pdh: reuse the row maps between samples (#2654)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

internal/pdh/collector.go

@@ -429,6 +429,12 @@ type collectState struct {
 	valid []bool
 
 	names instanceNameCache
+
+	// index, occurrences and seen back the maps of rowSet. They are cleared
+	// for every sample, so their buckets are not allocated again.
+	index       map[instanceKey]int
+	occurrences map[string]int
+	seen        map[string]int
 }
 
 // collect replaces the content of dst with one sample.
@@ -581,13 +587,24 @@ type rowSet[T any] struct {
 func newRowSet[T any](c *Collector[T], dst *[]T, state *collectState) rowSet[T] {
 	state.names.startSample()
 
+	if state.index == nil {
+		state.index = map[instanceKey]int{}
+		state.occurrences = map[string]int{}
+		state.seen = map[string]int{}
+	} else {
+		// A failed sample can leave entries behind.
+		clear(state.index)
+		clear(state.occurrences)
+		clear(state.seen)
+	}
+
 	return rowSet[T]{
 		c:           c,
 		dst:         dst,
-		index:       map[instanceKey]int{},
+		index:       state.index,
 		nameCache:   &state.names,
-		occurrences: map[string]int{},
-		seen:        map[string]int{},
+		occurrences: state.occurrences,
+		seen:        state.seen,
 		valid:       state.valid[:0],
 	}
 }

internal/pdh/collector_internal_test.go

@@ -482,3 +482,34 @@ func TestInstanceNameCache(t *testing.T) {
 	require.Len(t, cache.names, 1, "name of a vanished instance kept")
 	require.Contains(t, cache.names, "a\x00", "keyed by the UTF-16LE content")
 }
+
+// TestRowSetReusesState checks that a sample does not see the instances of an
+// earlier sample that shared its state, even if that sample did not finish.
+func TestRowSetReusesState(t *testing.T) {
+	t.Parallel()
+
+	c := newRowSetTestCollector(false)
+	c.counters[1].FieldIndexSecondValue = -1
+	state := &collectState{}
+
+	var dst []rowSetValues
+
+	// A sample that fails after its first counter array.
+	rows := newRowSet(c, &dst, state)
+	addDuplicateTestItems(&rows, CounterTypeRaw, 0, []string{"a", "a", "b"}, []float64{1, 2, 3})
+
+	for range 2 {
+		dst = nil
+		rows = newRowSet(c, &dst, state)
+		addDuplicateTestItems(&rows, CounterTypeRaw, 0, []string{"b", "c"}, []float64{4, 5})
+		addDuplicateTestItems(&rows, CounterTypeRaw, 1, []string{"b", "c"}, []float64{6, 7})
+		rows.finish()
+
+		state.valid = rows.valid
+
+		require.Equal(t, []rowSetValues{
+			{Name: "b", A: 4, B: 6},
+			{Name: "c", A: 5, B: 7},
+		}, dst)
+	}
+}