[PATCH] box: serialize calls to box.cfg

[Vladimir Davydov]

On Tue, Aug 07, 2018 at 12:43:26PM +0300, Vladimir Davydov wrote:
> It is dangerous to call box.cfg() concurrently from different fibers.
> For example, replication configuration uses static variables and yields
> so calling it concurrently can result in a crash. To make sure it never
> happens, let's protect box.cfg() with a lock.
> 
> Closes #3606
> ---
> https://github.com/tarantool/tarantool/issues/3606
> https://github.com/tarantool/tarantool/tree/dv/gh-3606-concurrent-replication-cfg-fix
> 
>  src/box/lua/load_cfg.lua       | 21 ++++++++++++++++++---
>  test/replication/misc.result   | 24 ++++++++++++++++++++++++
>  test/replication/misc.test.lua | 10 ++++++++++
>  3 files changed, 52 insertions(+), 3 deletions(-)
> 
> diff --git a/src/box/lua/load_cfg.lua b/src/box/lua/load_cfg.lua
> index 0b668cdc..4de79220 100644
> --- a/src/box/lua/load_cfg.lua
> +++ b/src/box/lua/load_cfg.lua
> @@ -5,6 +5,21 @@ local json = require('json')
>  local private = require('box.internal')
>  local urilib = require('uri')
>  local math = require('math')
> +local fiber = require('fiber')
> +
> +-- Function decorator that is used to prevent box.cfg() from
> +-- being called concurrently by different fibers.
> +local lock = fiber.channel(1)
> +local function locked(f)
> +    return function(...)
> +        lock:put(true)
> +        local status = pcall(f, ...)
> +        lock:get()
> +        if not status then
> +            box.error()
> +        end
> +    end
> +end

Georgy noted that we'd better use error() to re-throw the error returned
by box.cfg(). Updated on the branch, the incremental diff is below:

diff --git a/src/box/lua/load_cfg.lua b/src/box/lua/load_cfg.lua
index 4de79220..c68a3583 100644
--- a/src/box/lua/load_cfg.lua
+++ b/src/box/lua/load_cfg.lua
@@ -13,10 +13,10 @@ local lock = fiber.channel(1)
 local function locked(f)
     return function(...)
         lock:put(true)
-        local status = pcall(f, ...)
+        local status, err = pcall(f, ...)
         lock:get()
         if not status then
-            box.error()
+            error(err)
         end
     end
 end