Add helper for efficiently spawning commands requiring internet access

It ensures that the command will be run at most once during the desired
`interval`, even if AwesomeWM restarts multiple times during this time.

config/awesome/evil/weather.lua

@@ -4,6 +4,7 @@
 --      description (string)
 --      icon_code (string)
 local awful = require("awful")
+local helpers = require("helpers")
 
 -- Configuration
 local key = user.openweathermap_key
@@ -31,8 +32,7 @@ local weather_details_script = [[
     fi
   ']]
 
--- Periodically get weather info
-awful.widget.watch(weather_details_script, update_interval, function(widget, stdout)
+helpers.remote_watch(weather_details_script, update_interval, "/tmp/awesomewm-evil-weather", function(stdout)
     local icon_code = string.sub(stdout, 1, 3)
     local weather_details = string.sub(stdout, 5)
     weather_details = string.gsub(weather_details, '^%s*(.-)%s*$', '%1')

config/awesome/helpers.lua

@@ -436,5 +436,46 @@ function helpers.screen_mask(s, bg)
     return mask
 end
 
+-- Useful for periodically checking the output of a command that
+-- requires internet access.
+-- Ensures that `command` will be run only once during the desired
+-- `interval`, even if awesome restarts multiple times during this time.
+-- Saves output in `output_file` and checks its last modification
+-- time to determine whether to run the command again or not.
+-- Passes the output of `command` to `callback` function.
+function helpers.remote_watch(command, interval, output_file, callback)
+
+    local run_the_thing = function()
+        -- Pass output to callback AND write it to file
+        awful.spawn.easy_async_with_shell(command.." | tee "..output_file, function(out) callback(out) end)
+    end
+
+    gears.timer {
+        timeout = interval,
+        call_now = true,
+        autostart = true,
+        single_shot = false,
+        callback = function()
+            awful.spawn.easy_async_with_shell("date -r "..output_file.." +%s", function(last_update, _, __, exitcode)
+
+                -- Probably the file does not exist yet (first time
+                -- running after reboot)
+                if exitcode == 1 then
+                    run_the_thing()
+                    return
+                end
+
+                local diff = os.time() - tonumber(last_update)
+                if diff > interval then
+                    run_the_thing()
+                else
+                    -- Pass the date saved in the file since it is fresh enough
+                    awful.spawn.easy_async_with_shell("cat "..output_file, function(out) callback(out) end)
+                end
+            end)
+    end
+    }
+end
+
 
 return helpers