Improve test-coverage on blobplugin

pkg/blobplugin/main_test.go

@@ -17,11 +17,15 @@ limitations under the License.
 package main
 
 import (
+	"context"
 	"fmt"
+	"io"
 	"net"
+	"net/http"
 	"os"
 	"reflect"
 	"testing"
+	"time"
 )
 
 func TestMain(t *testing.T) {
@@ -80,3 +84,97 @@ func TestTrapClosedConnErr(t *testing.T) {
 		}
 	}
 }
+
+func TestServeMetrics(t *testing.T) {
+	// Open random test port
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+
+	// Start serveMetrics in background
+	errCh := make(chan error, 1)
+	go func() { errCh <- serveMetrics(l) }()
+
+	// Build URL
+	url := "http://" + l.Addr().String() + "/metrics"
+
+	// Client timeout for each request
+	client := &http.Client{Timeout: 500 * time.Millisecond}
+
+	// Poll with 3-second deadlines until server is ready
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	done := make(chan struct{})
+	go func(ctx context.Context) {
+		defer close(done)
+		for {
+			req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+			// Execute probe, expecting status 200
+			resp, err := client.Do(req)
+			if err == nil {
+				if resp.StatusCode == http.StatusOK {
+					resp.Body.Close()
+					return
+				}
+				resp.Body.Close()
+			}
+			// Abort probe if context deadline is expired or canceled
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				time.Sleep(20 * time.Millisecond)
+			}
+		}
+	}(ctx)
+
+	// Wait for readiness or fail on timeout
+	select {
+	case <-done:
+	case <-ctx.Done():
+		t.Fatalf("server not ready: %v", ctx.Err())
+	}
+
+	// Perform the request
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("Get /metrics: %v", err)
+	}
+	t.Cleanup(func() {
+		if resp != nil && resp.Body != nil {
+			_ = resp.Body.Close()
+		}
+	})
+
+	// Check for HTTP status 200
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("unexpected status: %d", resp.StatusCode)
+	}
+	// Validate response body is non-empty
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+	// Validate response body is non-empty
+	if len(body) == 0 {
+		t.Fatalf("empty metrics body")
+	}
+
+	// Trigger graceful shutdown
+	if err := l.Close(); err != nil {
+		t.Fatalf("close listener %v", err)
+	}
+
+	// Fail if errCh exits non-graceful or is not closed after 2 sec
+	select {
+	case err := <-errCh:
+		if err != nil {
+			t.Fatalf("serveMetrics error after close: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatalf("serveMetrics did not exit after listener close")
+	}
+}