// checkpointFPMappings persists the fingerprint mappings. This method is not
// goroutine-safe.
//
// Description of the file format, v1:
//
// (1) Magic string (const mappingsMagicString).
//
// (2) Uvarint-encoded format version (const mappingsFormatVersion).
//
// (3) Uvarint-encoded number of mappings in fpMappings.
//
// (4) Repeated once per mapping:
//
// (4.1) The raw fingerprint as big-endian uint64.
//
// (4.2) The uvarint-encoded number of sub-mappings for the raw fingerprint.
//
// (4.3) Repeated once per sub-mapping:
//
// (4.3.1) The uvarint-encoded length of the unique metric string.
// (4.3.2) The unique metric string.
// (4.3.3) The mapped fingerprint as big-endian uint64.
func (p *persistence) checkpointFPMappings(fpm fpMappings) (err error) {
	log.Info("Checkpointing fingerprint mappings...")
	begin := time.Now()
	f, err := os.OpenFile(p.mappingsTempFileName(), os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0640)
	if err != nil {
		return
	}

	defer func() {
		f.Sync()
		closeErr := f.Close()
		if err != nil {
			return
		}
		err = closeErr
		if err != nil {
			return
		}
		err = os.Rename(p.mappingsTempFileName(), p.mappingsFileName())
		duration := time.Since(begin)
		log.Infof("Done checkpointing fingerprint mappings in %v.", duration)
	}()

	w := bufio.NewWriterSize(f, fileBufSize)

	if _, err = w.WriteString(mappingsMagicString); err != nil {
		return
	}
	if _, err = codable.EncodeUvarint(w, mappingsFormatVersion); err != nil {
		return
	}
	if _, err = codable.EncodeUvarint(w, uint64(len(fpm))); err != nil {
		return
	}

	for fp, mappings := range fpm {
		if err = codable.EncodeUint64(w, uint64(fp)); err != nil {
			return
		}
		if _, err = codable.EncodeUvarint(w, uint64(len(mappings))); err != nil {
			return
		}
		for ms, mappedFP := range mappings {
			if _, err = codable.EncodeUvarint(w, uint64(len(ms))); err != nil {
				return
			}
			if _, err = w.WriteString(ms); err != nil {
				return
			}
			if err = codable.EncodeUint64(w, uint64(mappedFP)); err != nil {
				return
			}
		}
	}
	err = w.Flush()
	return
}

// checkpointSeriesMapAndHeads persists the fingerprint to memory-series mapping
// and all non persisted chunks. Do not call concurrently with
// loadSeriesMapAndHeads. This method will only write heads format v2, but
// loadSeriesMapAndHeads can also understand v1.
//
// Description of the file format (for both, v1 and v2):
//
// (1) Magic string (const headsMagicString).
//
// (2) Varint-encoded format version (const headsFormatVersion).
//
// (3) Number of series in checkpoint as big-endian uint64.
//
// (4) Repeated once per series:
//
// (4.1) A flag byte, see flag constants above. (Present but unused in v2.)
//
// (4.2) The fingerprint as big-endian uint64.
//
// (4.3) The metric as defined by codable.Metric.
//
// (4.4) The varint-encoded persistWatermark. (Missing in v1.)
//
// (4.5) The modification time of the series file as nanoseconds elapsed since
// January 1, 1970 UTC. -1 if the modification time is unknown or no series file
// exists yet. (Missing in v1.)
//
// (4.6) The varint-encoded chunkDescsOffset.
//
// (4.6) The varint-encoded savedFirstTime.
//
// (4.7) The varint-encoded number of chunk descriptors.
//
// (4.8) Repeated once per chunk descriptor, oldest to most recent, either
// variant 4.8.1 (if index < persistWatermark) or variant 4.8.2 (if index >=
// persistWatermark). In v1, everything is variant 4.8.1 except for a
// non-persisted head-chunk (determined by the flags).
//
// (4.8.1.1) The varint-encoded first time.
// (4.8.1.2) The varint-encoded last time.
//
// (4.8.2.1) A byte defining the chunk type.
// (4.8.2.2) The chunk itself, marshaled with the marshal() method.
//
func (p *persistence) checkpointSeriesMapAndHeads(fingerprintToSeries *seriesMap, fpLocker *fingerprintLocker) (err error) {
	log.Info("Checkpointing in-memory metrics and chunks...")
	begin := time.Now()
	f, err := os.OpenFile(p.headsTempFileName(), os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0640)
	if err != nil {
		return
	}

	defer func() {
		f.Sync()
		closeErr := f.Close()
		if err != nil {
			return
		}
		err = closeErr
		if err != nil {
			return
		}
		err = os.Rename(p.headsTempFileName(), p.headsFileName())
		duration := time.Since(begin)
		p.checkpointDuration.Set(float64(duration) / float64(time.Millisecond))
		log.Infof("Done checkpointing in-memory metrics and chunks in %v.", duration)
	}()

	w := bufio.NewWriterSize(f, fileBufSize)

	if _, err = w.WriteString(headsMagicString); err != nil {
		return
	}
	var numberOfSeriesOffset int
	if numberOfSeriesOffset, err = codable.EncodeVarint(w, headsFormatVersion); err != nil {
		return
	}
	numberOfSeriesOffset += len(headsMagicString)
	numberOfSeriesInHeader := uint64(fingerprintToSeries.length())
	// We have to write the number of series as uint64 because we might need
	// to overwrite it later, and a varint might change byte width then.
	if err = codable.EncodeUint64(w, numberOfSeriesInHeader); err != nil {
		return
	}

	iter := fingerprintToSeries.iter()
	defer func() {
		// Consume the iterator in any case to not leak goroutines.
		for range iter {
		}
	}()

	var realNumberOfSeries uint64
	for m := range iter {
		func() { // Wrapped in function to use defer for unlocking the fp.
			fpLocker.Lock(m.fp)
			defer fpLocker.Unlock(m.fp)

			if len(m.series.chunkDescs) == 0 {
				// This series was completely purged or archived in the meantime. Ignore.
//.........这里部分代码省略.........