// readSkews reads all clock data for the client's namespace.
func (client *client) readSkews(collection mongo.Collection) (map[string]Skew, error) {

	// Read the clock document, recording the time before and after completion.
	readBefore := client.config.Clock.Now()
	var clockDoc clockDoc
	if err := collection.FindId(client.clockDocId()).One(&clockDoc); err != nil {
		return nil, errors.Trace(err)
	}
	readAfter := client.config.Clock.Now()
	if err := clockDoc.validate(); err != nil {
		return nil, errors.Annotatef(err, "corrupt clock document")
	}

	// Create skew entries for each known writer...
	skews, err := clockDoc.skews(readBefore, readAfter)
	if err != nil {
		return nil, errors.Trace(err)
	}

	// If a writer was previously known to us, and has not written since last
	// time we read, we should keep the original skew, which is more accurate.
	for writer, skew := range client.skews {
		if skews[writer].LastWrite == skew.LastWrite {
			skews[writer] = skew
		}
	}

	// ...and overwrite our own with a zero skew, which will DTRT (assuming
	// nobody's reusing client ids across machines with different clocks,
	// which *should* never happen).
	skews[client.config.Id] = Skew{}
	return skews, nil
}

// exists returns whether the identified refcount doc exists.
func (nsRefcounts_) exists(coll mongo.Collection, key string) (bool, error) {
	count, err := coll.FindId(key).Count()
	if err != nil {
		return false, errors.Trace(err)
	}
	return count != 0, nil
}

// deleteOldPlaceholderCharmsOps returns the txn ops required to delete all placeholder charm
// records older than the specified charm URL.
func deleteOldPlaceholderCharmsOps(st *State, charms mongo.Collection, curl *charm.URL) ([]txn.Op, error) {
	// Get a regex with the charm URL and no revision.
	noRevURL := curl.WithRevision(-1)
	curlRegex := "^" + regexp.QuoteMeta(st.docID(noRevURL.String()))

	var docs []charmDoc
	query := bson.D{{"_id", bson.D{{"$regex", curlRegex}}}, {"placeholder", true}}
	err := charms.Find(query).Select(bson.D{{"_id", 1}, {"url", 1}}).All(&docs)
	if err != nil {
		return nil, errors.Trace(err)
	}
	var ops []txn.Op
	for _, doc := range docs {
		if doc.URL.Revision >= curl.Revision {
			continue
		}
		ops = append(ops, txn.Op{
			C:      charmsC,
			Id:     doc.DocID,
			Assert: stillPlaceholder,
			Remove: true,
		})
	}
	return ops, nil
}

// StrictIncRefOp returns a txn.Op that increments the value of a
// refcount doc, or returns an error if it does not exist.
func (ns nsRefcounts_) StrictIncRefOp(coll mongo.Collection, key string, n int) (txn.Op, error) {
	if exists, err := ns.exists(coll, key); err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if !exists {
		return txn.Op{}, errors.New("does not exist")
	}
	return ns.JustIncRefOp(coll.Name(), key, n), nil
}

// getEntitiesWithStatuses returns the ids for all entities that
// have history entries
func getEntitiesWithStatuses(coll mongo.Collection) ([]string, error) {
	var globalKeys []string
	err := coll.Find(nil).Distinct("globalkey", &globalKeys)
	if err != nil {
		return nil, errors.Trace(err)
	}
	return globalKeys, nil
}

// getEntitiesWithStatuses returns the ids for all entities that
// have history entries
func getEntitiesWithStatuses(coll mongo.Collection) ([]string, error) {
	var entityKeys []string
	err := coll.Find(nil).Distinct("entityid", &entityKeys)
	if err != nil {
		return nil, errors.Trace(err)
	}
	return entityKeys, nil
}

// AliveDecRefOp returns a txn.Op that decrements the value of a
// refcount doc, or an error if the doc does not exist or the count
// would go below 0.
func (ns nsRefcounts_) AliveDecRefOp(coll mongo.Collection, key string) (txn.Op, error) {
	if refcount, err := ns.read(coll, key); err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if refcount < 1 {
		return txn.Op{}, errors.New("cannot decRef below 0")
	}
	return ns.justDecRefOp(coll.Name(), key, 0), nil
}

// StrictCreateOp returns a txn.Op that creates a refcount document as
// configured, or an error if the document already exists.
func (ns nsRefcounts_) StrictCreateOp(coll mongo.Collection, key string, value int) (txn.Op, error) {
	if exists, err := ns.exists(coll, key); err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if exists {
		return txn.Op{}, errors.New("refcount already exists")
	}
	return ns.JustCreateOp(coll.Name(), key, value), nil
}

// LazyCreateOp returns a txn.Op that creates a refcount document; or
// false if the document already exists.
func (ns nsRefcounts_) LazyCreateOp(coll mongo.Collection, key string) (txn.Op, bool, error) {
	if exists, err := ns.exists(coll, key); err != nil {
		return txn.Op{}, false, errors.Trace(err)
	} else if exists {
		return txn.Op{}, false, nil
	}
	return ns.JustCreateOp(coll.Name(), key, 0), true, nil
}

// read returns the value stored in the identified refcount doc.
func (nsRefcounts_) read(coll mongo.Collection, key string) (int, error) {
	var doc refcountDoc
	if err := coll.FindId(key).One(&doc); err == mgo.ErrNotFound {
		return 0, errors.NotFoundf("refcount %q", key)
	} else if err != nil {
		return 0, errors.Trace(err)
	}
	return doc.RefCount, nil
}

func newStateCollection(collection mongo.Collection, envUUID string) mongo.Collection {
	if multiEnvCollections.Contains(collection.Name()) {
		return &envStateCollection{
			WriteCollection: collection.Writeable(),
			envUUID:         envUUID,
		}
	}
	return collection
}

// RemoveOp returns a txn.Op that removes a refcount doc so long as its
// refcount is the supplied value, or an error.
func (ns nsRefcounts_) RemoveOp(coll mongo.Collection, key string, value int) (txn.Op, error) {
	refcount, err := ns.read(coll, key)
	if err != nil {
		return txn.Op{}, errors.Trace(err)
	}
	if refcount != value {
		return txn.Op{}, errRefcountChanged
	}
	return ns.JustRemoveOp(coll.Name(), key, value), nil
}

// getOldestTimeToKeep returns the create time for the oldest
// status log to be kept.
func getOldestTimeToKeep(coll mongo.Collection, globalKey string, size int) (int64, bool, error) {
	result := historicalStatusDoc{}
	err := coll.Find(bson.D{{"globalkey", globalKey}}).Sort("-updated").Skip(size - 1).One(&result)
	if err == mgo.ErrNotFound {
		return -1, false, nil
	}
	if err != nil {
		return -1, false, errors.Trace(err)
	}
	return result.Updated, true, nil

}

// DyingDecRefOp returns a txn.Op that decrements the value of a
// refcount doc and deletes it if the count reaches 0; if the Op will
// cause a delete, the bool result will be true. It will return an error
// if the doc does not exist or the count would go below 0.
func (ns nsRefcounts_) DyingDecRefOp(coll mongo.Collection, key string) (txn.Op, bool, error) {
	refcount, err := ns.read(coll, key)
	if err != nil {
		return txn.Op{}, false, errors.Trace(err)
	}
	if refcount < 1 {
		return txn.Op{}, false, errors.New("cannot decRef below 0")
	} else if refcount > 1 {
		return ns.justDecRefOp(coll.Name(), key, 1), false, nil
	}
	return ns.JustRemoveOp(coll.Name(), key, 1), true, nil
}

// setStatusOp returns a txn.Op that updates the status of the
// identified payload. If the payload doesn't exist, it returns
// errAlreadyRemoved.
func (nsPayloads_) setStatusOp(payloads mongo.Collection, docID string, status string) (txn.Op, error) {
	count, err := payloads.FindId(docID).Count()
	if err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if count == 0 {
		return txn.Op{}, errAlreadyRemoved
	}
	return txn.Op{
		C:      payloads.Name(),
		Id:     docID,
		Assert: txn.DocExists,
		Update: bson.D{{"$set", bson.D{{"state", status}}}},
	}, nil
}

// untrackOp returns a txn.Op that will unconditionally remove the
// identified document. If the payload doesn't exist, it returns
// errAlreadyRemoved.
func (nsPayloads_) untrackOp(payloads mongo.Collection, docID string) (txn.Op, error) {
	count, err := payloads.FindId(docID).Count()
	if err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if count == 0 {
		return txn.Op{}, errAlreadyRemoved
	}
	return txn.Op{
		C:      payloads.Name(),
		Id:     docID,
		Assert: txn.DocExists,
		Remove: true,
	}, nil
}

// CurrentOp returns the current reference count value, and a txn.Op that
// asserts that the refcount has that value, or an error. If the refcount
// doc does not exist, then the op will assert that the document does not
// exist instead, and no error is returned.
func (ns nsRefcounts_) CurrentOp(coll mongo.Collection, key string) (txn.Op, int, error) {
	refcount, err := ns.read(coll, key)
	if errors.IsNotFound(err) {
		return txn.Op{
			C:      coll.Name(),
			Id:     key,
			Assert: txn.DocMissing,
		}, 0, nil
	}
	if err != nil {
		return txn.Op{}, -1, errors.Trace(err)
	}
	return txn.Op{
		C:      coll.Name(),
		Id:     key,
		Assert: bson.D{{"refcount", refcount}},
	}, refcount, nil
}

// trackOp returns a txn.Op that will either insert or update the
// supplied payload, and fail if the observed precondition changes.
func (nsPayloads_) trackOp(payloads mongo.Collection, doc payloadDoc) (txn.Op, error) {
	docID := nsPayloads.docID(doc.UnitID, doc.Name)
	payloadOp := txn.Op{
		C:  payloads.Name(),
		Id: docID,
	}
	count, err := payloads.FindId(docID).Count()
	if err != nil {
		return txn.Op{}, errors.Trace(err)
	} else if count == 0 {
		payloadOp.Assert = txn.DocMissing
		payloadOp.Insert = doc
	} else {
		payloadOp.Assert = txn.DocExists
		payloadOp.Update = bson.D{{"$set", doc}}
	}
	return payloadOp, nil
}

// readEntries reads all lease data for the client's namespace.
func (client *client) readEntries(collection mongo.Collection) (map[string]entry, error) {

	// Read all lease documents in the client's namespace.
	query := bson.M{
		fieldType:      typeLease,
		fieldNamespace: client.config.Namespace,
	}
	iter := collection.Find(query).Iter()

	// Extract valid entries for each one.
	entries := make(map[string]entry)
	var leaseDoc leaseDoc
	for iter.Next(&leaseDoc) {
		name, entry, err := leaseDoc.entry()
		if err != nil {
			return nil, errors.Annotatef(err, "corrupt lease document %q", leaseDoc.Id)
		}
		entries[name] = entry
	}
	if err := iter.Close(); err != nil {
		return nil, errors.Trace(err)
	}
	return entries, nil
}

func newBinaryStorage(uuid string, metadataCollection mongo.Collection, txnRunner jujutxn.Runner) binarystorage.Storage {
	db := metadataCollection.Writeable().Underlying().Database
	rs := blobstore.NewGridFS(blobstoreDB, blobstoreDB, db.Session)
	managedStorage := blobstore.NewManagedStorage(db, rs)
	return binarystorageNew(uuid, managedStorage, metadataCollection, txnRunner)
}