test: benchmark the installation token cache hit

benchmark_test.go

@@ -0,0 +1,67 @@
+package githubauth
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/json"
+	"encoding/pem"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// BenchmarkInstallationTokenSource_CacheHit measures the path almost every
+// Token() call takes: a cached token that is still valid. The call count is
+// asserted afterwards so a regression that turned cache hits back into HTTP
+// requests would fail the benchmark rather than quietly reporting a slower
+// number.
+func BenchmarkInstallationTokenSource_CacheHit(b *testing.B) {
+	var calls atomic.Int64
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		calls.Add(1)
+		w.WriteHeader(http.StatusCreated)
+		_ = json.NewEncoder(w).Encode(InstallationToken{
+			Token:     "ghs_benchmark",
+			ExpiresAt: time.Now().Add(time.Hour),
+		})
+	}))
+	defer srv.Close()
+
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		b.Fatalf("generating key: %v", err)
+	}
+	keyPEM := pem.EncodeToMemory(&pem.Block{
+		Type:  "RSA PRIVATE KEY",
+		Bytes: x509.MarshalPKCS1PrivateKey(key),
+	})
+
+	app, err := NewApplicationTokenSource("Iv1.testclientid", keyPEM)
+	if err != nil {
+		b.Fatalf("NewApplicationTokenSource() error = %v", err)
+	}
+	src := NewInstallationTokenSource(1, app, WithBaseURL(srv.URL))
+
+	// Prime the cache so the loop measures hits, not the initial mint.
+	if _, err := src.Token(); err != nil {
+		b.Fatalf("priming Token() error = %v", err)
+	}
+
+	b.ReportAllocs()
+
+	for b.Loop() {
+		if _, err := src.Token(); err != nil {
+			b.Fatalf("Token() error = %v", err)
+		}
+	}
+
+	b.StopTimer()
+
+	if got := calls.Load(); got != 1 {
+		b.Fatalf("benchmark made %d installation-token requests, want 1: the loop was not measuring cache hits", got)
+	}
+}