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gcs.go
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gcs.go
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// Copyright 2023 The glassdb Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package gcs
import (
"context"
"fmt"
"hash/crc32"
"io"
"net/http"
"time"
"cloud.google.com/go/storage"
"github.com/googleapis/gax-go/v2"
"google.golang.org/api/googleapi"
"google.golang.org/api/iterator"
"github.com/mbrt/glassdb/backend"
"github.com/mbrt/glassdb/internal/errors"
)
// TODO: Reimplement by using GCS REST APIs. The Go client is inefficient.
// See https://cloud.google.com/storage/docs/uploading-objects#uploading-an-object
// Even better, we can use GRPC only. See e.g.
// https://github.com/fsouza/fake-gcs-server/blob/4d841124144532e140ac3c3261d4a87593b6e3c1/main.go#L91-L93
var errRateLimited = errors.New("rate limited")
func New(bucket *storage.BucketHandle) Backend {
return Backend{bucket}
}
type Backend struct {
bucket *storage.BucketHandle
}
func (b Backend) GetMetadata(ctx context.Context, path string) (backend.Metadata, error) {
attr, err := b.object(path).Attrs(ctx)
if err != nil {
return backend.Metadata{}, annotate(fmt.Errorf("GetMetadata(%q): %w", path, err))
}
return backend.Metadata{
Tags: attr.Metadata,
Version: backend.Version{
Contents: attr.Generation,
Meta: attr.Metageneration,
},
}, nil
}
func (b Backend) ReadIfModified(ctx context.Context,
path string,
version int64,
) (backend.ReadReply, error) {
obj := b.object(path).If(storage.Conditions{
GenerationNotMatch: version,
})
return b.readFrom(ctx, obj)
}
func (b Backend) Read(ctx context.Context, path string) (backend.ReadReply, error) {
obj := b.object(path)
return b.readFrom(ctx, obj)
}
func (b Backend) SetTagsIf(
ctx context.Context,
path string,
expected backend.Version,
t backend.Tags,
) (backend.Metadata, error) {
obj := b.object(path).If(storage.Conditions{
GenerationMatch: expected.Contents,
MetagenerationMatch: expected.Meta,
})
attr, err := obj.Update(ctx, storage.ObjectAttrsToUpdate{
Metadata: t,
})
if err != nil {
return backend.Metadata{}, annotate(fmt.Errorf("set tags: %w", err))
}
return backend.Metadata{
Tags: attr.Metadata,
Version: backend.Version{
Contents: attr.Generation,
Meta: attr.Metageneration,
},
}, nil
}
func (b Backend) Write(ctx context.Context,
path string,
value []byte,
t backend.Tags,
) (meta backend.Metadata, err error) {
obj := b.object(path)
return b.writeTo(ctx, obj, value, t)
}
func (b Backend) WriteIf(
ctx context.Context,
path string,
value []byte,
expected backend.Version,
t backend.Tags,
) (meta backend.Metadata, err error) {
obj := b.object(path).If(storage.Conditions{
GenerationMatch: expected.Contents,
MetagenerationMatch: expected.Meta,
})
return b.writeTo(ctx, obj, value, t)
}
func (b Backend) WriteIfNotExists(
ctx context.Context,
path string,
value []byte,
t backend.Tags,
) (meta backend.Metadata, err error) {
obj := b.object(path).If(storage.Conditions{
DoesNotExist: true,
})
return b.writeTo(ctx, obj, value, t)
}
func (b Backend) Delete(ctx context.Context, path string) error {
if err := b.object(path).Delete(ctx); err != nil {
return annotate(fmt.Errorf("deleting object: %w", err))
}
return nil
}
func (b Backend) DeleteIf(ctx context.Context, path string, expected backend.Version) error {
obj := b.object(path).If(storage.Conditions{
GenerationMatch: expected.Contents,
MetagenerationMatch: expected.Meta,
})
if err := obj.Delete(ctx); err != nil {
return annotate(fmt.Errorf("deleting object: %w", err))
}
return nil
}
func (b Backend) List(ctx context.Context, dirPath string) (backend.ListIter, error) {
iter := b.bucket.Objects(ctx, &storage.Query{
Delimiter: "/",
Prefix: ensureTrailingSlash(dirPath),
Projection: storage.ProjectionNoACL,
IncludeTrailingDelimiter: false,
})
return &listIter{inner: iter}, nil
}
func (b Backend) readFrom(
ctx context.Context,
obj *storage.ObjectHandle,
) (backend.ReadReply, error) {
reader, err := obj.NewReader(ctx)
if err != nil {
return backend.ReadReply{}, annotate(fmt.Errorf("opening object in read: %w", err))
}
defer reader.Close()
buf, err := io.ReadAll(reader)
if err != nil {
return backend.ReadReply{}, annotate(fmt.Errorf("reading object: %w", err))
}
return backend.ReadReply{
Contents: buf,
Version: backend.Version{
Contents: reader.Attrs.Generation,
Meta: reader.Attrs.Metageneration,
},
}, nil
}
func (b Backend) writeTo(
ctx context.Context,
obj *storage.ObjectHandle,
value []byte,
t backend.Tags,
) (meta backend.Metadata, err error) {
writer := obj.NewWriter(ctx)
// If tags are nil, no updates to metadata.
// Try with no buffered reads. This is more performant, but doesn't allow
// for automatic retries.
writer.ChunkSize = 0
writer.ObjectAttrs.Metadata = t
writer.ContentType = "application/octet-stream"
// The GCS client has a race condition on context cancellation. The object
// is sent only partially but the request looks 100% the same as a legit
// one. This is visible when the cancellation happens between metadata and
// object data being sent. If this happens, the result is an empty object.
// TODO: Fix this somehow.
writer.SendCRC32C = true
writer.CRC32C = crc32.Checksum(value, crc32.MakeTable(crc32.Castagnoli))
meta, err = b.writeWith(writer, value)
if err == nil || !errors.Is(err, errRateLimited) {
return meta, err
}
// Here we were rate limited. Let's just do this again, but allow for
// automatic retries by setting a reasonable chunk size.
writer = obj.NewWriter(ctx)
writer.ChunkSize = len(value) + 100
writer.ObjectAttrs.Metadata = t
writer.ContentType = "application/octet-stream"
return b.writeWith(writer, value)
}
func (b Backend) writeWith(
writer *storage.Writer,
value []byte,
) (meta backend.Metadata, err error) {
_, err = writer.Write(value)
if err != nil {
return backend.Metadata{}, annotate(err)
}
if err := writer.Close(); err != nil {
return backend.Metadata{}, annotate(fmt.Errorf("writing object: %w", err))
}
return backend.Metadata{
Tags: writer.Attrs().Metadata,
Version: backend.Version{
Contents: writer.Attrs().Generation,
Meta: writer.Attrs().Metageneration,
},
}, nil
}
func (b Backend) object(path string) *storage.ObjectHandle {
// https://cloud.google.com/storage/docs/retry-strategy.
return b.bucket.Object(path).Retryer(
storage.WithBackoff(gax.Backoff{
Initial: 200 * time.Millisecond,
Max: 3 * time.Second,
Multiplier: 2,
}),
storage.WithPolicy(storage.RetryAlways),
)
}
func annotate(err error) error {
if err == nil {
return nil
}
var aerr *googleapi.Error
if errors.As(err, &aerr) {
switch aerr.Code {
case http.StatusNotFound:
return errors.WithCause(err, backend.ErrNotFound)
case http.StatusPreconditionFailed:
return errors.WithCause(err, backend.ErrPrecondition)
case http.StatusTooManyRequests:
return errors.WithCause(err, errRateLimited)
default:
return err
}
}
if errors.Is(err, storage.ErrObjectNotExist) {
return backend.ErrNotFound
}
return err
}
func ensureTrailingSlash(a string) string {
if len(a) > 0 && a[len(a)-1] == '/' {
return a
}
return a + "/"
}
type listIter struct {
inner *storage.ObjectIterator
err error
}
func (l *listIter) Next() (string, bool) {
attrs, err := l.inner.Next()
if err != nil {
// Filter out iterator.Done errors.
if !errors.Is(err, iterator.Done) {
l.err = err
}
return "", false
}
if attrs.Prefix != "" {
return attrs.Prefix, true
}
return attrs.Name, true
}
func (l *listIter) Err() error {
return l.err
}
// Ensure that Backend interface is implemented correctly.
var (
_ backend.Backend = (*Backend)(nil)
_ backend.ListIter = (*listIter)(nil)
)