test: fix tests on broken connections

[Totktonada]

Just observed that my test cases for broken connections actually act on healthy connections. I mean the following cases:

mysql/test/mysql.test.lua

Lines 96 to 114 in 2d22046

	-- Case: the same, but for broken connection.
	test:test('loss a broken connection', function(test)
	test:plan(2)
	assert(pool.size >= 1, 'test case precondition fails')
	-- Get a connection, make a bad query and loss the
	-- connection.
	local conn = pool:get()
	local ok = pcall(conn.execute, conn, 'bad query')
	test:ok(not ok, 'a query actually fails')
	conn = nil -- luacheck: no unused
	-- Collect the lost connection.
	collectgarbage('collect')
	-- Verify that a pool is aware of collected connections.
	test:is(pool.queue:count(), pool.size, 'all connections are put back')
	end)

mysql/test/mysql.test.lua

Lines 312 to 354 in 2d22046

	-- Case: get a connection, broke it and put back.
	test:test('get, broke and put a connection', function(test)
	test:plan(2)
	assert(pool.size >= 1, 'test case precondition fails')
	-- Get a connection and make a bad query.
	local conn = pool:get()
	local ok = pcall(conn.execute, conn, 'bad query')
	test:ok(not ok, 'a query actually fails')
	-- Put the connection back and verify that the pool is full.
	pool:put(conn)
	test:ok(pool.queue:is_full(), 'a broken connection was given back')
	end)
	-- Case: the same, but loss and collect a connection after
	-- put.
	test:test('get, broke, put and loss a connection', function(test)
	test:plan(3)
	assert(pool.size >= 1, 'test case precondition fails')
	-- Get a connection and make a bad query.
	local conn = pool:get()
	local ok = pcall(conn.execute, conn, 'bad query')
	test:ok(not ok, 'a query actually fails')
	-- Put the connection back, loss it and trigger GC.
	pool:put(conn)
	conn = nil -- luacheck: no unused
	collectgarbage('collect')
	-- Verify that the pool is full
	test:ok(pool.queue:is_full(), 'a broken connection was given back')
	-- Verify the pool will not be populated by a connection's
	-- GC callback. Otherwise :put() will hang.
	local item = pool.queue:get()
	pool.queue:put(item)
	test:ok(true, 'GC callback does not put back a connection that was ' ..
	'put manually')
	end)

My initial assumption was that any SQL error (including syntax error) will lead to marking the connection as broken, however it is not so. Only CR_SERVER_LOST or CR_SERVER_GONE_ERROR errors matter here:

mysql/mysql/driver.c

Lines 149 to 157 in 2d22046

	int err = mysql_errno(raw_conn);
	switch (err) {
	case CR_SERVER_LOST:
	case CR_SERVER_GONE_ERROR:
	lua_pushnumber(L, -1);
	break;
	default:
	lua_pushnumber(L, 1);
	}

mysql/mysql/init.lua

Lines 71 to 74 in 2d22046

	if status ~= 0 then
	self.queue:put(status > 0)
	return error(datas)
	end

It seems we should find a way to broke a connection from a test. It seems that support of an error injection is needed for Debug build.