feat(mlcache) 'peek()' only returns 0 when key has indefinite ttl

Any non-zero ttl now guarantees a non-zero return value for `peek()`.

README.md

@@ -745,8 +745,12 @@ If there is no value for the queried `key`, it returns `nil` and no error.
 If there is a value for the queried `key`, it returns a number indicating the
 remaining TTL of the cached value (in seconds) and no error. If the value for
 `key` has expired but is still in the L2 cache, returned TTL value will be
-negative. Finally, the third returned value in that case will be the cached
-value itself, for convenience.
+negative. The remaining TTL return value will only be `0` if the queried `key`
+has an indefinite ttl (`ttl=0`). Otherwise, this return value may be positive
+(`key` still valid), or negative (`key` is stale).
+
+The third returned value will be the cached value as stored in the L2 cache, if
+still available.
 
 This method is useful when you want to determine if a value is cached. A value
 stored in the L2 cache is considered cached regardless of whether or not it is

lib/resty/mlcache.lua

@@ -1262,7 +1262,16 @@ function _M:peek(key, stale)
                         "retrieval: " .. err
         end
 
-        local remaining_ttl = ttl - (now() - at)
+        local remaining_ttl = 0
+
+        if ttl > 0 then
+            remaining_ttl = ttl - (now() - at)
+
+            if remaining_ttl == 0 then
+                -- guarantee a non-zero remaining_ttl if ttl is set
+                remaining_ttl = 0.001
+            end
+        end
 
         return remaining_ttl, nil, value, went_stale
     end

t/03-peek.t

@@ -120,9 +120,7 @@ ttl: 18
 
 
 
-=== TEST 4: peek() returns a negative ttl when a key expired
---- main_config
-    timer_resolution 10ms;
+=== TEST 4: peek() returns a 0 remaining_ttl if the ttl was 0
 --- config
     location /t {
         content_by_lua_block {
@@ -152,8 +150,8 @@ ttl: 18
         }
     }
 --- response_body
-ttl: -1
-ttl: -2
+ttl: 0
+ttl: 0
 --- no_error_log
 [error]
 [crit]