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layered.go
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/
layered.go
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// Copyright 2016 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.
package binarystorage
import (
"io"
"github.com/juju/errors"
"github.com/juju/utils/set"
)
type layeredStorage []Storage
// NewLayeredStorage wraps multiple Storages such all of their metadata
// can be listed and fetched. The later entries in the list have lower
// precedence than the earlier ones. The first entry in the list is always
// used for mutating operations.
func NewLayeredStorage(s ...Storage) (Storage, error) {
if len(s) <= 1 {
return nil, errors.Errorf("expected multiple stores")
}
return layeredStorage(s), nil
}
// Add implements Storage.Add.
//
// This method operates on the first Storage passed to NewLayeredStorage.
func (s layeredStorage) Add(r io.Reader, m Metadata) error {
return s[0].Add(r, m)
}
// Open implements Storage.Open.
//
// This method calls Open for each Storage passed to NewLayeredStorage in
// the order given, and returns the first result where the error does not
// satisfy errors.IsNotFound.
func (s layeredStorage) Open(v string) (Metadata, io.ReadCloser, error) {
var m Metadata
var rc io.ReadCloser
var err error
for _, s := range s {
m, rc, err = s.Open(v)
if !errors.IsNotFound(err) {
break
}
}
return m, rc, err
}
// Metadata implements Storage.Metadata.
//
// This method calls Metadata for each Storage passed to NewLayeredStorage in
// the order given, and returns the first result where the error does not
// satisfy errors.IsNotFound.
func (s layeredStorage) Metadata(v string) (Metadata, error) {
var m Metadata
var err error
for _, s := range s {
m, err = s.Metadata(v)
if !errors.IsNotFound(err) {
break
}
}
return m, err
}
// AllMetadata implements Storage.AllMetadata.
//
// This method calls AllMetadata for each Storage passed to NewLayeredStorage
// in the order given, and accumulates the results. Any results from a Storage
// earlier in the list will take precedence over any others with the same
// version.
func (s layeredStorage) AllMetadata() ([]Metadata, error) {
seen := set.NewStrings()
var all []Metadata
for _, s := range s {
sm, err := s.AllMetadata()
if err != nil {
return nil, err
}
for _, m := range sm {
if seen.Contains(m.Version) {
continue
}
all = append(all, m)
seen.Add(m.Version)
}
}
return all, nil
}