How to add function output to a variable using luasql - lua

Question

I am trying to assign the output of a function to a variable, but I can’t do it. :(
I use Lua 5.3.4 and luarocks 3.2.1 (+luasql-postgres)
My script:
luasql = require "luasql.postgres"
value=arg[1]
current_time=os.date("%Y-%m-%d %H:%M:%S")
env = luasql.postgres()
con = assert (env:connect('postgres', 'postgres', 'postgres', '192.168.241.93','5432'))
function printvalues(res)
row = res:fetch ({}, "a")
while row do
print(string.format("%-12s", row.client_addr))
row = res:fetch (row, "a")
end
print()
end
res = assert (con:execute('select client_addr from pg_stat_replication order by replay_lag asc limit 1'))
--txn = res
a = {myfunc = printvalues(res)}
if a == '192.168.242.41' then
print("backend1")
elseif a == '192.168.241.76' then
print("backend2")
end
print(string.format("%-12s",a))
print(a)
Script result:
root#haproxy:/etc/haproxy# lua scripts/test.lua
192.168.1.76
table: 0x559fadf97080
table: 0x559fadf97080
Please tell me:
How can I assign the result of a function to a variable?
How to remove the empty line in the output of the function printvalues(res)

Answer 1

you need to make a return statement in the function body to return a value
the empty line in your example is the print() statement in the function printvalues
I see you are making a query using limit 1, since you will only get one value, the while loop statement is useless.
I hope this works for you
local luasql = require "luasql.postgres"
local env = luasql.postgres()
local con = assert (env:connect(
'postgres', 'postgres', 'postgres', '192.168.241.93', '5432'
))
local res = assert(con:execute(
'select client_addr from pg_stat_replication order by replay_lag asc limit 1'
))
local row = res:fetch({}, 'a')
local a = string.format('%-12s', row.client_addr)
res:close()
if a == '192.168.242.41' then
print('backend1')
elseif a == '192.168.241.76' then
print('backend2')
end
print(a)
but if you want an example with function:
local luasql = require "luasql.postgres"
local env = luasql.postgres()
local con = assert (env:connect(
'postgres', 'postgres', 'postgres', '192.168.241.93', '5432'
))
local res = assert(con:execute(
'select client_addr from pg_stat_replication order by replay_lag asc limit 1'
))
local function printvalues(res)
local row = res:fetch({}, 'a')
res:close()
return string.format('%-12s', row.client_addr)
end
local a = printvalues(res)
if a == '192.168.242.41' then
print('backend1')
elseif a == '192.168.241.76' then
print('backend2')
end
print(a)
try to use local keyword before variables, this will make them scope variables

Answer 2

Thank. I ended up using this script:
luasql = require "luasql.postgres"
env = luasql.postgres()
con = assert (env:connect('postgres', 'postgres', 'postgres','333.333.333.333','5432')) --master
backend = function(txn)
res = assert (con:execute('select client_addr from pg_stat_replication order by replay_lag asc limit 1'))
row = res:fetch ({}, "a")
while row do
ip = string.format("%-12s", row.client_addr)
row = res:fetch (row, "a")
end
result = "backend1"
if ip == '111.111.111.111' then
result = "backend1"
elseif ip == '222.222.222.222' then
result = "backend2"
end
return result
end
core.register_fetches("choose_backend", backend)