[client] fix port collision in TestUpload (#5950)

* [debug] fix port collision in TestUpload

TestUpload hardcoded :8080, so it failed deterministically when anything
was already on that port and collided across concurrent test runs.
Bind a :0 listener in the test to get a kernel-assigned free port, and
add Server.Serve so tests can hand the listener in without reaching
into unexported state.

* [debug] drop test-only Server.Serve, use SERVER_ADDRESS env

The previous commit added a Server.Serve method on the upload-server,
used only by TestUpload. That left production with an unused function.
Reserve an ephemeral loopback port in the test, release it, and pass
the address through SERVER_ADDRESS (which the server already reads).
A small wait helper ensures the server is accepting connections before
the upload runs, so the close/rebind gap does not cause a false failure.

client/internal/debug/upload_test.go

@@ -3,10 +3,12 @@ package debug
 import (
 	"context"
 	"errors"
+	"net"
 	"net/http"
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -19,8 +21,10 @@ func TestUpload(t *testing.T) {
 		t.Skip("Skipping upload test on docker ci")
 	}
 	testDir := t.TempDir()
-	testURL := "http://localhost:8080"
+	addr := reserveLoopbackPort(t)
+	testURL := "http://" + addr
 	t.Setenv("SERVER_URL", testURL)
+	t.Setenv("SERVER_ADDRESS", addr)
 	t.Setenv("STORE_DIR", testDir)
 	srv := server.NewServer()
 	go func() {
@@ -33,6 +37,7 @@ func TestUpload(t *testing.T) {
 			t.Errorf("Failed to stop server: %v", err)
 		}
 	})
+	waitForServer(t, addr)
 
 	file := filepath.Join(t.TempDir(), "tmpfile")
 	fileContent := []byte("test file content")
@@ -47,3 +52,30 @@ func TestUpload(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, fileContent, createdFileContent)
 }
+
+// reserveLoopbackPort binds an ephemeral port on loopback to learn a free
+// address, then releases it so the server under test can rebind. The close/
+// rebind window is racy in theory; on loopback with a kernel-assigned port
+// it's essentially never contended in practice.
+func reserveLoopbackPort(t *testing.T) string {
+	t.Helper()
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	addr := l.Addr().String()
+	require.NoError(t, l.Close())
+	return addr
+}
+
+func waitForServer(t *testing.T, addr string) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		c, err := net.DialTimeout("tcp", addr, 100*time.Millisecond)
+		if err == nil {
+			_ = c.Close()
+			return
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	t.Fatalf("server did not start listening on %s in time", addr)
+}