feat(peek) [breaking] use shm:ttl() in recent OpenResty versions

Use the new `shm:ttl()` API which was contributed in order to support
mlcache's `peek()` method. The existing `peek()` implementation was a
temporary solution until then. It is slightly unfortunate that we chose
to return the value along with the remaining TTL, as this behavior now
must break.

Pros/cons
---------
+ As a result, `peek()` is ligher and thus, faster
+ It does not touch the shm LRU queue
- It is a breaking change

Fix #39

lib/resty/mlcache.lua

@@ -20,6 +20,12 @@ local ngx_log      = ngx.log
 local WARN         = ngx.WARN
 
 
+local has_shm_ttl
+if ngx.config.ngx_lua_version >= 10011 then
+    has_shm_ttl = true
+end
+
+
 local CACHE_MISS_SENTINEL_LRU = {}
 local SHM_SET_DEFAULT_TRIES = 3
 local LOCK_KEY_PREFIX = "lua-resty-mlcache:lock:"
@@ -713,40 +719,65 @@ function _M:get(key, opts, cb, ...)
 end
 
 
-function _M:peek(key)
-    if type(key) ~= "string" then
-        error("key must be a string", 2)
-    end
+if has_shm_ttl then
+    function _M:peek(key)
+        if type(key) ~= "string" then
+            error("key must be a string", 2)
+        end
 
-    -- restrict this key to the current namespace, so we isolate this
-    -- mlcache instance from potential other instances using the same
-    -- shm
-    local namespaced_key = self.name .. key
+        local namespaced_key = self.name .. key
 
-    local v, err = self.dict:get(namespaced_key)
-    if err then
-        return nil, "could not read from lua_shared_dict: " .. err
+        local ttl, err = self.dict:ttl(namespaced_key)
+        if not ttl and err ~= "not found" then
+            return nil, "could not read from lua_shared_dict: " .. err
+        end
+
+        if self.dict_miss and ttl == nil then
+            ttl, err = self.dict_miss:ttl(namespaced_key)
+            if not ttl and err ~= "not found" then
+                return nil, "could not read from lua_shared_dict: " .. err
+            end
+        end
+
+        return ttl
     end
 
-    -- if we specified shm_miss, it might be a negative hit cached
-    -- there
-    if self.dict_miss and v == nil then
-        v, err = self.dict_miss:get(namespaced_key)
+else
+    function _M:peek(key)
+        if type(key) ~= "string" then
+            error("key must be a string", 2)
+        end
+
+        -- restrict this key to the current namespace, so we isolate this
+        -- mlcache instance from potential other instances using the same
+        -- shm
+        local namespaced_key = self.name .. key
+
+        local v, err = self.dict:get(namespaced_key)
         if err then
             return nil, "could not read from lua_shared_dict: " .. err
         end
-    end
 
-    if v ~= nil then
-        local value, err, at, ttl = unmarshall_from_shm(v)
-        if err then
-            return nil, "could not deserialize value after lua_shared_dict " ..
-                        "retrieval: " .. err
+        -- if we specified shm_miss, it might be a negative hit cached
+        -- there
+        if self.dict_miss and v == nil then
+            v, err = self.dict_miss:get(namespaced_key)
+            if err then
+                return nil, "could not read from lua_shared_dict: " .. err
+            end
         end
 
-        local remaining_ttl = ttl - (now() - at)
+        if v ~= nil then
+            local value, err, at, ttl = unmarshall_from_shm(v)
+            if err then
+                return nil, "could not deserialize value after " ..
+                            "lua_shared_dict retrieval: " .. err
+            end
+
+            local remaining_ttl = ttl - (now() - at)
 
-        return remaining_ttl, nil, value
+            return remaining_ttl, nil, value
+        end
     end
 end
 

t/03-peek.t

@@ -2,6 +2,8 @@
 
 use Test::Nginx::Socket::Lua;
 use Cwd qw(cwd);
+use lib '.';
+use t::Util;
 
 workers(2);
 
@@ -202,6 +204,7 @@ ttl: 18
 
 
 === TEST 5: peek() returns the value if a key has been fetched before
+--- skip_eval: 3: t::Util::skip_openresty('>=', '1.13.6.1')
 --- http_config eval: $::HttpConfig
 --- config
     location = /t {
@@ -261,7 +264,50 @@ ttl: \d* nil_val: nil
 
 
 
-=== TEST 6: peek() returns the value if shm_miss is specified
+=== TEST 6: peek() returns no value for recent OpenResty versions
+--- skip_eval: 3: t::Util::skip_openresty('<', '1.13.6.1')
+--- http_config eval: $::HttpConfig
+--- config
+    location = /t {
+        content_by_lua_block {
+            local mlcache = require "resty.mlcache"
+
+            local cache, err = mlcache.new("my_mlcache", "cache_shm")
+            if not cache then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            local function cb_number()
+                return 123
+            end
+
+            local val, err = cache:get("my_key", nil, cb_number)
+            if err then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            local ttl, err, val = cache:peek("my_key")
+            if err then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            ngx.say("ttl: ", math.ceil(ttl), " val: ", val)
+        }
+    }
+--- request
+GET /t
+--- response_body_like
+ttl: \d* val: nil
+--- no_error_log
+[error]
+
+
+
+=== TEST 7: peek() returns the value if shm_miss is specified
+--- skip_eval: 3: t::Util::skip_openresty('>=', '1.13.6.1')
 --- http_config eval: $::HttpConfig
 --- config
     location = /t {
@@ -300,7 +346,7 @@ ttl: \d* nil_val: nil
 
 
 
-=== TEST 7: peek() JITs on hit
+=== TEST 8: peek() JITs on hit
 --- http_config eval: $::HttpConfig
 --- config
     location = /t {
@@ -332,7 +378,7 @@ qr/\[TRACE\s+\d+ content_by_lua\(nginx\.conf:\d+\):13 loop\]/
 
 
 
-=== TEST 8: peek() JITs on miss
+=== TEST 9: peek() JITs on miss
 --- http_config eval: $::HttpConfig
 --- config
     location = /t {