database/sql: add SetConnMaxIdleTime

Allow removing a connection from the connection pool after
it has been idle for a period of time, without regard to the
total lifespan of the connection.

Fixes #25232

Change-Id: Icff157b906769a2d2d45c67525e04a72feb8d832
Reviewed-on: https://go-review.googlesource.com/c/go/+/145758
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -3593,6 +3593,61 @@ func TestStatsMaxIdleClosedTen(t *testing.T) {
 	}
 }
 
+func TestMaxIdleTime(t *testing.T) {
+	list := []struct {
+		wantMaxIdleTime time.Duration
+		wantIdleClosed  int64
+		timeOffset      time.Duration
+	}{
+		{time.Nanosecond, 1, 10 * time.Millisecond},
+		{time.Hour, 0, 10 * time.Millisecond},
+	}
+	baseTime := time.Unix(0, 0)
+	defer func() {
+		nowFunc = time.Now
+	}()
+	for _, item := range list {
+		nowFunc = func() time.Time {
+			return baseTime
+		}
+		t.Run(fmt.Sprintf("%v", item.wantMaxIdleTime), func(t *testing.T) {
+			db := newTestDB(t, "people")
+			defer closeDB(t, db)
+
+			db.SetMaxOpenConns(1)
+			db.SetMaxIdleConns(1)
+			db.SetConnMaxIdleTime(item.wantMaxIdleTime)
+			db.SetConnMaxLifetime(0)
+
+			preMaxIdleClosed := db.Stats().MaxIdleTimeClosed
+
+			if err := db.Ping(); err != nil {
+				t.Fatal(err)
+			}
+
+			nowFunc = func() time.Time {
+				return baseTime.Add(item.timeOffset)
+			}
+
+			db.mu.Lock()
+			closing := db.connectionCleanerRunLocked()
+			db.mu.Unlock()
+			for _, c := range closing {
+				c.Close()
+			}
+			if g, w := int64(len(closing)), item.wantIdleClosed; g != w {
+				t.Errorf("got: %d; want %d closed conns", g, w)
+			}
+
+			st := db.Stats()
+			maxIdleClosed := st.MaxIdleTimeClosed - preMaxIdleClosed
+			if g, w := maxIdleClosed, item.wantIdleClosed; g != w {
+				t.Errorf(" got: %d; want %d max idle closed conns", g, w)
+			}
+		})
+	}
+}
+
 type nvcDriver struct {
 	fakeDriver
 	skipNamedValueCheck bool