1 files changed, 41 insertions, 0 deletions
diff --git a/internal/cli/pprof_test.go b/internal/cli/pprof_test.go
new file mode 100644
index 0000000..aa1cd05
--- /dev/null
+++ b/internal/cli/pprof_test.go
@@ -0,0 +1,41 @@
+package cli
+
+import (
+	"runtime"
+	"testing"
+)
+
+// TestEnableProfilingRatesSetsMutexFraction verifies that EnableProfilingRates
+// actually turns on mutex profiling. Without this the /debug/pprof/mutex
+// endpoint reports an empty profile. runtime.SetMutexProfileFraction(-1) reads
+// the current fraction without changing it, so it lets us assert the state.
+func TestEnableProfilingRatesSetsMutexFraction(t *testing.T) {
+	// Save and restore global runtime state so this test does not leak into
+	// other tests in the package. The block profile rate has no getter, so we
+	// simply disable it again on cleanup.
+	prevMutex := runtime.SetMutexProfileFraction(-1)
+	t.Cleanup(func() {
+		runtime.SetMutexProfileFraction(prevMutex)
+		runtime.SetBlockProfileRate(0)
+	})
+
+	// Start from a known-disabled state.
+	runtime.SetMutexProfileFraction(0)
+
+	EnableProfilingRates()
+
+	if got := runtime.SetMutexProfileFraction(-1); got != mutexProfileFraction {
+		t.Errorf("mutex profile fraction = %d, want %d", got, mutexProfileFraction)
+	}
+}
+
+// TestProfilingRateConstants pins the ss0-validated values so a change is a
+// conscious decision rather than an accident.
+func TestProfilingRateConstants(t *testing.T) {
+	if mutexProfileFraction != 5 {
+		t.Errorf("mutexProfileFraction = %d, want 5", mutexProfileFraction)
+	}
+	if blockProfileRateNanos != 100000 {
+		t.Errorf("blockProfileRateNanos = %d, want 100000", blockProfileRateNanos)
+	}
+}