[pair.GetValue()] = struct{}{}
				}
			}
		}
	}

	return seen
}

// scrapeLines renders the registry through the same promhttp handler
// /metrics is mounted on and counts the sample lines it produced.
//
// This is the quantity the issue measured and the one a Prometheus
// server pays for on every scrape: one histogram label set is a
// single gathered series but around 25 lines of exposition, which is
// why 300 method tokens cost thousands of lines rather than hundreds.
func scrapeLines(t *testing.T, reg *prometheus.Registry) int {
	t.Helper()

	h := promhttp.HandlerFor(reg, promhttp.HandlerOpts{})
	req := httptest.NewRequestWithContext(
		t.Context(), http.MethodGet, "/metrics", nil,
	)
	w := httptest.NewRecorder()
	h.ServeHTTP(w, req)
	require.Equal(t, http.StatusOK, w.Code)

	lines := 0

	for line := range strings.SplitSeq(w.Body.String(), "\n") {
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}

		lines++
	}

	return lines
}

// TestMetrics_MethodSentinelIsTheRouteSentinel pins the convention
// rather than the mechanism. An unroutable method and an unmatched
// path are the same fact — a client-chosen token matching nothing
// this service registers — so they carry one spelling. Two spellings
// would read in a scrape as two different unmatched states.
func TestMetrics_MethodSentinelIsTheRouteSentinel(t *testing.T) {
	t.Parallel()

	assert.Equal(
		t,
		middleware.UnmatchedRouteConst,
		middleware.UnmatchedMethodConst,
		"the unmatched sentinel must have exactly one spelling",
	)
}

// TestMetrics_InventedMethodsMintOneLabelSet is the direct assertion
// the issue asks for: N requests carrying N distinct invented method
// tokens must produce exactly ONE method label. Before the fix this
// produced N of them, on an unauthenticated route with no rate
// limiter.
func TestMetrics_InventedMethodsMintOneLabelSet(t *testing.T) {
	t.Parallel()

	h, reg := metricsTestRouter(t, generousReceiverLimit)

	methods := inventedMethods(metricsProbeMethods)

	codes := driveMethods(t, h, methodProbePath(), methods)
	require.Equal(
		t, metricsProbeMethods, codes[http.StatusMethodNotAllowed],
		"every invented token should have been unroutable",
	)

	labels := methodLabels(gatherMetrics(t, reg))

	// Asserted on the count rather than on the set, so that a
	// regression reports one number instead of dumping every token it
	// minted.
	distinct := len(labels)

	assert.Equal(
		t, 1, distinct,
		"invented methods must collapse onto one label",
	)
	assert.Contains(
		t, keys(labels), middleware.UnmatchedMethodConst,
		"that one label must be the unmatched sentinel",
	)

	// The scrape must not republish the tokens it was driven with
	// either: a label that merely looks bounded while still echoing
	// client bytes is the same defect wearing a different name.
	echoed := 0

	for _, m := range methods {
		for label := range labels {
			if strings.Contains(label, m) {
				echoed++
			}
		}
	}

	assert.Equal(
		t, 0, echoed,
		"invented method tokens reached the metrics labels",
	)
}

// TestMetrics_MethodSeriesCountIsFlatUnderAFlood reproduces the
// measurement on the issue in miniature: scrape, drive several
// hundred distinct method tokens, scrape again, and require the
// second scrape to be no larger than the first. The first batch
// establishes every label set the route can produce; a flood five
// times its size must land on exactly those.
func TestMetrics_MethodSeriesCountIsFlatUnderAFlood(t *testing.T) {
	t.Parallel()

	h, reg := metricsTestRouter(t, generousReceiverLimit)

	path := methodProbePath()

	driveMethods(t, h, path, inventedMethods(metricsProbeMethods))
	seededSeries := seriesCount(gatherMetrics(t, reg))
	seededLines := scrapeLines(t, reg)

	driveMethods(t, h, path, inventedMethods(metricsProbeMethods*4))
	floodedSeries := seriesCount(gatherMetrics(t, reg))
	floodedLines := scrapeLines(t, reg)

	t.Logf(
		"after %d invented methods: %d series, %d lines; "+
			"after %d more: %d series, %d lines",
		metricsProbeMethods, seededSeries, seededLines,
		metricsProbeMethods*4, floodedSeries, floodedLines,
	)

	assert.Equal(
		t, seededSeries, floodedSeries,
		"a flood of invented methods must not mint series",
	)
	assert.Equal(
		t, seededLines, floodedLines,
		"a flood of invented methods must not grow the scrape",
	)
}

// TestMetrics_RealMethodsStayDistinct is the positive control. The
// bound is worth nothing if it is bought by flattening the metric:
// every method the router can route must still carry a series of its
// own, one sample each, under the route pattern it was sent to.
func TestMetrics_RealMethodsStayDistinct(t *testing.T) {
	t.Parallel()

	h, reg := metricsTestRouter(t, generousReceiverLimit)

	methods := realMethods()

	codes := driveMethods(t, h, methodProbePath(), methods)
	require.Equal(
		t, len(methods), codes[http.StatusNotFound],
		"every real method should have reached the receiver",
	)

	families := gatherMetrics(t, reg)

	want := make(map[string]struct{}, len(methods))
	for _, m := range methods {
		want[m] = struct{}{}
	}

	assert.Equal(
		t, want, methodLabels(families),
		"real methods must remain distinguishable",
	)

	// Appearing somewhere in the scrape is not enough: each method
	// must own its duration series, holding the one sample it sent.
	observed := 0

	for _, fam := range families {
		if !strings.HasSuffix(fam.GetName(), "request_duration_seconds") {
			continue
		}

		for _, m := range fam.GetMetric() {
			observed++

			assert.Equal(
				t, receiverRoutePattern,
				labelValue(m, "handler"),
			)
			assert.Equal(
				t, uint64(1),
				m.GetHistogram().GetSampleCount(),
				"method %q shares a series",
				labelValue(m, methodLabel),
			)
		}
	}

	assert.Equal(
		t, len(methods), observed,
		"one duration series per routable method",
	)
}