func (db *Database) AddToChangeLogs(changedChannels base.Set, channelMap ChannelMap, entry channels.LogEntry, parentRevID string) error {
	var err error
	base.LogTo("Changes", "Updating #%d %q/%q in channels %s", entry.Sequence, entry.DocID, entry.RevID, changedChannels)
	for channel, removal := range channelMap {
		if removal != nil && removal.Seq != entry.Sequence {
			continue
		}
		// Set Removed flag appropriately for this channel:
		if removal != nil {
			entry.Flags |= channels.Removed
		} else {
			entry.Flags = entry.Flags &^ channels.Removed
		}
		if err1 := db.AddToChangeLog(channel, entry, parentRevID); err != nil {
			err = err1
		}
	}

	// Finally, add to the universal "*" channel.
	if EnableStarChannelLog {
		entry.Flags = entry.Flags &^ channels.Removed
		if err1 := db.AddToChangeLog("*", entry, parentRevID); err != nil {
			err = err1
		}
	}

	return err
}

// Adds a new change to a channel log.
func (db *Database) AddToChangeLog(channelName string, entry channels.LogEntry, parentRevID string) error {
	if channelName == "*" && !EnableStarChannelLog {
		return nil
	}
	var fullUpdate bool
	var removedCount int
	fullUpdateAttempts := 0

	logDocID := channelLogDocID(channelName)
	err := db.Bucket.WriteUpdate(logDocID, 0, func(currentValue []byte) ([]byte, walrus.WriteOptions, error) {
		// (Be careful: this block can be invoked multiple times if there are races!)
		// Should I do a full update of the change log, removing older entries to limit its size?
		// This has to be done occasionaly, but it's slower than simply appending to it. This
		// test is a heuristic that seems to strike a good balance in practice:
		fullUpdate = AlwaysCompactChangeLog ||
			(len(currentValue) > 20000 && (rand.Intn(100) < len(currentValue)/5000))
		removedCount = 0

		if len(currentValue) == 0 {
			channelLog := channels.ChangeLog{}
			channelLog.Add(entry)
			return encodeChannelLog(&channelLog), walrus.Raw, nil
		}

		if fullUpdate {
			fullUpdateAttempts++
			var newValue bytes.Buffer
			removedCount = channels.TruncateEncodedChangeLog(bytes.NewReader(currentValue),
				MaxChangeLogLength-1, &newValue)
			if removedCount > 0 {
				entry.Encode(&newValue, parentRevID)
				return newValue.Bytes(), walrus.Raw, nil
			}
		}

		w := bytes.NewBuffer(make([]byte, 0, 50000))
		entry.Encode(w, parentRevID)
		currentValue = append(currentValue, w.Bytes()...)
		return currentValue, walrus.Raw, nil
	})

	/*if fullUpdate {
		base.Log("Removed %d entries from %q", removedCount, channelName)
	} else if fullUpdateAttempts > 0 {
		base.Log("Attempted to remove entries %d times but failed", fullUpdateAttempts)
	}*/
	return err
}

// Returns a list of all the changes made on a channel.
// Does NOT handle the Wait option. Does NOT check authorization.
func (db *Database) changesFeed(channel string, options ChangesOptions) (<-chan *ChangeEntry, error) {
	since := options.Since[channel]
	channelLog, err := db.changesWriter.getChangeLog(channel, since)
	if err != nil {
		base.Warn("Error reading channel-log %q (using view instead): %v", channel, err)
		channelLog = nil
	}
	rebuildLog := channelLog == nil && err == nil && (EnableStarChannelLog || channel != "*")
	var log []*channels.LogEntry
	if channelLog != nil {
		log = channelLog.Entries
	}

	var viewFeed <-chan *ChangeEntry
	if channelLog == nil || channelLog.Since > since {
		var upToSeq uint64
		if channelLog != nil {
			upToSeq = channelLog.Since
		}
		// Channel log may not go back far enough, so also fetch view-based change feed:
		viewFeed, err = db.changesFeedFromView(channel, options, upToSeq)
		if err != nil {
			return nil, err
		}
	}

	feed := make(chan *ChangeEntry, 5)
	go func() {
		defer close(feed)

		// First, if we need to backfill from the view, write its early entries to the channel:
		if viewFeed != nil {
			newLog := channels.ChangeLog{Since: since}
			for change := range viewFeed {
				if channelLog != nil && change.seqNo > channelLog.Since {
					// TODO: Close the view-based feed somehow
					break
				}

				select {
				case <-options.Terminator:
					base.LogTo("Changes+", "Aborting changesFeed (reading from view)")
					return
				case feed <- change:
				}

				if rebuildLog {
					// If there wasn't any channel log, build up a new one from the view:
					entry := channels.LogEntry{
						Sequence: change.seqNo,
						DocID:    change.ID,
						RevID:    change.Changes[0]["rev"],
					}
					if change.Deleted {
						entry.Flags |= channels.Deleted
					}
					if change.Removed != nil {
						entry.Flags |= channels.Removed
					}
					newLog.Add(entry)
					newLog.TruncateTo(MaxChangeLogLength)
				}
			}

			if rebuildLog {
				// Save the missing channel log we just rebuilt:
				base.LogTo("Changes", "Saving rebuilt channel log %q with %d sequences",
					channel, len(newLog.Entries))
				db.changesWriter.addChangeLog(channel, &newLog)
			}
		}

		// Now write each log entry to the 'feed' channel in turn:
		for _, logEntry := range log {
			if !options.Conflicts && (logEntry.Flags&channels.Hidden) != 0 {
				//continue  // FIX: had to comment this out.
				// This entry is shadowed by a conflicting one. We would like to skip it.
				// The problem is that if this is the newest revision of this doc, then the
				// doc will appear under this sequence # in the changes view, which means
				// we won't emit the doc at all because we already stopped emitting entries
				// from the view before this point.
			}
			change := ChangeEntry{
				seqNo:   logEntry.Sequence,
				ID:      logEntry.DocID,
				Deleted: (logEntry.Flags & channels.Deleted) != 0,
				Changes: []ChangeRev{{"rev": logEntry.RevID}},
			}
			if logEntry.Flags&channels.Removed != 0 {
				change.Removed = channels.SetOf(channel)
			} else if options.IncludeDocs || options.Conflicts {
				doc, _ := db.GetDoc(logEntry.DocID)
				db.addDocToChangeEntry(doc, &change, options.IncludeDocs, false)
			}

			select {
			case <-options.Terminator:
				base.LogTo("Changes+", "Aborting changesFeed")
//.........这里部分代码省略.........

//.........这里部分代码省略.........
			// need to update the doc's top-level Channels and Access properties to correspond
			// to the current rev's state.
			if newRevID != doc.CurrentRev {
				// In some cases an older revision might become the current one. If so, get its
				// channels & access, for purposes of updating the doc:
				var curBody Body
				if curBody, err = db.getAvailableRev(doc, doc.CurrentRev); curBody != nil {
					base.LogTo("CRUD+", "updateDoc(%q): Rev %q causes %q to become current again",
						docid, newRevID, doc.CurrentRev)
					curParent := doc.History[doc.CurrentRev].Parent
					channels, access, roles, err = db.getChannelsAndAccess(doc, curBody, curParent)
					if err != nil {
						return
					}
				} else {
					// Shouldn't be possible (CurrentRev is a leaf so won't have been compacted)
					base.Warn("updateDoc(%q): Rev %q missing, can't call getChannelsAndAccess "+
						"on it (err=%v)", docid, doc.CurrentRev, err)
					channels = nil
					access = nil
					roles = nil
				}
			}

			// Update the document struct's channel assignment and user access.
			// (This uses the new sequence # so has to be done after updating doc.Sequence)
			changedChannels = doc.updateChannels(channels) //FIX: Incorrect if new rev is not current!
			changedPrincipals = doc.Access.updateAccess(doc, access)
			changedRoleUsers = doc.RoleAccess.updateAccess(doc, roles)
			if len(changedPrincipals) > 0 || len(changedRoleUsers) > 0 {
				// If this update affects user/role access privileges, make sure the write blocks till
				// the new value is indexable, otherwise when a User/Role updates (using a view) it
				// might not incorporate the effects of this change.
				writeOpts |= walrus.Indexable
			}
		} else {
			base.LogTo("CRUD+", "updateDoc(%q): Rev %q leaves %q still current",
				docid, newRevID, prevCurrentRev)
		}

		// Prune old revision history to limit the number of revisions:
		if pruned := doc.History.pruneRevisions(db.RevsLimit); pruned > 0 {
			base.LogTo("CRUD+", "updateDoc(%q): Pruned %d old revisions", docid, pruned)
		}

		// Return the new raw document value for the bucket to store.
		raw, err = json.Marshal(doc)
		return
	})

	if err == couchbase.UpdateCancel {
		return "", nil
	} else if err == couchbase.ErrOverwritten {
		// ErrOverwritten is ok; if a later revision got persisted, that's fine too
		base.LogTo("CRUD+", "Note: Rev %q/%q was overwritten in RAM before becoming indexable",
			docid, newRevID)
	} else if err != nil {
		return "", err
	}

	dbExpvars.Add("revs_added", 1)

	// Store the new revision in the cache
	history := doc.History.getHistory(newRevID)
	if doc.History[newRevID].Deleted {
		body["_deleted"] = true
	}
	revChannels := doc.History[newRevID].Channels
	db.revisionCache.Put(body, encodeRevisions(history), revChannels)

	// Now that the document has successfully been stored, we can make other db changes:
	base.LogTo("CRUD", "Stored doc %q / %q", docid, newRevID)

	// Mark affected users/roles as needing to recompute their channel access:
	for _, name := range changedPrincipals {
		db.invalUserOrRoleChannels(name)
	}
	for _, name := range changedRoleUsers {
		db.invalUserRoles(name)
	}

	// Add the new revision to the change logs of all affected channels:
	newEntry := channels.LogEntry{
		Sequence: doc.Sequence,
		DocID:    docid,
		RevID:    newRevID,
	}
	if doc.History[newRevID].Deleted {
		newEntry.Flags |= channels.Deleted
	}
	if newRevID != doc.CurrentRev {
		newEntry.Flags |= channels.Hidden
	}
	if inConflict {
		newEntry.Flags |= channels.Conflict
	}
	db.changesWriter.addToChangeLogs(changedChannels, doc.Channels, newEntry, parentRevID)

	return newRevID, nil
}

// Returns a list of all the changes made on a channel.
// Does NOT handle the Wait option. Does NOT check authorization.
func (db *Database) changesFeed(channel string, options ChangesOptions) (<-chan *ChangeEntry, error) {
	since := options.Since[channel]
	channelLog, err := db.GetChangeLog(channel, since)
	if err != nil {
		base.Warn("Error reading channel-log %q (using view instead) %v", channel, err)
		channelLog = nil
	}
	rebuildLog := channelLog == nil && err == nil && (EnableStarChannelLog || channel != "*")
	var log []*channels.LogEntry
	if channelLog != nil {
		log = channelLog.Entries
	}

	var viewFeed <-chan *ChangeEntry
	if channelLog == nil || channelLog.Since > since {
		// Channel log may not go back far enough, so also fetch view-based change feed:
		viewFeed, err = db.changesFeedFromView(channel, options)
		if err != nil {
			return nil, err
		}
	}

	feed := make(chan *ChangeEntry, 5)
	go func() {
		defer close(feed)

		// First, if we need to backfill from the view, write its early entries to the channel:
		if viewFeed != nil {
			newLog := channels.ChangeLog{Since: since}
			for change := range viewFeed {
				if len(log) > 0 && change.seqNo >= log[0].Sequence {
					// TODO: Close the view-based feed somehow
					break
				}
				feed <- change
				if rebuildLog {
					// If there wasn't any channel log, build up a new one from the view:
					entry := channels.LogEntry{
						Sequence: change.seqNo,
						DocID:    change.ID,
						RevID:    change.Changes[0]["rev"],
					}
					if change.Deleted {
						entry.Flags |= channels.Deleted
					}
					if change.Removed != nil {
						entry.Flags |= channels.Removed
					}
					newLog.Add(entry)
					newLog.TruncateTo(MaxChangeLogLength)
				}
			}

			if rebuildLog {
				// Save the missing channel log we just rebuilt:
				base.LogTo("Changes", "Saving rebuilt channel log %q with %d sequences",
					channel, len(newLog.Entries))
				if _, err := db.AddChangeLog(channel, &newLog); err != nil {
					base.Warn("ChangesFeed: AddChangeLog failed, %v", err)
				}
			}
		}

		// Now write each log entry to the 'feed' channel in turn:
		for _, logEntry := range log {
			hidden := (logEntry.Flags & channels.Hidden) != 0
			if logEntry.RevID == "" || (hidden && !options.Conflicts) {
				continue
			}
			change := ChangeEntry{
				seqNo:   logEntry.Sequence,
				ID:      logEntry.DocID,
				Deleted: (logEntry.Flags & channels.Deleted) != 0,
				Changes: []ChangeRev{{"rev": logEntry.RevID}},
			}
			if logEntry.Flags&channels.Removed != 0 {
				change.Removed = channels.SetOf(channel)
			} else if options.IncludeDocs || options.Conflicts {
				doc, _ := db.getDoc(logEntry.DocID)
				db.addDocToChangeEntry(doc, &change, options.IncludeDocs, false)
			}
			feed <- &change

			if options.Limit > 0 {
				options.Limit--
				if options.Limit == 0 {
					break
				}
			}
		}
	}()
	return feed, nil
}