memcached: add an API method that shows slab info

Before switching to our own slab arena user can control usage and
information about slab arena using Tarantool Lua API (specifically
box.slab.{check,info,stat}() functions), see [1]. Example of output
box.slab.info() is below:

tarantool> box.slab.info()
---
- items_size: 146896
  items_used_ratio: 26.05%
  quota_size: 268435456
  quota_used_ratio: 12.50%
  arena_used_ratio: 3.7%
  items_used: 38264
  quota_used: 33554432
  arena_size: 33554432
  arena_used: 1234296
...

With our own slab arena we need a way to obtain information about arena.
It's a reason why new method slab() was introduced. Output of slab() is
similar to output from `box.slab.info()`, but without items_size,
items_used, items_used_ratio and arena_used is not implemented.

tarantool> require('memcached'):slab()
---
- info:
    quota_used: 4194304
    arena_used_ratio: 0
    quota_used_ratio: 0.097656249999998
    arena_size: 4194304
    quota_size: 4294967296
    arena_used: 0
...

1. https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_slab/slab_info/

Follows up #96

README.md

@@ -83,6 +83,7 @@ END
   - `name` - a string with instance name
   - `uri`  - a string with uri to bind to, for example: `0.0.0.0:11211` (only TCP is supported now)
   - `opts` - a table with options, the list of possible options is described in the configuration section
+* `local slab = memcached.slab()` - show information about slab arena and quota.
 * `local instance = instance:cfg(<opts>)` - (re)configure an existing instance.
   `opts` - a table with options, same as in `create`
 * `local instance = instance:start()` - start an instance

memcached/init.lua

@@ -54,6 +54,16 @@ struct memcached_stat {
     uint64_t      auth_errors;
 };
 
+struct slab_arena_info {
+	size_t arena_size;
+	size_t arena_used;
+	double arena_used_ratio;
+
+	size_t quota_size;
+	size_t quota_used;
+	double quota_used_ratio;
+};
+
 void
 memcached_set_opt (struct memcached_service *srv, int opt, ...);
 
@@ -82,6 +92,9 @@ memcached_get_stat (struct memcached_service *);
 struct memcached_service *
 memcached_create(const char *, uint32_t);
 
+struct slab_arena_info *
+memcached_slab_arena_info();
+
 int    memcached_start     (struct memcached_service *);
 void   memcached_stop      (struct memcached_service *);
 void   memcached_free      (struct memcached_service *);
@@ -233,6 +246,17 @@ local RUNNING = 'r'
 local STOPPED = 's'
 local ERRORED = 'e'
 
+
+local slab_arena_info_table = {
+    'arena_size',
+    'arena_used',
+    'arena_used_ratio',
+
+    'quota_size',
+    'quota_used',
+    'quota_used_ratio',
+}
+
 local stat_table = {
     'total_items', 'curr_items',
     'curr_conns', 'total_conns',
@@ -386,7 +410,20 @@ local function memcached_get(name)
     return memcached_services[name]
 end
 
+local function memcached_slab()
+    local slab_arena_info = memcached_internal.memcached_slab_arena_info()
+    local slab = {
+        info = {}
+    }
+    for _, v in pairs(slab_arena_info_table) do
+	slab.info[v] = slab_arena_info[0][v]
+    end
+
+    return slab
+end
+
 return {
+    slab   = memcached_slab,
     create = memcached_init,
     get    = memcached_get,
     server = setmetatable({}, {

memcached/internal/memcached.c

@@ -51,6 +51,20 @@
 __thread struct slab_cache slabc;
 struct quota quota;
 struct slab_arena arena;
+struct slab_arena_info slab_info;
+
+struct slab_arena_info *
+memcached_slab_arena_info()
+{
+	slab_info.arena_size = arena.used;
+
+	slab_info.quota_used = quota_used(arena.quota);
+	slab_info.quota_size = quota_total(arena.quota);
+	slab_info.quota_used_ratio = 100 * ((double) slab_info.quota_used /
+					((double) slab_info.quota_size + 0.0001));
+
+	return &slab_info;
+}
 
 void
 memcached_slab_arena_create()

memcached/internal/memcached.h

@@ -27,6 +27,16 @@ enum memcached_proto_type {
 	MEMCACHED_PROTO_MAX
 };
 
+struct slab_arena_info {
+	size_t arena_size;
+	size_t arena_used;
+	double arena_used_ratio;
+
+	size_t quota_size;
+	size_t quota_used;
+	double quota_used_ratio;
+};
+
 struct memcached_stat {
 	/* connection informations */
 	unsigned int  curr_items;

test/common/instance_api.test.lua

@@ -121,3 +121,19 @@ local self_instance = mc:grant('Vera')
 test:isnt(self_instance, nil, 'instance:grant() returns self object')
 
 os.exit(test:check() and 0 or 1)
+
+-- instance:slab()
+
+local slab = mc:slab()
+test:istable(slab, 'instance:slab() returns a table')
+test:istable(slab.info, 'instance:slab().info returns a table')
+test:isnumber(slab.info.arena_size, 'instance:slab().info.arena_size returns a number')
+test:isnumber(slab.info.arena_used, 'instance:slab().info.arena_used returns a number')
+test:isnumber(slab.info.arena_used_ratio, 'instance:slab().info.arena_used_ratio returns a number')
+test:isnumber(slab.info.quota_size, 'instance:slab().info.quota_size returns a number')
+test:isnumber(slab.info.quota_used, 'instance:slab().info.quota_used returns a number')
+test:isnumber(slab.info.quota_used_ratio, 'instance:slab().info.quota_used_ratio returns a number')
+local quota_used_ratio = 100 * slab.info.quota_used / slab.info.quota_used
+test:is(slab.info.quota_used_ratio, quota_used_ratio, 'instance:slab().info.quota_used_ratio is correct')
+
+os.exit(test:check() and 0 or 1)