FIX weird blackout when certin batteries start discharging

src/modules/power.lua

@@ -26,6 +26,8 @@ return function(update_freq, config, common, width, point)
 
    local mk_rate_plot = function(label, address, y)
       local read_joules = sys.intel_powercap_reader(address)
+      local read_joules0 = function() return read_joules() or 0 end
+      local read_joulesNA = function() return read_joules() or 'N/A' end
       local obj = common.make_rate_timeseries(
          point.x,
          y,
@@ -38,12 +40,12 @@ return function(update_freq, config, common, width, point)
          label,
          0,
          update_freq,
-         read_joules()
+         read_joules0()
       )
       return common.mk_acc(
          width,
          plot_height + plot_sec_break,
-         function(_) common.update_rate_timeseries(obj, read_joules()) end,
+         function(_) common.update_rate_timeseries(obj, read_joulesNA()) end,
          mk_static(obj),
          mk_dynamic(obj)
       )
@@ -65,8 +67,11 @@ return function(update_freq, config, common, width, point)
    local mk_bat = function(y)
       local _read_battery_power = sys.battery_power_reader(config.battery)
 
+      -- TODO this is actually telling the plot to say it is on AC when the
+      -- battery status can't be read (in which case it should say ERROR or
+      -- something)
       local read_battery_power = function(is_using_ac)
-         return is_using_ac and 0 or _read_battery_power()
+         return is_using_ac and 0 or (_read_battery_power() or 0)
       end
       local read_bat_status = sys.battery_status_reader(config.battery)
       local obj = common.make_tagged_scaled_timeseries(

src/sys.lua

@@ -13,7 +13,12 @@ local dirname = function(s)
 end
 
 local read_micro = function(path)
-   return i_o.read_file(path, nil, '*n') * 0.000001
+   local j = i_o.read_file(path, nil, '*n')
+   if j == nil then
+      return nil
+   else
+      return j * 0.000001
+   end
 end
 
 local gmatch_to_table1 = function(pat, s)
@@ -110,7 +115,15 @@ end
 M.battery_power_reader = function(battery)
    local current = format_power_path(battery, 'current_now')
    local voltage = format_power_path(battery, 'voltage_now')
-   return function() return read_micro(current) * read_micro(voltage) end
+   return function()
+      local c = read_micro(current)
+      local v = read_micro(voltage)
+      if c == nil or v == nil then
+         return nil
+      else
+         return c * v
+      end
+   end
 end
 
 M.battery_status_reader = function(battery)