Naughty: Catch invalid markup in notifications

Previously, an invalid markup caused an empty popup. Since the C core now throws
a lua error on invalid markup, we have a way to notice that something is wrong.

This patch first tries to set the notification's text the same way we did
previously. If that fails, everything is escaped and the result is used as the
text for the notification.

Thanks to farhaven/Gregor Best for the initial version of this and for the
string.gsub() call I stole from him. :)

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

lib/naughty.lua.in

@@ -9,6 +9,7 @@ local pairs = pairs
 local table = table
 local type = type
 local string = string
+local pcall = pcall
 local capi = { screen = screen,
                awesome = awesome,
                dbus = dbus,
@@ -354,9 +355,19 @@ function notify(args)
     local textbox = capi.widget({ type = "textbox", align = "flex" })
     textbox:buttons(util.table.join(button({ }, 1, run), button({ }, 3, die)))
     layout.margins[textbox] = { right = margin, left = margin, bottom = margin, top = margin }
-    textbox.text = string.format('<span font_desc="%s"><b>%s</b>%s</span>', font, title, text:gsub("<br.->", "\n"))
     textbox.valign = "middle"
 
+    local function setText(pattern, replacements)
+        textbox.text = string.format('<span font_desc="%s"><b>%s</b>%s</span>', font, title, text:gsub(pattern, replacements))
+    end
+
+    -- First try to set the text while only interpreting <br>.
+    -- (Setting a textbox' .text to an invalid pattern throws a lua error)
+    if not pcall(setText, "<br.->", "\n") then
+        -- That failed, escape everything which might cause an error from pango
+        setText("[<>&]", { ['<'] = "&lt;", ['>'] = "&gt;", ['&'] = "&amp;" })
+    end
+
     -- create iconbox
     local iconbox = nil
     if icon then