[BUGFIX] Scraping: bump cache iteration after error

We use the cache iteration number to detect when the same metric has
occurred twice in a scrape. We need to bump this number at the end of
every scrape, not just successful scrapes.

The `iter` number is also used:
* After a successful scrape, to delete older metrics and metadata.
* To detect when metadata changed in this scrape.

None of those additional cases is broken by incrementing the number
on error.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

scrape/scrape.go

@@ -1034,9 +1034,8 @@ func (c *scrapeCache) iterDone(flushCache bool) {
 			}
 		}
 		c.metaMtx.Unlock()
-
-		c.iter++
 	}
+	c.iter++
 
 	// Swap current and previous series.
 	c.seriesPrev, c.seriesCur = c.seriesCur, c.seriesPrev

scrape/scrape_test.go

@@ -5153,3 +5153,38 @@ func TestScrapePoolScrapeAfterReload(t *testing.T) {
 
 	<-time.After(1 * time.Second)
 }
+
+// Regression test against https://github.com/prometheus/prometheus/issues/16160.
+func TestScrapeAppendWithParseError(t *testing.T) {
+	const (
+		scrape1 = `metric_a 1
+`
+		scrape2 = `metric_a 11
+# EOF`
+	)
+
+	sl := newBasicScrapeLoop(t, context.Background(), nil, nil, 0)
+	sl.cache = newScrapeCache(sl.metrics)
+
+	now := time.Now()
+	capp := &collectResultAppender{next: nopAppender{}}
+	_, _, _, err := sl.append(capp, []byte(scrape1), "application/openmetrics-text", now)
+	require.Error(t, err)
+	_, _, _, err = sl.append(capp, nil, "application/openmetrics-text", now)
+	require.NoError(t, err)
+	require.Empty(t, capp.resultFloats)
+
+	capp = &collectResultAppender{next: nopAppender{}}
+	_, _, _, err = sl.append(capp, []byte(scrape2), "application/openmetrics-text", now.Add(15*time.Second))
+	require.NoError(t, err)
+	require.NoError(t, capp.Commit())
+
+	want := []floatSample{
+		{
+			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
+			t:      timestamp.FromTime(now.Add(15 * time.Second)),
+			f:      11,
+		},
+	}
+	requireEqual(t, want, capp.resultFloats, "Appended samples not as expected:\n%s", capp)
+}