console: add line carrying backslash

When using interactive console(stdin) instead of \set delimiter <delimiter>
with "\", "\" in the end if line can be used.

Closes #4317

src/box/lua/console.lua

@@ -573,14 +573,20 @@ local function local_read(self)
             break
         end
         if delim == "" then
-            local lang = box.session.language
-            if not lang or lang == 'lua' then
-                -- stop once a complete Lua statement is entered
-                if local_check_lua(buf) then
+            -- if no delim is set and line ends with the backslash
+            -- continue reading
+            if buf:sub(-1, -1) == '\\' then
+                buf = buf:sub(0, #buf - 1)
+            else
+                local lang = box.session.language
+                if not lang or lang == 'lua' then
+                    -- stop once a complete Lua statement is entered
+                    if local_check_lua(buf) then
+                        break
+                    end
+                else
                     break
                 end
-            else
-                break
             end
         elseif #buf >= #delim and buf:sub(#buf - #delim + 1) == delim then
             buf = buf:sub(0, #buf - #delim)

test/app-tap/gh-4317.test.lua

@@ -0,0 +1,53 @@
+#!/usr/bin/env tarantool
+
+local tap = require('tap')
+local console = require('console')
+local fio = require('fio')
+
+test = tap.test("console")
+test:plan(1)
+-- The following cases use LD_PRELOAD, so will not work under
+    -- Mac OS X.
+    if jit.os == 'OSX' then return end
+
+    -- Allow to run the test from the repository root without
+    -- test-run:
+    --
+    -- $ ./src/tarantool test/app-tap/console.test.lua
+--
+-- It works at least for in-source build.
+local is_under_test_run = pcall(require, 'test_run')
+local isatty_lib_dir = is_under_test_run and '../..' or 'test'
+local isatty_lib_path = fio.pathjoin(isatty_lib_dir, 'libisatty.so')
+local saved_path = os.getenv('PATH')
+if not is_under_test_run then
+    os.setenv('PATH', './src:', saved_path)
+end
+
+local tarantool_command = "local a = 0 \\\nfor i = 1, 10 do\na = a + i\nend \\\nprint(a)"
+local result_str =  [[tarantool> local a = 0 \
+         > for i = 1, 10 do
+         > a = a + i
+         > end \
+         > print(a)
+55
+---
+...
+
+tarantool> ]]
+local cmd = ([[printf '%s\n' | tarantool -i ]] ..
+[[2>/dev/null]]):format(tarantool_command)
+local fh, err = io.popen(cmd, 'r')
+-- Readline on CentOS 7 produces \e[?1034h escape
+-- sequence before tarantool> prompt, remove it.
+local result = fh:read('*a'):gsub('\x1b%[%?1034h', '')
+print(err)
+fh:close()
+print("!!!!!!!")
+print(result)
+print("!!!!!!!")
+print(tra_str)
+test:is(result, result_str, "Backslash")
+
+test:check()
+os.exit(0)