profiler: test random execution trace collection with a fixed seed

To de-flake TestExecutionTraceRandom, provide a fixed-seed random number
generator so that the results are deterministic. This is done through a
non-exported profiler option so it's easy to provide in specific test
cases (only one so far). Developers should remove this option while
working on anything that might rely on real randomness, verify that it
works as intended, and then add the option back to get reliable tests in
CI.

Fixes #2529

profiler/options.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"math/rand"
 	"net"
 	"net/http"
 	"net/url"
@@ -111,6 +112,7 @@ type config struct {
 	logStartup           bool
 	traceConfig          executionTraceConfig
 	endpointCountEnabled bool
+	rng                  *rand.Rand
 }
 
 // logStartup records the configuration to the configured logger in JSON format
@@ -574,3 +576,10 @@ func WithCustomProfilerLabelKeys(keys ...string) Option {
 		cfg.customProfilerLabels = append(cfg.customProfilerLabels, keys...)
 	}
 }
+
+// withRNG provides a seeded RNG, for reliable testing
+func withRNG(r *rand.Rand) Option {
+	return func(cfg *config) {
+		cfg.rng = r
+	}
+}

profiler/profiler.go

@@ -126,6 +126,13 @@ func (p *profiler) lookupProfile(name string, w io.Writer, debug int) error {
 	return prof.WriteTo(w, debug)
 }
 
+func (p *profiler) randFloat64() float64 {
+	if p.cfg.rng != nil {
+		return p.cfg.rng.Float64()
+	}
+	return rand.Float64()
+}
+
 // newProfiler creates a new, unstarted profiler.
 func newProfiler(opts ...Option) (*profiler, error) {
 	if os.Getenv("AWS_LAMBDA_FUNCTION_NAME") != "" {
@@ -319,7 +326,7 @@ func (p *profiler) collect(ticker <-chan time.Time) {
 		// we will always record a trace
 		// We do multiplication here instead of division to defensively guard against
 		// division by 0
-		shouldTraceRandomly := rand.Float64()*float64(p.cfg.traceConfig.Period) < float64(p.cfg.period)
+		shouldTraceRandomly := p.randFloat64()*float64(p.cfg.traceConfig.Period) < float64(p.cfg.period)
 		// As a special case, we want to trace during the first
 		// profiling cycle since startup activity is generally much
 		// different than regular operation

profiler/profiler_test.go

@@ -531,14 +531,13 @@ func TestExecutionTraceMisconfiguration(t *testing.T) {
 }
 
 func TestExecutionTraceRandom(t *testing.T) {
-	t.Skip("flaky test, see: https://github.com/DataDog/dd-trace-go/issues/2529")
-
 	collectTraces := func(t *testing.T, profilePeriod, tracePeriod time.Duration, count int) int {
 		t.Setenv("DD_PROFILING_EXECUTION_TRACE_ENABLED", "true")
 		t.Setenv("DD_PROFILING_EXECUTION_TRACE_PERIOD", tracePeriod.String())
 		profiles := startTestProfiler(t, 10,
 			WithProfileTypes(),
 			WithPeriod(profilePeriod),
+			withRNG(rand.New(rand.NewSource(0))),
 		)
 
 		seenTraces := 0