Add a mock.func() helper for mocking functions

library/lua/test_util/mock.lua

@@ -1,6 +1,5 @@
 local mock = mkmodule('test_util.mock')
 
-
 --[[
 Usage:
     patch(table, key, value, callback)
@@ -49,4 +48,22 @@ function mock.patch(...)
     )
 end
 
+function mock.func(return_value)
+    local f = {
+        return_value = return_value,
+        call_count = 0,
+        call_args = {},
+    }
+
+    setmetatable(f, {
+        __call = function(self, ...)
+            self.call_count = self.call_count + 1
+            table.insert(self.call_args, {...})
+            return self.return_value
+        end,
+    })
+
+    return f
+end
+
 return mock

test/library/test_util/mock.lua

@@ -98,3 +98,39 @@ function test.patch_callback_return_value()
     expect.eq(a, 3)
     expect.eq(b, 4)
 end
+
+function test.func_call_count()
+    local f = mock.func()
+    expect.eq(f.call_count, 0)
+    f()
+    expect.eq(f.call_count, 1)
+    f()
+    expect.eq(f.call_count, 2)
+end
+
+function test.func_call_args()
+    local f = mock.func()
+    expect.eq(#f.call_args, 0)
+    f()
+    f(7)
+    expect.eq(#f.call_args, 2)
+    expect.eq(#f.call_args[1], 0)
+    expect.eq(#f.call_args[2], 1)
+    expect.eq(f.call_args[2][1], 7)
+end
+
+function test.func_call_args_nil()
+    local f = mock.func()
+    f(nil)
+    f(2, nil, 4)
+    expect.table_eq(f.call_args[1], {nil})
+    expect.table_eq(f.call_args[2], {2, nil, 4})
+    expect.eq(#f.call_args[2], 3)
+end
+
+function test.func_call_return_value()
+    local f = mock.func(7)
+    expect.eq(f(), 7)
+    f.return_value = 9
+    expect.eq(f(), 9)
+end