database/sql: close connection if db.numOpen > db.maxOpen

Bug Description:
When reduce db.maxOpen via db.SetMaxOpenConns, the unnecssary
connections won't been released until all other connections are free.

Fixes #9453

Change-Id: I9afb2e4b184139b31029ae53d7f5fd1fdb8d8d7e
Reviewed-on: https://go-review.googlesource.com/2200Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -782,6 +782,9 @@ func (db *DB) putConn(dc *driverConn, err error) {
 // If a connRequest was fulfilled or the *driverConn was placed in the
 // freeConn list, then true is returned, otherwise false is returned.
 func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
+	if db.maxOpen > 0 && db.numOpen > db.maxOpen {
+		return false
+	}
 	if c := len(db.connRequests); c > 0 {
 		req := db.connRequests[0]
 		// This copy is O(n) but in practice faster than a linked list.

src/database/sql/sql_test.go

@@ -1070,6 +1070,57 @@ func TestMaxOpenConns(t *testing.T) {
 	}
 }
 
+// Issue 9453: tests that SetMaxOpenConns can be lowered at runtime
+// and affects the subsequent release of connections.
+func TestMaxOpenConnsOnBusy(t *testing.T) {
+	defer setHookpostCloseConn(nil)
+	setHookpostCloseConn(func(_ *fakeConn, err error) {
+		if err != nil {
+			t.Errorf("Error closing fakeConn: %v", err)
+		}
+	})
+
+	db := newTestDB(t, "magicquery")
+	defer closeDB(t, db)
+
+	db.SetMaxOpenConns(3)
+
+	conn0, err := db.conn()
+	if err != nil {
+		t.Fatalf("db open conn fail: %v", err)
+	}
+
+	conn1, err := db.conn()
+	if err != nil {
+		t.Fatalf("db open conn fail: %v", err)
+	}
+
+	conn2, err := db.conn()
+	if err != nil {
+		t.Fatalf("db open conn fail: %v", err)
+	}
+
+	if g, w := db.numOpen, 3; g != w {
+		t.Errorf("free conns = %d; want %d", g, w)
+	}
+
+	db.SetMaxOpenConns(2)
+	if g, w := db.numOpen, 3; g != w {
+		t.Errorf("free conns = %d; want %d", g, w)
+	}
+
+	conn0.releaseConn(nil)
+	conn1.releaseConn(nil)
+	if g, w := db.numOpen, 2; g != w {
+		t.Errorf("free conns = %d; want %d", g, w)
+	}
+
+	conn2.releaseConn(nil)
+	if g, w := db.numOpen, 2; g != w {
+		t.Errorf("free conns = %d; want %d", g, w)
+	}
+}
+
 func TestSingleOpenConn(t *testing.T) {
 	db := newTestDB(t, "people")
 	defer closeDB(t, db)