Set quantile value to NAN if there are no samples provided for an age

Closes #303

CHANGELOG.md

@@ -16,6 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Throw an error when http_middelware is processing a wrong handler [#199](https://github.com/tarantool/metrics/issues/199)
 - cartridge issues metric fails before cartridge.cfg() call [#298](https://github.com/tarantool/metrics/issues/298)
 
+### Changed
+- quantile metric is NAN if no samples provided for an age [#303](https://github.com/tarantool/metrics/issues/303)
+
 ## [0.10.0] - 2021-08-03
 ### Changed
 - metrics registry refactoring to search with `O(1)` [#188](https://github.com/tarantool/metrics/issues/188)

metrics-scm-1.rockspec

@@ -31,6 +31,7 @@ build = {
         ['metrics.collectors.counter']      = 'metrics/collectors/counter.lua',
         ['metrics.collectors.gauge']        = 'metrics/collectors/gauge.lua',
         ['metrics.collectors.histogram']    = 'metrics/collectors/histogram.lua',
+        ['metrics.const']                   = 'metrics/const.lua',
         ['metrics.plugins.graphite']        = 'metrics/plugins/graphite.lua',
         ['metrics.plugins.prometheus']      = 'metrics/plugins/prometheus.lua',
         ['metrics.plugins.json']            = 'metrics/plugins/json.lua',

metrics/const.lua

@@ -0,0 +1,4 @@
+return {
+    INF = math.huge,
+    NAN = math.huge * 0,
+}

metrics/init.lua

@@ -3,6 +3,7 @@
 local checks = require('checks')
 local log = require('log')
 
+local Const = require('metrics.const')
 local Registry = require('metrics.registry')
 
 local Counter = require('metrics.collectors.counter')
@@ -108,8 +109,8 @@ return {
     histogram = histogram,
     summary = summary,
 
-    INF = math.huge,
-    NAN = math.huge * 0,
+    INF = Const.INF,
+    NAN = Const.NAN,
 
     clear = clear,
     collectors = collectors,

metrics/quantile.lua

@@ -1,4 +1,5 @@
 local ffi = require('ffi')
+local const = require('metrics.const')
 
 local quantile = {}
 
@@ -270,6 +271,10 @@ function quantile.Query(stream_obj, q)
 		-- Fast path when there hasn't been enough data for a flush;
 		-- this also yields better accuracy for small sets of data.
 		local l = stream_obj.b_len
+		if l == 0 then
+			return const.NAN
+		end
+
 		local i = math.modf(l * q)
 		stream_obj:maybe_sort()
 		return stream_obj.b[i]

test/quantile_test.lua

@@ -146,7 +146,7 @@ g.test_query_on_empty_quantile = function()
 
     local res = quantile.Query(emptyQuantile, 0.99)
 
-    t.assert_equals(res, math.huge)
+    t.assert_nan(res)
 end
 
 g.test_reset = function()
@@ -156,12 +156,12 @@ g.test_reset = function()
     end
 
     local res = quantile.Query(Quantile, 0.99)
-    t.assert_not_equals(res, math.huge)
+    t.assert_not_nan(res)
 
     quantile.Reset(Quantile)
 
     res = quantile.Query(Quantile, 0.99)
-    t.assert_equals(res, math.huge)
+    t.assert_nan(res)
 end
 
 g.test_quantile_insert_works_after_reset = function()