database/sql: correctly report MaxIdleClosed stat

Previously the MaxIdleClosed counter was incremented when added
to the free connection list, rather then when it wasn't added
to the free connection list. Flip this logic to correct.

Fixes #27792

Change-Id: I405302c14fb985369dab48fbe845e5651afc4ccf
Reviewed-on: https://go-review.googlesource.com/c/138578
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.go

@@ -1322,12 +1322,14 @@ func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
 			err:  err,
 		}
 		return true
-	} else if err == nil && !db.closed && db.maxIdleConnsLocked() > len(db.freeConn) {
+	} else if err == nil && !db.closed {
+		if db.maxIdleConnsLocked() > len(db.freeConn) {
 			db.freeConn = append(db.freeConn, dc)
-		db.maxIdleClosed++
 			db.startCleanerLocked()
 			return true
 		}
+		db.maxIdleClosed++
+	}
 	return false
 }
 

src/database/sql/sql_test.go

@@ -3415,6 +3415,58 @@ func TestConnectionLeak(t *testing.T) {
 	wg.Wait()
 }
 
+func TestStatsMaxIdleClosedZero(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	db.SetMaxOpenConns(1)
+	db.SetMaxIdleConns(1)
+	db.SetConnMaxLifetime(0)
+
+	preMaxIdleClosed := db.Stats().MaxIdleClosed
+
+	for i := 0; i < 10; i++ {
+		rows, err := db.Query("SELECT|people|name|")
+		if err != nil {
+			t.Fatal(err)
+		}
+		rows.Close()
+	}
+
+	st := db.Stats()
+	maxIdleClosed := st.MaxIdleClosed - preMaxIdleClosed
+	t.Logf("MaxIdleClosed: %d", maxIdleClosed)
+	if maxIdleClosed != 0 {
+		t.Fatal("expected 0 max idle closed conns, got: ", maxIdleClosed)
+	}
+}
+
+func TestStatsMaxIdleClosedTen(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	db.SetMaxOpenConns(1)
+	db.SetMaxIdleConns(0)
+	db.SetConnMaxLifetime(0)
+
+	preMaxIdleClosed := db.Stats().MaxIdleClosed
+
+	for i := 0; i < 10; i++ {
+		rows, err := db.Query("SELECT|people|name|")
+		if err != nil {
+			t.Fatal(err)
+		}
+		rows.Close()
+	}
+
+	st := db.Stats()
+	maxIdleClosed := st.MaxIdleClosed - preMaxIdleClosed
+	t.Logf("MaxIdleClosed: %d", maxIdleClosed)
+	if maxIdleClosed != 10 {
+		t.Fatal("expected 0 max idle closed conns, got: ", maxIdleClosed)
+	}
+}
+
 type nvcDriver struct {
 	fakeDriver
 	skipNamedValueCheck bool