Fix awful.spawn to work with more LGI versions

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

lib/awful/spawn.lua

@@ -29,6 +29,45 @@ local protected_call = require("gears.protected_call")
 local spawn = {}
 
 
+local end_of_file
+do
+    -- API changes, bug fixes and lots of fun. Figure out how a EOF is signalled.
+    local input
+    if not pcall(function()
+        -- No idea when this API changed, but some versions expect a string,
+        -- others a table with some special(?) entries
+        input = Gio.DataInputStream.new(Gio.MemoryInputStream.new_from_data(""))
+    end) then
+        input = Gio.DataInputStream.new(Gio.MemoryInputStream.new_from_data({}))
+    end
+    local line, length = input:read_line()
+    if not line then
+        -- Fixed in 2016: NULL on the C side is transformed to nil in Lua
+        end_of_file = function(arg)
+            return not arg
+        end
+    elseif tostring(line) == "" and #line ~= length then
+        -- "Historic" behaviour for end-of-file:
+        -- - NULL is turned into an empty string
+        -- - The length variable is not initialized
+        -- It's highly unlikely that the uninitialized variable has value zero.
+        -- Use this hack to detect EOF.
+        end_of_file = function(arg1, arg2)
+            return #arg1 ~= arg2
+        end
+    else
+        assert(tostring(line) == "", "Cannot determine how to detect EOF")
+        -- The above uninitialized variable was fixed and thus length is
+        -- always 0 when line is NULL in C. We cannot tell apart an empty line and
+        -- EOF in this case.
+        require("gears.debug").print_warning("Cannot reliably detect EOF on an "
+                .. "GIOInputStream with this LGI version")
+        end_of_file = function(arg)
+            return tostring(arg) == ""
+        end
+    end
+end
+
 spawn.snid_buffer = {}
 
 function spawn.on_snid_callback(c)
@@ -219,7 +258,7 @@ function spawn.read_lines(input_stream, line_callback, done_callback, close)
             -- Error
             print("Error in awful.spawn.read_lines:", tostring(length))
             done()
-        elseif #line ~= length then
+        elseif end_of_file(line, length) then
             -- End of file
             done()
         else