[WIP] Partial bot skeleton, non-functional though

vendor/github.com/cockroachdb/pebble/objstorage/shared_readable.go

@@ -0,0 +1,97 @@
+// Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use
+// of this source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package objstorage
+
+import (
+	"io"
+
+	"github.com/cockroachdb/pebble/objstorage/shared"
+)
+
+// sharedReadable is a very simple implementation of Readable on top of the
+// ReadCloser returned by shared.Storage.CreateObject.
+type sharedReadable struct {
+	storage shared.Storage
+	objName string
+	size    int64
+
+	// rh is used for direct ReadAt calls without a read handle.
+	rh sharedReadHandle
+}
+
+var _ Readable = (*sharedReadable)(nil)
+
+func newSharedReadable(storage shared.Storage, objName string, size int64) *sharedReadable {
+	r := &sharedReadable{
+		storage: storage,
+		objName: objName,
+		size:    size,
+	}
+	r.rh.readable = r
+	return r
+}
+
+func (r *sharedReadable) ReadAt(p []byte, offset int64) (n int, err error) {
+	return r.rh.ReadAt(p, offset)
+}
+
+func (r *sharedReadable) Close() error {
+	err := r.rh.Close()
+	r.storage = nil
+	return err
+}
+
+func (r *sharedReadable) Size() int64 {
+	return r.size
+}
+
+func (r *sharedReadable) NewReadHandle() ReadHandle {
+	// TODO(radu): use a pool.
+	return &sharedReadHandle{readable: r}
+}
+
+type sharedReadHandle struct {
+	readable   *sharedReadable
+	lastReader io.ReadCloser
+	lastOffset int64
+}
+
+var _ ReadHandle = (*sharedReadHandle)(nil)
+
+func (r *sharedReadHandle) ReadAt(p []byte, offset int64) (n int, err error) {
+	// See if this continues the previous read so that we can reuse the last reader.
+	if r.lastReader == nil || r.lastOffset != offset {
+		// We need to create a new reader.
+		if r.lastReader != nil {
+			if err := r.lastReader.Close(); err != nil {
+				return 0, err
+			}
+			r.lastReader = nil
+		}
+		reader, _, err := r.readable.storage.ReadObjectAt(r.readable.objName, offset)
+		if err != nil {
+			return 0, err
+		}
+		r.lastReader = reader
+		r.lastOffset = offset
+	}
+	n, err = io.ReadFull(r.lastReader, p)
+	r.lastOffset += int64(n)
+	return n, err
+}
+
+func (r *sharedReadHandle) Close() error {
+	var err error
+	if r.lastReader != nil {
+		err = r.lastReader.Close()
+		r.lastReader = nil
+	}
+	r.readable = nil
+	return err
+}
+
+func (r *sharedReadHandle) MaxReadahead() {}
+
+func (r *sharedReadHandle) RecordCacheHit(offset, size int64) {}