fix(tsm1): "snapshot in progress" error during backup

Test the skipCacheOk flag to tsdb.Shard.CreateSnapshot() and
tsdb.Engine.CreateSnapshot()
A value of true allows the backup to proceed even if a cache
snapshot cannot be taken.

influxdata/plutonium#3227
(cherry picked from commit 0dcff81)

tsdb/engine/tsm1/engine_internal_test.go

@@ -0,0 +1,116 @@
+package tsm1
+
+import (
+	"github.com/influxdata/influxdb/logger"
+	"github.com/influxdata/influxdb/models"
+	"github.com/influxdata/influxdb/tsdb"
+	"github.com/influxdata/influxdb/tsdb/index/inmem"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+)
+
+func TestEngine_ConcurrentShardSnapshots(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Skipping on windows")
+	}
+
+	tmpDir, err := ioutil.TempDir("", "shard_test")
+	if err != nil {
+		t.Fatalf("error creating temporary directory: %s", err.Error())
+	}
+	defer os.RemoveAll(tmpDir)
+	tmpShard := filepath.Join(tmpDir, "shard")
+	tmpWal := filepath.Join(tmpDir, "wal")
+
+	sfile := NewSeriesFile(tmpDir)
+	defer sfile.Close()
+
+	opts := tsdb.NewEngineOptions()
+	opts.Config.WALDir = filepath.Join(tmpDir, "wal")
+	opts.InmemIndex = inmem.NewIndex(filepath.Base(tmpDir), sfile)
+	opts.SeriesIDSets = seriesIDSets([]*tsdb.SeriesIDSet{})
+
+	sh := tsdb.NewShard(1, tmpShard, tmpWal, sfile, opts)
+	if err := sh.Open(); err != nil {
+		t.Fatalf("error opening shard: %s", err.Error())
+	}
+	defer sh.Close()
+
+	points := make([]models.Point, 0, 10000)
+	for i := 0; i < cap(points); i++ {
+		points = append(points, models.MustNewPoint(
+			"cpu",
+			models.NewTags(map[string]string{"host": "server"}),
+			map[string]interface{}{"value": 1.0},
+			time.Unix(int64(i), 0),
+		))
+	}
+	err = sh.WritePoints(points)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	engineInterface, err := sh.Engine()
+	if err != nil {
+		t.Fatalf("error retrieving shard.Engine(): %s", err.Error())
+	}
+
+	// Get the struct underlying the interface. Not a recommended practice.
+	realEngineStruct, ok := (engineInterface).(*Engine)
+	if !ok {
+		t.Log("Engine type does not permit simulating Cache race conditions")
+		return
+	}
+	// fake a race condition in snapshotting the cache.
+	realEngineStruct.Cache.snapshotting = true
+	defer func() {
+		realEngineStruct.Cache.snapshotting = false
+	}()
+
+	snapshotFunc := func(skipCacheOk bool) {
+		if f, err := sh.CreateSnapshot(skipCacheOk); err == nil {
+			if err = os.RemoveAll(f); err != nil {
+				t.Fatalf("Failed to clean up in TestEngine_ConcurrentShardSnapshots: %s", err.Error())
+			}
+		} else if err == ErrSnapshotInProgress {
+			if skipCacheOk {
+				t.Fatalf("failing to ignore this error,: %s", err.Error())
+			}
+		} else if err != nil {
+			t.Fatalf("error creating shard snapshot: %s", err.Error())
+		}
+	}
+
+	// Permit skipping cache in the snapshot
+	snapshotFunc(true)
+	// do not permit skipping the cache in the snapshot
+	snapshotFunc(false)
+	realEngineStruct.Cache.snapshotting = false
+}
+
+// NewSeriesFile returns a new instance of SeriesFile with a temporary file path.
+func NewSeriesFile(tmpDir string) *tsdb.SeriesFile {
+	dir, err := ioutil.TempDir(tmpDir, "tsdb-series-file-")
+	if err != nil {
+		panic(err)
+	}
+	f := tsdb.NewSeriesFile(dir)
+	f.Logger = logger.New(os.Stdout)
+	if err := f.Open(); err != nil {
+		panic(err)
+	}
+	return f
+}
+
+type seriesIDSets []*tsdb.SeriesIDSet
+
+func (a seriesIDSets) ForEach(f func(ids *tsdb.SeriesIDSet)) error {
+	for _, v := range a {
+		f(v)
+	}
+	return nil
+}