dashboard: share Spanner clients

Instead of creating a new Spanner client/connection per each DB
operation, cache the clients per each appID. In normal operation,
there will be just one appID and one client. For tests, we create
a different appID per each NewSpannerCtx anyway.

Author: a-nogikh

dashboard/app/aidb/crud.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"reflect"
 	"strings"
+	"sync"
 	"time"
 
 	"cloud.google.com/go/spanner"
@@ -236,17 +237,37 @@ func selectOne[T any](ctx context.Context, stmt spanner.Statement) (*T, error) {
 	return all[0], nil
 }
 
+var clients sync.Map // map[string]*spanner.Client
+
 func dbClient(ctx context.Context) (*spanner.Client, error) {
+	appID := appengine.AppID(ctx)
+	if v, ok := clients.Load(appID); ok {
+		return v.(*spanner.Client), nil
+	}
 	path := fmt.Sprintf("projects/%v/instances/%v/databases/%v",
-		appengine.AppID(ctx), Instance, Database)
-	// TODO(dvyukov): create a persistent client with a pool of connections for prod,
-	// but keep transient/per-test clients for tests.
-	return spanner.NewClientWithConfig(ctx, path, spanner.ClientConfig{
+		appID, Instance, Database)
+	client, err := spanner.NewClientWithConfig(context.Background(), path, spanner.ClientConfig{
 		SessionPoolConfig: spanner.SessionPoolConfig{
 			MinOpened: 1,
-			MaxOpened: 1,
+			MaxOpened: 20,
 		},
 	})
+	if err != nil {
+		return nil, err
+	}
+	if actual, loaded := clients.LoadOrStore(appID, client); loaded {
+		// The client was already created in parallel, close it.
+		client.Close()
+		return actual.(*spanner.Client), nil
+	}
+	return client, nil
+}
+
+func CloseClient(ctx context.Context) {
+	appID := appengine.AppID(ctx)
+	if v, ok := clients.LoadAndDelete(appID); ok {
+		v.(*spanner.Client).Close()
+	}
 }
 
 var TimeNow = func(ctx context.Context) time.Time {

dashboard/app/util_test.go

@@ -301,6 +301,7 @@ func (ctx *Ctx) Close() {
 			}
 		}
 	}
+	aidb.CloseClient(ctx.ctx)
 	unregisterContext(ctx)
 	validateGlobalConfig()
 }