From 4f35952cf3fa6160da47161cbc508376b347ae2f Mon Sep 17 00:00:00 2001
From: Brian Brazil <brian.brazil@robustperception.io>
Date: Thu, 13 Apr 2017 18:07:23 +0100
Subject: [PATCH] Inject a stale NaN when sample disappears between scrapes.

---
 retrieval/scrape.go      | 49 ++++++++++++++++++++++++++++++++--------
 retrieval/scrape_test.go |  5 ++--
 2 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/retrieval/scrape.go b/retrieval/scrape.go
index 8258892b8..4cdcf5a28 100644
--- a/retrieval/scrape.go
+++ b/retrieval/scrape.go
@@ -19,6 +19,7 @@ import (
 	"compress/gzip"
 	"fmt"
 	"io"
+	"math"
 	"net/http"
 	"sync"
 	"time"
@@ -35,6 +36,7 @@ import (
 	"github.com/prometheus/prometheus/pkg/labels"
 	"github.com/prometheus/prometheus/pkg/textparse"
 	"github.com/prometheus/prometheus/pkg/timestamp"
+	"github.com/prometheus/prometheus/pkg/value"
 	"github.com/prometheus/prometheus/storage"
 	"github.com/prometheus/prometheus/util/httputil"
 )
@@ -410,13 +412,20 @@ type loop interface {
 	stop()
 }
 
+type lsetCacheEntry struct {
+	lset labels.Labels
+	str  string
+}
+
 type scrapeLoop struct {
 	scraper scraper
 
 	appender       func() storage.Appender
 	reportAppender func() storage.Appender
 
-	cache map[string]uint64
+	refCache                map[string]uint64         // Parsed string to ref.
+	lsetCache               map[uint64]lsetCacheEntry // Ref to labelset and string
+	samplesInPreviousScrape map[string]labels.Labels
 
 	done   chan struct{}
 	ctx    context.Context
@@ -428,7 +437,8 @@ func newScrapeLoop(ctx context.Context, sc scraper, app, reportApp func() storag
 		scraper:        sc,
 		appender:       app,
 		reportAppender: reportApp,
-		cache:          map[string]uint64{},
+		refCache:       map[string]uint64{},
+		lsetCache:      map[uint64]lsetCacheEntry{},
 		done:           make(chan struct{}),
 	}
 	sl.ctx, sl.cancel = context.WithCancel(ctx)
@@ -525,9 +535,10 @@ func (s samples) Less(i, j int) bool {
 
 func (sl *scrapeLoop) append(b []byte, ts time.Time) (total, added int, err error) {
 	var (
-		app     = sl.appender()
-		p       = textparse.New(b)
-		defTime = timestamp.FromTime(ts)
+		app            = sl.appender()
+		p              = textparse.New(b)
+		defTime        = timestamp.FromTime(ts)
+		samplesScraped = map[string]labels.Labels{}
 	)
 
 loop:
@@ -541,10 +552,11 @@ loop:
 		}
 
 		mets := yoloString(met)
-		ref, ok := sl.cache[mets]
+		ref, ok := sl.refCache[mets]
 		if ok {
 			switch err = app.AddFast(ref, t, v); err {
 			case nil:
+				samplesScraped[sl.lsetCache[ref].str] = sl.lsetCache[ref].lset
 			case storage.ErrNotFound:
 				ok = false
 			case errSeriesDropped:
@@ -568,13 +580,31 @@ loop:
 			}
 			// Allocate a real string.
 			mets = string(met)
-			sl.cache[mets] = ref
+			sl.refCache[mets] = ref
+			str := lset.String()
+			sl.lsetCache[ref] = lsetCacheEntry{lset: lset, str: str}
+			samplesScraped[str] = lset
 		}
 		added++
 	}
 	if err == nil {
 		err = p.Err()
 	}
+	if err == nil {
+		for metric, lset := range sl.samplesInPreviousScrape {
+			if _, ok := samplesScraped[metric]; !ok {
+				// Sample no longer exposed, mark it stale.
+				_, err = app.Add(lset, defTime, math.Float64frombits(value.StaleNaN))
+				switch err {
+				case nil:
+				case errSeriesDropped:
+					continue
+				default:
+					break
+				}
+			}
+		}
+	}
 	if err != nil {
 		app.Rollback()
 		return total, 0, err
@@ -582,6 +612,7 @@ loop:
 	if err := app.Commit(); err != nil {
 		return total, 0, err
 	}
+	sl.samplesInPreviousScrape = samplesScraped
 	return total, added, nil
 }
 
@@ -621,7 +652,7 @@ func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scraped, a
 }
 
 func (sl *scrapeLoop) addReportSample(app storage.Appender, s string, t int64, v float64) error {
-	ref, ok := sl.cache[s]
+	ref, ok := sl.refCache[s]
 
 	if ok {
 		if err := app.AddFast(ref, t, v); err == nil {
@@ -637,7 +668,7 @@ func (sl *scrapeLoop) addReportSample(app storage.Appender, s string, t int64, v
 	if err != nil {
 		return err
 	}
-	sl.cache[s] = ref
+	sl.refCache[s] = ref
 
 	return nil
 }
diff --git a/retrieval/scrape_test.go b/retrieval/scrape_test.go
index 23097215b..7ebac1e3e 100644
--- a/retrieval/scrape_test.go
+++ b/retrieval/scrape_test.go
@@ -18,10 +18,10 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
-	"math"
 	"reflect"
 	"strings"
 	"sync"
@@ -442,7 +442,8 @@ func TestScrapeLoopAppend(t *testing.T) {
 	sl := &scrapeLoop{
 		appender:       func() storage.Appender { return app },
 		reportAppender: func() storage.Appender { return nopAppender{} },
-		cache:          map[string]uint64{},
+		refCache:       map[string]uint64{},
+		lsetCache:      map[uint64]lsetCacheEntry{},
 	}
 
 	now := time.Now()