collector: let the collection own the label and metric caches (#2740)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

internal/scrapecache/scrapecache_test.go

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package scrapecache_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/prometheus-community/windows_exporter/internal/scrapecache"
+	"github.com/stretchr/testify/require"
+)
+
+type cache struct {
+	name   string
+	events *[]string
+}
+
+func (c cache) BeginScrape() {
+	*c.events = append(*c.events, c.name+" begin")
+}
+
+func (c cache) EndScrape(err error) {
+	*c.events = append(*c.events, fmt.Sprintf("%s end %v", c.name, err))
+}
+
+func (c cache) Reset() {
+	*c.events = append(*c.events, c.name+" reset")
+}
+
+type owner struct {
+	events []string
+	calls  int
+}
+
+func (o *owner) ScrapeCaches() []scrapecache.Cache {
+	o.calls++
+
+	return []scrapecache.Cache{cache{name: "a", events: &o.events}, cache{name: "b", events: &o.events}}
+}
+
+func TestOwner(t *testing.T) {
+	t.Parallel()
+
+	o := &owner{}
+
+	scrapecache.Reset(o)
+
+	cacheScrape := scrapecache.Begin(o)
+	require.Equal(t, []string{"a reset", "b reset", "a begin", "b begin"}, o.events)
+
+	cacheScrape.End(errors.New("failed"))
+	require.Equal(t, []string{"a end failed", "b end failed"}, o.events[4:])
+	require.Equal(t, 2, o.calls, "Begin asks for the caches once per scrape")
+}
+
+func TestNotAnOwner(t *testing.T) {
+	t.Parallel()
+
+	scrapecache.Reset(struct{}{})
+	scrapecache.Begin(struct{}{}).End(errors.New("ignored"))
+}