promhttp: add regression test for concurrent map writes (#1274) (#2000)

* promhttp: add regression test for concurrent map writes (#1274)

Without the fix in #1318, instrumenting a metric vec partitioned only by
labels supplied via WithLabelFromCtx (i.e. with no "code" or "method"
label) would cause concurrent requests to share and mutate a single
'emptyLabels' map, producing 'fatal error: concurrent map writes' under
load. The fix made labels() allocate a fresh map per call but no
regression test was added at the time.

This adds a test that exercises both InstrumentHandlerCounter and
InstrumentHandlerDuration from many goroutines so the bug would be
caught either by the race detector or the runtime's concurrent-map-
writes detection.

Verified: the test fails (DATA RACE / concurrent map writes panic) when
the pre-#1318 labels() implementation is reintroduced, and passes on
current main with 'go test -race'.

Closes #1274

Signed-off-by: Pedram <pedram@arpdigital.io>

* Update prometheus/promhttp/instrument_server_test.go

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>
Signed-off-by: Pedram Pd <eragon.pedy@gmail.com>

---------

Signed-off-by: Pedram <pedram@arpdigital.io>
Signed-off-by: Pedram Pd <eragon.pedy@gmail.com>
Co-authored-by: Pedram <pedram@arpdigital.io>
Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

Author: 3 people

prometheus/promhttp/instrument_server_test.go

@@ -19,6 +19,7 @@ import (
 	"log"
 	"net/http"
 	"net/http/httptest"
+	"sync"
 	"testing"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -523,6 +524,60 @@ func TestInterfaceUpgrade(t *testing.T) {
 	}
 }
 
+// Regression test against https://github.com/prometheus/client_golang/pull/1318
+func TestInstrumentHandlerLabelFromCtxConcurrent(t *testing.T) {
+	const (
+		workers           = 32
+		requestsPerWorker = 200
+	)
+
+	counter := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "test_requests_total",
+			Help: "A counter for requests to the wrapped handler.",
+		},
+		[]string{"dyn"},
+	)
+	histogram := prometheus.NewHistogramVec(
+		prometheus.HistogramOpts{
+			Name: "test_request_duration_seconds",
+			Help: "A histogram of latencies for requests to the wrapped handler.",
+		},
+		[]string{"dyn"},
+	)
+
+	reg := prometheus.NewRegistry()
+	reg.MustRegister(counter, histogram)
+
+	next := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	dyn := WithLabelFromCtx("dyn", func(_ context.Context) string {
+		return "v"
+	})
+
+	chain := InstrumentHandlerCounter(
+		counter,
+		InstrumentHandlerDuration(histogram, next, dyn),
+		dyn,
+	)
+
+	var wg sync.WaitGroup
+	wg.Add(workers)
+	for i := 0; i < workers; i++ {
+		go func() {
+			defer wg.Done()
+			for j := 0; j < requestsPerWorker; j++ {
+				req := httptest.NewRequest(http.MethodGet, "http://example.com/", nil)
+				rec := httptest.NewRecorder()
+				chain.ServeHTTP(rec, req)
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 func ExampleInstrumentHandlerDuration() {
 	inFlightGauge := prometheus.NewGauge(prometheus.GaugeOpts{
 		Name: "in_flight_requests",