internal/pkg/casmsg/assembler.go

// Copyright 2026 Google LLC
//
// 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 casmsg

import (
	"sync"

	bspb "google.golang.org/genproto/googleapis/bytestream"
)

// ByteStreamAssembler is a stateful pipe that populates "Sticky ResourceNames"
// in a Bytestream Write stream.
type ByteStreamAssembler struct {
	mu sync.Mutex

	// activeNames maps stream IDs to the last seen resource name for that stream.
	// In the Bytestream Write protocol, the ResourceName is only required on the
	// first message; subsequent messages in the same stream are assumed to belong
	// to the same resource. This map allows the assembler to restore (rehydrate)
	// that name into subsequent messages.
	activeNames map[uint64]string
}

// NewByteStreamAssembler creates a new ByteStreamAssembler.
func NewByteStreamAssembler() *ByteStreamAssembler {
	return &ByteStreamAssembler{
		activeNames: make(map[uint64]string),
	}
}

// Assemble is a transformation function that restores missing resource names
// in a Bytestream Write sequence based on the stream's first message.
func (a *ByteStreamAssembler) Assemble(raw *RawWriteRequest) (*bspb.WriteRequest, bool) {
	if raw == nil {
		return nil, false
	}

	a.mu.Lock()
	defer a.mu.Unlock()

	// Handle Stream Closure: Clean up the remembered name for this stream.
	if raw.IsClose {
		delete(a.activeNames, raw.StreamID)
		return nil, false
	}

	// Restore Sticky Name:
	// If this message has a name, remember it. If it doesn't, populate it
	// from the previously remembered name for this stream.
	if raw.Req.ResourceName != "" {
		a.activeNames[raw.StreamID] = raw.Req.ResourceName
	} else {
		raw.Req.ResourceName = a.activeNames[raw.StreamID]
	}

	return raw.Req, true
}