pool: reopen connection closed by idle timeout

Signed-off-by: Harshit Gangal <harshit@planetscale.com>

go/pools/smartconnpool/pool.go

@@ -503,8 +503,16 @@ func (pool *ConnPool[C]) connReopen(ctx context.Context, dbconn *Pooled[C], now
 		return err
 	}
 
-	dbconn.timeUsed.set(now)
+	if dbconn.Conn.Setting() != nil {
+		err = dbconn.Conn.ApplySetting(ctx, dbconn.Conn.Setting())
+		if err != nil {
+			dbconn.Close()
+			return err
+		}
+	}
+
 	dbconn.timeCreated.set(now)
+	dbconn.timeUsed.set(now)
 	return nil
 }
 
@@ -764,7 +772,9 @@ func (pool *ConnPool[C]) closeIdleResources(now time.Time) {
 			if conn.timeUsed.expired(mono, timeout) {
 				pool.Metrics.idleClosed.Add(1)
 				conn.Close()
-				pool.closedConn()
+				if err := pool.connReopen(context.Background(), conn, monotonicNow()); err != nil {
+					pool.closedConn()
+				}
 			}
 		}
 	}

go/pools/smartconnpool/pool_test.go

@@ -608,6 +608,7 @@ func TestIdleTimeout(t *testing.T) {
 
 			conns = append(conns, r)
 		}
+		assert.GreaterOrEqual(t, state.open.Load(), int64(5))
 
 		// wait a long while; ensure that none of the conns have been closed
 		time.Sleep(1 * time.Second)
@@ -628,9 +629,15 @@ func TestIdleTimeout(t *testing.T) {
 				t.Fatalf("Connections remain open after 1 second")
 			}
 		}
+		// At least 5 connections should have been closed by now.
+		assert.GreaterOrEqual(t, p.Metrics.IdleClosed(), int64(5), "At least 5 connections should have been closed by now.")
 
-		// no need to assert anything: all the connections in the pool should are idle-closed
-		// now and if they're not the test will timeout and fail
+		// At any point, at least 4 connections should be open, with 1 either in the process of opening or already opened.
+		// The idle connection closer shuts down one connection at a time.
+		assert.GreaterOrEqual(t, state.open.Load(), int64(4))
+
+		// The number of available connections in the pool should remain at 5.
+		assert.EqualValues(t, 5, p.Available())
 	}
 
 	t.Run("WithoutSettings", func(t *testing.T) { testTimeout(t, nil) })