`Server:grep_log()` doesn't work with stopped server

[locker]

Consider the following test case:

local t = require('luatest')
local server = require('luatest.server')

local g = t.group()

g.before_each(function(cg)
    cg.server = server.new()
    cg.server:start()
end)

g.after_each(function(cg)
    cg.server:drop()
end)

g.test_grep_log = function(cg)
    server:exec(function() require('log').info('test grep_log') end)
    t.assert(server:grep_log('test grep_log'))
    server:stop()
    t.assert(server:grep_log('test grep_log'))
end

It fails with:

Tarantool version is 3.3.0-entrypoint-169-gd8ebc818c936
Started on Tue Oct 22 14:10:17 2024
    server.test_grep_log_offline ... (0.126s) error
/home/vlad/src/tarantool/luatest/luatest/server.lua:883: net_box is not connected
=========================================================

Tests with errors:
------------------

1) server.test_grep_log_offline
/home/vlad/src/tarantool/luatest/luatest/server.lua:883: net_box is not connected
stack traceback:
        ...
        [C]: in function 'xpcall'
artifacts:
        server -> /tmp/t/artifacts/server-PvqKU6hNIooc
Ran 1 tests in 0.444 seconds, 0 succeeded, 1 errored, 29 not selected

=========================================================
Failed tests:

server.test_grep_log_offline

It's possible to work around this issue by explicitly passing the path to the log file in opts.filename but it's inconvenient. Server:grep_log() should work even if the server is stopped, at least with the default instance.

[Totktonada]

BTW, I've created a helper for my own test. Maybe it worth to consider it for inclusion into <luatest.server object>:exec().

-- Execute the given function on the given server and collect logs
-- produced during its execution.
--
-- Comparing to <server object>:exec() it has a limited
-- functionality:
--
-- * no arguments passing
-- * no return value receiving
-- * no luatest upvalue autorebinding
-- * no check that there are no other upvalues
--
-- Though, it is enough for the test cases below.
--
-- It would be nice to integrate the log capturing functionality
-- right into <server object>:exec() in a future. It would also be
-- nice to support logging to stderr, not just file, because
-- luatest anyway captures it.
--
-- Unlike <server object>:grep_log() this function is bounded to
-- a particular given function and leans on unique log markers, so
-- it is suitable in a situation, when several testing scenarios
-- are producing a similar output.
--
-- Also, it just feels more user-friendly.
local function log_capturing_exec(server, f)
    -- Generate markers to issue them before/after the execution
    -- of the provided function and look them up inside logs
    -- later.
    local start_marker = digest.urandom(40):hex()
    local end_marker = digest.urandom(40):hex()

    -- Execute the provided function (and issue the markers
    -- before/after it).
    local func_body = string.dump(f)
    server:exec(function(func_body, start_marker, end_marker)
        local log = require('log')
        local func = loadstring(func_body)
        log.info(start_marker)
        func()
        log.info(end_marker)
    end, {func_body, start_marker, end_marker})

    -- Determine a path to the log file of the given server.
    local file = fio.pathjoin(server.chdir, 'var/log', server.alias,
        'tarantool.log')

    -- Cut off a text between start and end markers: the outout
    -- of the provided function.
    local raw_log = t.helpers.retrying({timeout = 60}, function()
        -- Read the log file.
        local fh = assert(fio.open(file, {'O_RDONLY'}))
        local data = fh:read()
        fh:close()

        -- Look up positions of the start and the end markers.
        --
        -- We need the last position of the start marker and the
        -- first position of the end marker.
        local _, s = data:find(('%s\n'):format(start_marker))
        assert(s ~= nil, ('no test start marker: %s'):format(start_marker))
        local e = data:find(('\n[^\n]+%s'):format(end_marker), s)
        assert(e ~= nil, ('no test end marker: %s'):format(end_marker))

        -- All the text between them is what we're searching for.
        return data:sub(s + 1, e - 1)
    end)

    -- Split per line.
    --
    -- Remove all the columns except the log level and the
    -- message.
    local res = {}
    for _, line in ipairs(raw_log:split('\n')) do
        table.insert(res, line:match('.> .*$'))
    end
    return res
end