Add initial implementation of mock.restore()

Like mock.patch() but only restores original values instead of also setting
new values initially.

library/lua/test_util/mock.lua

@@ -1,27 +1,8 @@
 local mock = mkmodule('test_util.mock')
 
---[[
-Usage:
-    patch(table, key, value, callback)
-    patch({
-        {table, key, value},
-        {table2, key2, value2}
-    }, callback)
-]]
-function mock.patch(...)
-    local args = {...}
-    if #args == 4 then
-        args = {{
-            {args[1], args[2], args[3]}
-        }, args[4]}
-    end
-    if #args ~= 2 then
-        error('expected 2 or 4 arguments')
-    end
-
-    local callback = args[2]
+function _patch_impl(patches_raw, callback, restore_only)
     local patches = {}
-    for _, v in ipairs(args[1]) do
+    for _, v in ipairs(patches_raw) do
         local p = {
             table = v[1],
             key = v[2],
@@ -40,14 +21,66 @@ function mock.patch(...)
             end
         end,
         function()
+            if not restore_only then
                 for _, p in ipairs(patches) do
                     p.table[p.key] = p.new_value
                 end
+            end
             return callback()
         end
     )
 end
 
+--[[
+Usage:
+    patch(table, key, value, callback)
+    patch({
+        {table, key, value},
+        {table2, key2, value2},
+    }, callback)
+]]
+function mock.patch(...)
+    local args = {...}
+    local patches
+    local callback
+    if #args == 4 then
+        patches = {{args[1], args[2], args[3]}}
+        callback = args[4]
+    elseif #args == 2 then
+        patches = args[1]
+        callback = args[2]
+    else
+        error('expected 2 or 4 arguments')
+    end
+
+    return _patch_impl(patches, callback)
+end
+
+--[[
+Usage:
+    restore(table, key, callback)
+    restore({
+        {table, key},
+        {table2, key2},
+    }, callback)
+]]
+function mock.restore(...)
+    local args = {...}
+    local patches
+    local callback
+    if #args == 3 then
+        patches = {{args[1], args[2]}}
+        callback = args[3]
+    elseif #args == 2 then
+        patches = args[1]
+        callback = args[2]
+    else
+        error('expected 2 or 3 arguments')
+    end
+
+    return _patch_impl(patches, callback, true)
+end
+
 function mock.func(return_value)
     local f = {
         return_value = return_value,

test/library/test_util/mock.lua

@@ -99,6 +99,33 @@ function test.patch_callback_return_value()
     expect.eq(b, 4)
 end
 
+function test.restore_single()
+    local t = {k = 1}
+    mock.restore(t, 'k', function()
+        expect.eq(t.k, 1)
+        t.k = 2
+        expect.eq(t.k, 2)
+    end)
+    expect.eq(t.k, 1)
+end
+
+function test.restore_multiple()
+    local t = {a = 1, b = 2}
+    mock.restore({
+        {t, 'a'},
+        {t, 'b'},
+    }, function()
+        expect.eq(t.a, 1)
+        expect.eq(t.b, 2)
+        t.a = 3
+        t.b = 4
+        expect.eq(t.a, 3)
+        expect.eq(t.b, 4)
+    end)
+    expect.eq(t.a, 1)
+    expect.eq(t.b, 2)
+end
+
 function test.func_call_count()
     local f = mock.func()
     expect.eq(f.call_count, 0)