Add syntax sugar for more convenient logging

You can use `luatest.log` for logging. This is convenient in case of debugging
in several places at the same time:

    local luatest = require('luatest')

    luatest.log('outside')
    g.test_foo = function()
        luatest.log('inside')
        g.server:exec(function() luatest.log('hi!') end)
    end

    I> outside
    I> inside
    I> hi!

The pretty conversion of arguments of any type will be performed
automatically:

    luatest.log('My structure is %s', {a = 1, b = 2, c = {cc = 1}})
    I> My structure is {a = 1, b = 2, c = {cc = 1}}

Closes #326

CHANGELOG.md

@@ -5,6 +5,7 @@
   * Fixed incorrent unix socket path length check (gh-341).
   * Now net_box_uri can be accepted as table (gh-342).
   * Fixed returning values from `Server:exec()` if some of them are nil (gh-350).
+  * Introduce `luatest.log` helper (gh-326).
 
 ## 1.0.0
 

luatest/init.lua

@@ -33,6 +33,11 @@ luatest.ReplicaSet = require('luatest.replica_set')
 local Group = require('luatest.group')
 local hooks = require('luatest.hooks')
 local parametrizer = require('luatest.parametrizer')
+local utils = require('luatest.utils')
+
+--- Add syntax sugar for logging.
+--
+luatest.log = utils.log
 
 --- Add before suite hook.
 --

luatest/pp.lua

@@ -21,7 +21,7 @@ local TABLE_TOSTRING_SEP_LEN = string.len(TABLE_TOSTRING_SEP)
 
 -- Final function called in format_table() to format the resulting list of
 -- string describing the table.
-local function _table_tostring_format_result(tbl, result, indentLevel, printTableRefs)
+local function _table_tostring_format_result(tbl, result, indentLevel, printTableRefs, isLogLine)
     local dispOnMultLines = false
 
     -- set dispOnMultLines to true if the maximum LINE_LENGTH would be exceeded with the values
@@ -46,7 +46,7 @@ local function _table_tostring_format_result(tbl, result, indentLevel, printTabl
     end
 
     -- now reformat the result table (currently holding element strings)
-    if dispOnMultLines then
+    if dispOnMultLines and not isLogLine then
         local indentString = string.rep("    ", indentLevel - 1)
         result = {
             "{\n    ",
@@ -77,7 +77,7 @@ function Formatter.mt:initialize(printTableRefs)
     self.recursionTable = {}
 end
 
-function Formatter.mt:format_table(tbl, indentLevel)
+function Formatter.mt:format_table(tbl, indentLevel, isLogLine)
     indentLevel = indentLevel or 1
     self.recursionTable[tbl] = true
 
@@ -133,16 +133,16 @@ function Formatter.mt:format_table(tbl, indentLevel)
             count = count + 1
             result[count] = entry
         end
-        return _table_tostring_format_result(tbl, result, indentLevel, self.printTableRefs)
+        return _table_tostring_format_result(tbl, result, indentLevel, self.printTableRefs, isLogLine)
     end
 end
 
-function Formatter.mt:format(v, indentLevel)
+function Formatter.mt:format(v, indentLevel, isLogLine)
     local type_v = type(v)
     if "string" == type_v  then
         return string.format("%q", v)
     elseif "table" == type_v then
-        return self:format_table(v, indentLevel)
+        return self:format_table(v, indentLevel, isLogLine)
     elseif "number" == type_v then
         -- eliminate differences in formatting between various Lua versions
         if v ~= v then
@@ -169,18 +169,24 @@ end
 --
 -- * string are enclosed with " by default, or with ' if string contains a "
 -- * tables are expanded to show their full content, with indentation in case of nested tables
-function pp.tostring(value)
+function pp.tostring(value, is_logline)
     local formatter = Formatter:new(pp.TABLE_REF_IN_ERROR_MSG)
-    local result = formatter:format(value)
+    local result = formatter:format(value, nil, is_logline)
     if formatter.recursionDetected and not pp.TABLE_REF_IN_ERROR_MSG then
         -- some table contain recursive references,
         -- so we must recompute the value by including all table references
         -- else the result looks like crap
-        return Formatter:new(true):format(value)
+        return Formatter:new(true):format(value, nil, is_logline)
     end
     return result
 end
 
+-- This function helps with displaying `value` of any type without line breaks ('\n')
+-- for logging. It is a simple wrapper over the tostring() function.
+function pp.tostringlog(value)
+    return pp.tostring(value, true)
+end
+
 local function has_new_line(s)
     return (string.find(s, '\n', 1, true) ~= nil)
 end

luatest/utils.lua

@@ -1,6 +1,9 @@
 local digest = require('digest')
 local fun = require('fun')
 local yaml = require('yaml')
+local log = require('log')
+
+local pp = require('luatest.pp')
 
 local utils = {}
 
@@ -191,4 +194,12 @@ function utils.is_tarantool_binary(path)
     return path:find('^.*/tarantool[^/]*$') ~= nil
 end
 
+function utils.log(msg, ...)
+    local args = {...}
+    for k, v in pairs(args) do
+        args[k] = pp.tostringlog(v)
+    end
+    log.info(msg, unpack(args))
+end
+
 return utils