Add a benchmark to the test suite

This (ab)uses the integration tests to run a benchmark. This currently only
measures wibox drawing performance. To avoid wasting CPU-cycles, this does only
a quick run under travis while on "normal" runs the function under test is
executed in a loop to improve the precision of the measurement.

This benchmarks hopefully allow to optimize things in a clear fashion instead of
things like "it feels faster to me".

Results when run against the previous commit:

== Running test-benchmark.lua ==
        create wibox: 0.0788958  sec/iter ( 13 iters, 1.103 sec for benchmark)
    update textclock: 0.019493   sec/iter ( 56 iters, 2.507 sec for benchmark)
  relayout textclock: 0.0160725  sec/iter ( 63 iters, 1.029 sec for benchmark)
    redraw textclock: 0.0015601  sec/iter (647 iters, 1.875 sec for benchmark)
W: awesome: a_glib_poll:291: Last main loop iteration took 6.593912 seconds! Increasing limit for this warning to that value.

Results right before the new widget layouts were merged (commit 52154d0f15):

== Running test-benchmark.lua ==
        create wibox: 0.0782874  sec/iter ( 13 iters, 1.095 sec for benchmark)
    update textclock: 0.00736755 sec/iter (136 iters, 1.346 sec for benchmark)
W: awesome: luaA_dofunction:77: error while running function
[...]
error: /home/psychon/projects/awesome/build/lib/gears/object.lua:30: Trying to emit non-existent signal 'widget::layout_changed'

Closes https://github.com/awesomeWM/awesome/pull/451.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/test-benchmark.lua

@@ -0,0 +1,95 @@
+-- Some benchmarks that aren't really tests, but are included here anyway so
+-- that we notice if they break.
+
+local awful = require("awful")
+local wibox = require("wibox")
+local lgi = require("lgi")
+local GLib = lgi.GLib
+local cairo = lgi.cairo
+
+local not_under_travis = not os.getenv("CI")
+
+local measure, benchmark
+do
+    local timer_measure = GLib.Timer()
+    measure = function(f, iter)
+        timer_measure:start()
+        for i = 1, iter do
+            f()
+        end
+        local elapsed = timer_measure:elapsed()
+        return elapsed / iter, elapsed
+    end
+
+    local timer_benchmark = GLib.Timer()
+    benchmark = function(f, msg)
+        timer_benchmark:start()
+        local iters = 1
+        local time_per_iter, time_total = measure(f, iters)
+        -- To improve precision, we want to loop for this long
+        local target_time = 1
+        while time_total < target_time and not_under_travis do
+            iters = math.ceil(target_time / time_per_iter)
+            time_per_iter, time_total = measure(f, iters)
+        end
+        print(string.format("%20s: %-10.6g sec/iter (%3d iters, %.4g sec for benchmark)",
+                msg, time_per_iter, iters, timer_benchmark:elapsed()))
+    end
+end
+
+local function do_pending_repaint()
+    awesome.emit_signal("refresh")
+end
+
+local function create_wibox()
+    local img = cairo.ImageSurface(cairo.Format.ARGB32, 20, 20)
+
+    -- Widgets that are aligned to the left
+    local left_layout = wibox.layout.fixed.horizontal()
+    left_layout:add(awful.widget.launcher({ image = img, command = "bash" }))
+    left_layout:add(awful.widget.taglist(1, awful.widget.taglist.filter.all))
+    left_layout:add(awful.widget.prompt())
+
+    -- Widgets that are aligned to the right
+    local right_layout = wibox.layout.fixed.horizontal()
+    local textclock = awful.widget.textclock()
+    right_layout:add(textclock)
+    right_layout:add(awful.widget.layoutbox(1))
+
+    -- Now bring it all together (with the tasklist in the middle)
+    local layout = wibox.layout.align.horizontal()
+    layout:set_left(left_layout)
+    layout:set_middle(awful.widget.tasklist(1, awful.widget.tasklist.filter.currenttags))
+    layout:set_right(right_layout)
+
+    -- Create wibox
+    local wb = wibox({ width = 1024, height = 20, screen = 1 })
+    wb.visible = true
+    wb:set_widget(layout)
+
+    return wb, textclock
+end
+
+local wb, textclock = create_wibox()
+
+local function relayout_textclock()
+    textclock:emit_signal("widget::layout_changed")
+    do_pending_repaint()
+end
+
+local function redraw_textclock()
+    textclock:emit_signal("widget::redraw_needed")
+    do_pending_repaint()
+end
+
+local function update_textclock()
+    textclock:emit_signal("widget::updated")
+    do_pending_repaint()
+end
+
+benchmark(create_wibox, "create wibox")
+benchmark(update_textclock, "update textclock")
+benchmark(relayout_textclock, "relayout textclock")
+benchmark(redraw_textclock, "redraw textclock")
+
+require("_runner").run_steps({ function() return true end })