config: extract univeral_read() into a helper

The plan is to re-use it in a next commit.

Part of #9506

NO_DOC=no public API changes
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC

src/box/CMakeLists.txt

@@ -58,6 +58,7 @@ lua_source(lua_sources lua/config/instance_config.lua     config_instance_config
 lua_source(lua_sources lua/config/source/env.lua          config_source_env_lua)
 lua_source(lua_sources lua/config/source/file.lua         config_source_file_lua)
 lua_source(lua_sources lua/config/utils/expression.lua    config_utils_expression_lua)
+lua_source(lua_sources lua/config/utils/file.lua          config_utils_file_lua)
 lua_source(lua_sources lua/config/utils/log.lua           config_utils_log_lua)
 lua_source(lua_sources lua/config/utils/schema.lua        config_utils_schema_lua)
 lua_source(lua_sources lua/config/utils/snapshot.lua      config_utils_snapshot_lua)

src/box/lua/config/source/file.lua

-local ffi = require('ffi')
-local buffer = require('buffer')
-local fio = require('fio')
 local yaml = require('yaml')
+local file = require('internal.config.utils.file')
 
 local methods = {}
 local mt = {
     __index = methods,
 }
 
--- {{{ Helpers
-
--- Read the file block by block till EOF.
-local function stream_read(fh)
-    local block_size = 1024
-    local buf = buffer.ibuf(block_size)
-    while true do
-        local rc, err = fh:read(buf:reserve(block_size), block_size)
-        if err ~= nil then
-            buf:recycle()
-            return nil, err
-        end
-        if rc == 0 then
-            -- EOF.
-            break
-        end
-        buf:alloc(rc)
-    end
-    local res = ffi.string(buf.rpos, buf:size())
-    buf:recycle()
-    return res
-end
-
--- Read all the file content disregarding whether its size is
--- known from the stat(2) file information.
-local function universal_read(file_name, file_kind)
-    local fh, err = fio.open(file_name)
-    if fh == nil then
-        error(('Unable to open %s %q: %s'):format(file_kind, file_name, err))
-    end
-
-    -- A FIFO file has zero size in the stat(2) file information.
-    --
-    -- However, it is useful for manual testing from a shell:
-    --
-    -- $ tarantool --name <...> --config <(echo '{<...>}')
-    --
-    -- Let's read such a file block by block till EOF.
-    local data
-    local err
-    if fio.path.is_file(file_name) then
-        data, err = fh:read()
-    else
-        data, err = stream_read(fh)
-    end
-
-    fh:close()
-
-    if err ~= nil then
-        error(('Unable to read %s %q: %s'):format(file_kind, file_name, err))
-    end
-
-    return data
-end
-
--- }}} Helpers
-
 function methods.sync(self, config_module, _iconfig)
     assert(config_module._config_file ~= nil)
 
-    local data = universal_read(config_module._config_file, 'config file')
+    local data = file.universal_read(config_module._config_file, 'config file')
 
     -- Integrity module is available only in Tarantool Enterprise Edition
     -- builds.

src/box/lua/config/utils/file.lua

+local ffi = require('ffi')
+local buffer = require('buffer')
+local fio = require('fio')
+
+-- Read the file block by block till EOF.
+local function stream_read(fh)
+    local block_size = 1024
+    local buf = buffer.ibuf(block_size)
+    while true do
+        local rc, err = fh:read(buf:reserve(block_size), block_size)
+        if err ~= nil then
+            buf:recycle()
+            return nil, err
+        end
+        if rc == 0 then
+            -- EOF.
+            break
+        end
+        buf:alloc(rc)
+    end
+    local res = ffi.string(buf.rpos, buf:size())
+    buf:recycle()
+    return res
+end
+
+-- Read all the file content disregarding whether its size is
+-- known from the stat(2) file information.
+local function universal_read(file_name, file_kind)
+    local fh, err = fio.open(file_name)
+    if fh == nil then
+        error(('Unable to open %s %q: %s'):format(file_kind, file_name, err), 0)
+    end
+
+    -- A FIFO file has zero size in the stat(2) file information.
+    --
+    -- However, it is useful for manual testing from a shell:
+    --
+    -- $ tarantool --name <...> --config <(echo '{<...>}')
+    --
+    -- Let's read such a file block by block till EOF.
+    local data
+    local err
+    if fio.path.is_file(file_name) then
+        data, err = fh:read()
+    else
+        data, err = stream_read(fh)
+    end
+
+    fh:close()
+
+    if err ~= nil then
+        error(('Unable to read %s %q: %s'):format(file_kind, file_name, err), 0)
+    end
+
+    return data
+end
+
+return {
+    universal_read = universal_read,
+}

src/box/lua/init.c

@@ -161,6 +161,7 @@ extern char session_lua[],
 	config_source_env_lua[],
 	config_source_file_lua[],
 	config_utils_expression_lua[],
+	config_utils_file_lua[],
 	config_utils_log_lua[],
 	config_utils_schema_lua[],
 	config_utils_snapshot_lua[],
@@ -337,6 +338,10 @@ static const char *lua_sources[] = {
 	"internal.config.utils.expression",
 	config_utils_expression_lua,
 
+	"config/utils/file",
+	"internal.config.utils.file",
+	config_utils_file_lua,
+
 	"config/utils/log",
 	"internal.config.utils.log",
 	config_utils_log_lua,