Add unified diff support for luatest.assert_equals

This patch adds unified diff output for `t.assert_equals()` failures when
the test suite is run with the `--diff` option, using a vendored Lua
implementation of google/diff-match-patch (`luatest/vendor/diff_match_patch.lua`
taken from [^1]).

Closes #412

[^1]: https://github.com/google/diff-match-patch/blob/master/lua/diff_match_patch.lua

Author: mandesero

.luacheckrc

@@ -1,4 +1,4 @@
 include_files = {"**/*.lua", "*.rockspec", "*.luacheckrc"}
-exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/"}
+exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/", "luatest/vendor/"}
 
 max_line_length = 120

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Added support for unified diff output in `t.assert_equals()` failure messages.
+  The diff is shown only when the test suite is run with the `--diff` option (gh-412).
+
 ## 1.3.0
 
 - Fixed a bug when `assert_covers` treats arrays as maps (gh-405).

README.rst

@@ -123,6 +123,10 @@ You can configure luatest or run any bootstrap code there.
 See the `getting-started example <https://github.com/tarantool/cartridge-cli/tree/master/examples/getting-started-app/test>`_
 in cartridge-cli repo.
 
+Enable unified diffs for equality assertions by starting ``luatest`` with ``--diff``.
+When enabled, failing comparisons of strings, tables, or msgpack-encoded tables will
+include a unified diff alongside the existing mismatch analysis to make debugging easier.
+
 ---------------------------------
 Tests order
 ---------------------------------

luatest/assertions.lua

@@ -6,6 +6,7 @@
 local math = require('math')
 
 local comparator = require('luatest.comparator')
+local diff = require('luatest.diff')
 local mismatch_formatter = require('luatest.mismatch_formatter')
 local pp = require('luatest.pp')
 local log = require('luatest.log')
@@ -20,12 +21,17 @@ local prettystr_pairs = pp.tostring_pair
 local M = {}
 
 local xfail = false
+local diff_enabled = false
 
 local box_error_type = ffi.typeof(box.error.new(box.error.UNKNOWN))
 
 -- private exported functions (for testing)
 M.private = {}
 
+function M.private.set_diff_enabled(value)
+    diff_enabled = value and true or false
+end
+
 function M.private.is_xfail()
     local xfail_status = xfail
     xfail = false
@@ -83,6 +89,14 @@ local function error_msg_equality(actual, expected, deep_analysis)
         if success then
             result = table.concat({result, mismatchResult}, '\n')
         end
+
+        if diff_enabled then
+            local diff_result = diff.unified(expected, actual)
+            if diff_result then
+                result = table.concat({result, 'diff:', diff_result}, '\n')
+            end
+        end
+
         return result
     end
     return string.format("expected: %s, actual: %s",

luatest/diff.lua

@@ -0,0 +1,97 @@
+local yaml = require('yaml')
+local msgpack = require('msgpack')
+
+-- diff_match_patch expects bit32
+if not rawget(_G, 'bit32') then
+    _G.bit32 = require('bit')
+end
+
+local diff_match_patch = require('luatest.vendor.diff_match_patch')
+
+diff_match_patch.settings({
+    Diff_Timeout = 0,
+    Patch_Margin = 1e9,
+})
+
+local M = {}
+
+local function encode_yaml(value)
+    local ok, encoded = pcall(yaml.encode, value)
+    if ok then
+        return encoded
+    end
+end
+
+local function msgpack_to_yaml(value)
+    if type(value) ~= 'string' then
+        return nil
+    end
+
+    local ok, decoded = pcall(msgpack.decode, value)
+    if not ok or type(decoded) ~= 'table' then
+        return nil
+    end
+
+    return encode_yaml(decoded)
+end
+
+local function as_text(value)
+    if type(value) == 'table' then
+        return encode_yaml(value)
+    end
+
+    if type(value) == 'string' then
+        return msgpack_to_yaml(value) or value
+    end
+end
+
+local function url_unescape(s)
+    return s:gsub("%%(%x%x)", function(h)
+        return string.char(tonumber(h, 16))
+    end)
+end
+
+local function prettify_patch(patch_text)
+    patch_text = url_unescape(patch_text)
+
+    local out = {}
+
+    for line in (patch_text .. '\n'):gmatch('(.-)\n') do
+        if line ~= '' and line ~= ' ' then
+            local first = line:sub(1, 1)
+
+            if first ~= '@' and first ~= '+'
+               and first ~= '-' and first ~= ' ' then
+                line = ' ' .. line
+            end
+
+            table.insert(out, line)
+        end
+    end
+
+    return table.concat(out, '\n')
+end
+
+function M.unified(expected, actual)
+    local expected_text = as_text(expected)
+    local actual_text = as_text(actual)
+
+    if expected_text == nil or actual_text == nil then
+        return nil
+    end
+
+    local diffs = diff_match_patch.diff_main(expected_text, actual_text)
+    diff_match_patch.diff_cleanupSemantic(diffs)
+
+    local patches = diff_match_patch.patch_make(expected_text,
+                                                actual_text, diffs)
+    local patch_text = diff_match_patch.patch_toText(patches)
+
+    if patch_text == '' then
+        return nil
+    end
+
+    return prettify_patch(patch_text)
+end
+
+return M

luatest/runner.lua

@@ -57,6 +57,8 @@ function Runner.run(args, options)
         end
         options = utils.merge(options.luatest.configure(), Runner.parse_cmd_line(args), options)
 
+        assertions.private.set_diff_enabled(options.diff)
+
         log.initialize({
             vardir = Server.vardir,
             log_file = options.log_file,
@@ -118,6 +120,7 @@ Options:
                           May be repeated to exclude several patterns
                           Make sure you escape magic chars like +? with %
   --coverage:             Use luacov to collect code coverage.
+  --diff:                 Enable diff output for equality assertion failures.
   --no-clean:             Disable the var directory (default: /tmp/t) deletion before
                           running tests.
   --list-test-cases       List all test cases.
@@ -190,6 +193,8 @@ function Runner.parse_cmd_line(args)
             result.enable_capture = false
         elseif arg == '--coverage' then
             result.coverage_report = true
+        elseif arg == '--diff' then
+            result.diff = true
         elseif arg == '--no-clean' then
             result.no_clean = true
         elseif arg == '--list-test-cases' then