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// Package chain provides an adaptive wrapper over multiple object storage
// backends.
package chain
import (
"errors"
"fmt"
"io"
"sync"
"codeberg.org/lindenii/furgit/objectid"
"codeberg.org/lindenii/furgit/objectstore"
"codeberg.org/lindenii/furgit/objecttype"
)
// Chain queries multiple object databases with an MRU backend preference.
type Chain struct {
mu sync.RWMutex
backendHead *backendNode
backendTail *backendNode
backendNodeByStore map[objectstore.Store]*backendNode
}
// New creates a Chain from backends.
func New(backends ...objectstore.Store) *Chain {
nodeByStore := make(map[objectstore.Store]*backendNode, len(backends))
var head *backendNode
var tail *backendNode
for _, backend := range backends {
if backend == nil {
continue
}
node := &backendNode{
backend: backend,
prev: tail,
}
if tail != nil {
tail.next = node
}
if head == nil {
head = node
}
tail = node
nodeByStore[backend] = node
}
return &Chain{
backendHead: head,
backendTail: tail,
backendNodeByStore: nodeByStore,
}
}
// ReadBytesFull reads a full serialized object from one backend that has it.
func (chain *Chain) ReadBytesFull(id objectid.ObjectID) ([]byte, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
full, err := backend.ReadBytesFull(id)
if err == nil {
chain.touchBackend(backend)
return full, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return nil, fmt.Errorf("objectstore: backend %d read bytes full: %w", i, err)
}
return nil, objectstore.ErrObjectNotFound
}
// ReadBytesContent reads an object's type and content bytes from one backend
// that has it.
func (chain *Chain) ReadBytesContent(id objectid.ObjectID) (objecttype.Type, []byte, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
ty, content, err := backend.ReadBytesContent(id)
if err == nil {
chain.touchBackend(backend)
return ty, content, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return objecttype.TypeInvalid, nil, fmt.Errorf("objectstore: backend %d read bytes content: %w", i, err)
}
return objecttype.TypeInvalid, nil, objectstore.ErrObjectNotFound
}
// ReadReaderFull reads a full serialized object stream from one backend that
// has it.
func (chain *Chain) ReadReaderFull(id objectid.ObjectID) (io.ReadCloser, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
reader, err := backend.ReadReaderFull(id)
if err == nil {
chain.touchBackend(backend)
return reader, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return nil, fmt.Errorf("objectstore: backend %d read reader full: %w", i, err)
}
return nil, objectstore.ErrObjectNotFound
}
// ReadReaderContent reads an object's type, declared content length, and
// content stream from one backend that has it.
func (chain *Chain) ReadReaderContent(id objectid.ObjectID) (objecttype.Type, int64, io.ReadCloser, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
ty, size, reader, err := backend.ReadReaderContent(id)
if err == nil {
chain.touchBackend(backend)
return ty, size, reader, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return objecttype.TypeInvalid, 0, nil, fmt.Errorf("objectstore: backend %d read reader content: %w", i, err)
}
return objecttype.TypeInvalid, 0, nil, objectstore.ErrObjectNotFound
}
// ReadSize reads object content length from one backend that has it.
func (chain *Chain) ReadSize(id objectid.ObjectID) (int64, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
size, err := backend.ReadSize(id)
if err == nil {
chain.touchBackend(backend)
return size, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return 0, fmt.Errorf("objectstore: backend %d read size: %w", i, err)
}
return 0, objectstore.ErrObjectNotFound
}
// ReadHeader reads object header data from one backend that has it.
func (chain *Chain) ReadHeader(id objectid.ObjectID) (objecttype.Type, int64, error) {
for i, backend := 0, chain.firstBackend(); backend != nil; i, backend = i+1, chain.nextBackend(backend) {
ty, size, err := backend.ReadHeader(id)
if err == nil {
chain.touchBackend(backend)
return ty, size, nil
}
if errors.Is(err, objectstore.ErrObjectNotFound) {
continue
}
return objecttype.TypeInvalid, 0, fmt.Errorf("objectstore: backend %d read header: %w", i, err)
}
return objecttype.TypeInvalid, 0, objectstore.ErrObjectNotFound
}
// Close closes all backends and joins close errors.
func (chain *Chain) Close() error {
chain.mu.RLock()
backends := make([]objectstore.Store, 0, len(chain.backendNodeByStore))
for node := chain.backendHead; node != nil; node = node.next {
backends = append(backends, node.backend)
}
chain.mu.RUnlock()
var errs []error
for _, backend := range backends {
if err := backend.Close(); err != nil {
errs = append(errs, err)
}
}
return errors.Join(errs...)
}
type backendNode struct {
backend objectstore.Store
prev *backendNode
next *backendNode
}
func (chain *Chain) firstBackend() objectstore.Store {
chain.mu.RLock()
defer chain.mu.RUnlock()
if chain.backendHead == nil {
return nil
}
return chain.backendHead.backend
}
func (chain *Chain) nextBackend(current objectstore.Store) objectstore.Store {
chain.mu.RLock()
defer chain.mu.RUnlock()
node := chain.backendNodeByStore[current]
if node == nil || node.next == nil {
return nil
}
return node.next.backend
}
func (chain *Chain) touchBackend(backend objectstore.Store) {
if backend == nil {
return
}
if !chain.mu.TryLock() {
return
}
defer chain.mu.Unlock()
node := chain.backendNodeByStore[backend]
if node == nil || node == chain.backendHead {
return
}
if node.prev != nil {
node.prev.next = node.next
}
if node.next != nil {
node.next.prev = node.prev
}
if chain.backendTail == node {
chain.backendTail = node.prev
}
node.prev = nil
node.next = chain.backendHead
if chain.backendHead != nil {
chain.backendHead.prev = node
}
chain.backendHead = node
if chain.backendTail == nil {
chain.backendTail = node
}
}
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