--- a/hglib.lua	Fri Sep 09 12:21:28 2016 +0800
+++ b/hglib.lua	Fri Sep 09 12:24:59 2016 +0800
@@ -2,6 +2,15 @@
 
 local sizes = { i4 = 4, u4 = 4, c = 1 }
 
+local function write_u4(wh, value)
+	local bytes = ''
+	for _ = 1, sizes.u4 do
+		bytes = string.char(value % (2 ^ 8)) .. bytes
+		value = math.floor(value / (2 ^ 8))
+	end
+	wh:write(bytes)
+end
+
 local function read_u4(rh)
 	local value = 0
 	for i = 1, sizes.u4 do
@@ -37,6 +46,12 @@
 	return value
 end
 
+local function write_block(wh, ...)
+	local block = table.concat({...}, '\0')
+	write_u4(wh, #block)
+	wh:write(block)
+end
+
 local Client = {}
 Client.__index = Client
 
@@ -111,10 +126,38 @@
 	end
 end
 
+function Client:runcommand(...)
+	if not self.capabilities.runcommand then
+		return nil, '', 'runcommand is not supported by this command server', ''
+	end
+	self.wh:write('runcommand\n')
+	write_block(self.wh, ...)
+	self.wh:flush()
+	local o = ''
+	local e = ''
+	local d = ''
+	while true do
+		local channel, message = read_channel(self.rh)
+		if channel == 'r' then
+			return decode_i4(message), o, e, d
+		elseif channel == 'o' then
+			o = o .. message
+		elseif channel == 'e' then
+			e = e .. message
+		elseif channel == 'd' then
+			d = d .. message
+		elseif channel:lower() ~= channel then
+			return nil, o, e, d
+		end
+	end
+end
+
 return {
+	write_u4 = write_u4,
 	read_u4 = read_u4,
 	read_c = read_c,
 	read_channel = read_channel,
 	decode_i4 = decode_i4,
+	write_block = write_block,
 	Client = Client
 }

--- a/spec/hglib_spec.lua	Fri Sep 09 12:21:28 2016 +0800
+++ b/spec/hglib_spec.lua	Fri Sep 09 12:24:59 2016 +0800
@@ -48,4 +48,24 @@
 		assert.is_nil(err)
 		assert.are_equal('string', type(encoding))
 	end)
+
+	it('can use runcommand to request version', function()
+		local client = hglib.Client.open()
+		local code, o, e, d = client:runcommand('version')
+		client:close()
+		assert.are_equal(0, code)
+		assert.is_not_nil(o:find('Mercurial Distributed SCM'))
+		assert.are_equal('', e)
+		assert.are_equal('', d)
+	end)
+
+	it('can use runcommand to run nonexistent command', function()
+		local client = hglib.Client.open()
+		local code, o, e, d = client:runcommand('extirpate')
+		client:close()
+		assert.are_equal(255, code)
+		assert.is_not_nil(o:find('use "hg help" for the full list of commands'))
+		assert.is_not_nil(e:find('unknown command'))
+		assert.are_equal('', d)
+	end)
 end)