battery-widget: Sanitize total charge computation

The code to parse the output of `acpi -i` is fragile as it does not
guarantee that the same number of battery status and capacities are
read. This causes a wrong computation of the total charge level when
batteries of status "Unknown" are reported.

For instance, this is what `acpi -i` reports with a Wacom Intuos S
attached to a Thinkpad T460s:

  $ acpi -i
  Battery 0: Unknown, 0%, rate information unavailable
  Battery 1: Not charging, 98%
  Battery 1: design capacity 1857 mAh, last full capacity 1513 mAh = 81%
  Battery 2: Not charging, 98%
  Battery 2: design capacity 2051 mAh, last full capacity 1711 mAh = 83%

Fix the code by guaranteeing that for each battery status parsed there
is also a capacity, which is zero if no capacity can be parsed. This
effectively causes the exclusion of such batteries when computing the
total charge level.

battery-widget/battery.lua

@@ -125,12 +125,21 @@ local function worker(user_args)
         local battery_info = {}
         local capacities = {}
         for s in stdout:gmatch("[^\r\n]+") do
+            -- Match a line with status and charge level
             local status, charge_str, _ = string.match(s, '.+: ([%a%s]+), (%d?%d?%d)%%,?(.*)')
             if status ~= nil then
+                -- Enforce that for each entry in battery_info there is an
+                -- entry in capacities of zero. If a battery has status
+                -- "Unknown" then there is no capacity reported and we treat it
+                -- as zero capactiy for later calculations.
                 table.insert(battery_info, {status = status, charge = tonumber(charge_str)})
-            else
+                table.insert(capacities, 0)
+            end
+
+            -- Match a line where capacity is reported
             local cap_str = string.match(s, '.+:.+last full capacity (%d+)')
-                table.insert(capacities, tonumber(cap_str))
+            if cap_str ~= nil then
+                capacities[#capacities] = tonumber(cap_str) or 0
             end
         end
 
@@ -144,6 +153,9 @@ local function worker(user_args)
                     -- this is arbitrary, and maybe another metric should be used
                 end
 
+                -- Adds up total (capacity-weighted) charge and total capacity.
+                -- It effectively ignores batteries with status "Unknown" as we
+                -- treat them with capacity zero.
                 charge = charge + batt.charge * capacities[i]
                 capacity = capacity + capacities[i]
             end