console: add line carrying backslash

When using interactive console(stdin) instead of `\set delimiter '\'`,
slash at the end of the line can be used. Works on both server and client
side.

Co-authored-by: Olga Arkhangelskaia <arkholga@tarantool.org>

Closes tarantool#4317
Requires tarantool#7357

@TarantoolBot document
Title: introduce line carrying slash

Now we can use multiline commands with lines ending with '\' by default.
Works only without a set delimiter.

Consider the example:
```
tarantool> a = 10 \
         > + 12
---
...

tarantool> a
---
- 22
...

```

changelogs/unreleased/gh-4317-console-add-line-carrying-backslash.md

@@ -0,0 +1,5 @@
+## feature/console
+
+* Now multiline commands with each line ending with "\" can be used instead
+  of `\set delimiter "\"`, is active only without a set delimiter. Works on
+  both server and client side.

src/box/lua/console.lua

@@ -606,7 +606,10 @@ local function local_read(self)
         if buf:sub(1, 1) == '\\' then
             break
         end
-        if delim == "" then
+        if delim == "" and line:endswith("\\") then
+            buf = buf:sub(1, -2)
+            -- Continue reading.
+        elseif delim == "" then
             local lang = box.session.language
             if not lang or lang == 'lua' then
                 -- stop once a complete Lua statement is entered
@@ -642,9 +645,12 @@ local function local_print(self, output)
 end
 
 --
--- Read command from connected client console.listen()
+-- Read a line or a chunk from connected client console.listen().
 --
-local function client_read(self)
+-- The value is returned without delimiter (if any) and trailing
+-- newline character.
+--
+local function client_read_line(self)
     --
     -- Byte sequences that come over the network and come from
     -- the local client console may have a different terminal
@@ -665,6 +671,30 @@ local function client_read(self)
     return buf:sub(1, -#self.delimiter-2)
 end
 
+--
+-- Read a command from connected client console.listen().
+--
+local function client_read(self)
+    local buf = ""
+    while true do
+        local line = client_read_line(self)
+        if line == nil then
+            -- EOF or error.
+            --
+            -- Note: Even if `buf` is not empty, skip unfinished
+            -- command.
+            return nil
+        end
+        buf = buf .. line
+        if self.delimiter == "" and line:endswith("\\") then
+            buf = buf:sub(1, -2)
+        else
+            break
+        end
+    end
+    return buf
+end
+
 --
 -- Print result to connected client from console.listen()
 --

test/app-luatest/gh_4317_console_backslash_test.lua

@@ -0,0 +1,51 @@
+local t = require('luatest')
+local g = t.group()
+
+local result_str =  [[tarantool> local a = 0 \
+         > for i = 1, 10 do
+         > a = a + i
+         > end \
+         > print(a)
+55
+---
+...
+
+tarantool> ]]
+
+g.test_local_console = function()
+    local build_dir = arg[-1]:gsub('tarantool$', '')
+    local tarantool_command = [[local a = 0 \\\nfor i = 1, 10 do\na = a + i\nend \\\nprint(a)\n]]
+
+    local cmd = ("printf '%s' | tarantool -i 2>/dev/null"):format(tarantool_command)
+    local fh = 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', '')
+
+    fh:close()
+    t.assert_equals(result, result_str)
+end
+
+g.test_remote_console = function()
+    local console = require('console')
+    local socket = require('socket')
+    local log = require('log')
+
+    -- Suppress console log messages.
+    log.level(4)
+    local CONSOLE_SOCKET = '/tmp/tarantool-test-console.sock'
+    local IPROTO_SOCKET  = '/tmp/tarantool-test-iproto.sock'
+    local EOL = '\n...\n'
+    console.listen(CONSOLE_SOCKET)
+    socket.tcp_connect('unix/', CONSOLE_SOCKET)
+    local tarantool_command = 'local a = 0 \\\nfor i = 1, 10 do\\\n a = a + i \\\n end \\\n print(a)'
+
+    local client = socket.tcp_connect('unix/', CONSOLE_SOCKET)
+    client:read(128)
+    client:write(('%s\n'):format(tarantool_command))
+    local result = client:read(EOL)
+    t.assert_not_str_contains(result, 'error')
+    client:close()
+    os.remove(CONSOLE_SOCKET)
+    os.remove(IPROTO_SOCKET)
+end