coder · ibetitsmike · Aug 7, 2025 · Aug 12, 2025 · Aug 12, 2025 · Aug 13, 2025
diff --git a/agent/immortalstreams/backedpipe/backed_pipe.go b/agent/immortalstreams/backedpipe/backed_pipe.go
@@ -0,0 +1,293 @@
+package backedpipe
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"golang.org/x/sync/errgroup"
+	"golang.org/x/sync/singleflight"
+	"golang.org/x/xerrors"
+)
+
+var (
+	ErrPipeClosed             = xerrors.New("pipe is closed")
+	ErrPipeAlreadyConnected   = xerrors.New("pipe is already connected")
+	ErrReconnectionInProgress = xerrors.New("reconnection already in progress")
+	ErrReconnectFailed        = xerrors.New("reconnect failed")
+	ErrInvalidSequenceNumber  = xerrors.New("remote sequence number exceeds local sequence")
+	ErrReconnectWriterFailed  = xerrors.New("reconnect writer failed")
+)
+
+const (
+	// Default buffer capacity used by the writer - 64MB
+	DefaultBufferSize = 64 * 1024 * 1024
+)
+
+// Reconnector is an interface for establishing connections when the BackedPipe needs to reconnect.
+// Implementations should:
+// 1. Establish a new connection to the remote side
+// 2. Exchange sequence numbers with the remote side
+// 3. Return the new connection and the remote's current sequence number
+//
+// The writerSeqNum parameter is the local writer's current sequence number,
+// which should be sent to the remote side so it knows where to resume reading from.
+//
+// The returned readerSeqNum should be the remote side's current sequence number,
+// which indicates where the local reader should resume from.
+type Reconnector interface {
+	Reconnect(ctx context.Context, writerSeqNum uint64) (conn io.ReadWriteCloser, readerSeqNum uint64, err error)
+}
+
+// BackedPipe provides a reliable bidirectional byte stream over unreliable network connections.
+// It orchestrates a BackedReader and BackedWriter to provide transparent reconnection
+// and data replay capabilities.
+type BackedPipe struct {
+	ctx         context.Context
+	cancel      context.CancelFunc
+	mu          sync.RWMutex
+	reader      *BackedReader
+	writer      *BackedWriter
+	reconnector Reconnector
+	conn        io.ReadWriteCloser
+	connected   bool
+	closed      bool
+
+	// Reconnection state
+	reconnecting bool
+
+	// Error channels for receiving connection errors from reader/writer separately
+	readerErrorChan chan error
+	writerErrorChan chan error
+
+	// singleflight group to dedupe concurrent ForceReconnect calls
+	sf singleflight.Group
+}
+
+// NewBackedPipe creates a new BackedPipe with default options and the specified reconnector.
+// The pipe starts disconnected and must be connected using Connect().
+func NewBackedPipe(ctx context.Context, reconnector Reconnector) *BackedPipe {
+	pipeCtx, cancel := context.WithCancel(ctx)
+
+	readerErrorChan := make(chan error, 1) // Buffer for reader errors
+	writerErrorChan := make(chan error, 1) // Buffer for writer errors
+	bp := &BackedPipe{
+		ctx:             pipeCtx,
+		cancel:          cancel,
+		reader:          NewBackedReader(readerErrorChan),
+		writer:          NewBackedWriter(DefaultBufferSize, writerErrorChan),
+		reconnector:     reconnector,
+		readerErrorChan: readerErrorChan,
+		writerErrorChan: writerErrorChan,
+	}
+
+	// Start error handler goroutine
+	go bp.handleErrors()
+
+	return bp
+}
+
+// Connect establishes the initial connection using the reconnect function.
+func (bp *BackedPipe) Connect() error {
+	bp.mu.Lock()
+	defer bp.mu.Unlock()
+
+	if bp.closed {
+		return ErrPipeClosed
+	}
+
+	if bp.connected {
+		return ErrPipeAlreadyConnected
+	}
+
+	// Use internal context for the actual reconnect operation to ensure
+	// Close() reliably cancels any in-flight attempt.
+	return bp.reconnectLocked()
+}
+
+// Read implements io.Reader by delegating to the BackedReader.
+func (bp *BackedPipe) Read(p []byte) (int, error) {
+	return bp.reader.Read(p)
+}
+
+// Write implements io.Writer by delegating to the BackedWriter.
+func (bp *BackedPipe) Write(p []byte) (int, error) {
+	bp.mu.RLock()
+	writer := bp.writer
+	closed := bp.closed
+	bp.mu.RUnlock()
+
+	if closed {
+		return 0, io.EOF
+	}
+
+	return writer.Write(p)
+}
+
+// Close closes the pipe and all underlying connections.
+func (bp *BackedPipe) Close() error {
+	bp.mu.Lock()
+	defer bp.mu.Unlock()
+
+	if bp.closed {
+		return nil
+	}
+
+	bp.closed = true
+	bp.cancel() // Cancel main context
+
+	// Close all components in parallel to avoid deadlocks
+	//
+	// IMPORTANT: The connection must be closed first to unblock any
+	// readers or writers that might be holding the mutex on Read/Write
+	var g errgroup.Group
+
+	if bp.conn != nil {
+		conn := bp.conn
+		g.Go(func() error {
+			return conn.Close()
+		})
+		bp.conn = nil
+	}
+
+	if bp.reader != nil {
+		reader := bp.reader
+		g.Go(func() error {
+			return reader.Close()
+		})
+	}
+
+	if bp.writer != nil {
+		writer := bp.writer
+		g.Go(func() error {
+			return writer.Close()
+		})
+	}
+
+	bp.connected = false
+
+	// Wait for all close operations to complete and return any error
+	return g.Wait()
+}
+
+// Connected returns whether the pipe is currently connected.
+func (bp *BackedPipe) Connected() bool {
+	bp.mu.RLock()
+	defer bp.mu.RUnlock()
+	return bp.connected
+}
+
+// reconnectLocked handles the reconnection logic. Must be called with write lock held.
+func (bp *BackedPipe) reconnectLocked() error {
+	if bp.reconnecting {
+		return ErrReconnectionInProgress
+	}
+
+	bp.reconnecting = true
+	defer func() {
+		bp.reconnecting = false
+	}()
+
+	// Close existing connection if any
+	if bp.conn != nil {
+		_ = bp.conn.Close()
+		bp.conn = nil
+	}
+
+	bp.connected = false
+
+	// Get current writer sequence number to send to remote
+	writerSeqNum := bp.writer.SequenceNum()
+
+	conn, readerSeqNum, err := bp.reconnector.Reconnect(bp.ctx, writerSeqNum)
+	if err != nil {
+		return ErrReconnectFailed
+	}
+
+	// Validate sequence numbers
+	if readerSeqNum > writerSeqNum {
+		_ = conn.Close()
+		return ErrInvalidSequenceNumber
+	}
+
+	// Reconnect reader and writer
+	seqNum := make(chan uint64, 1)
+	newR := make(chan io.Reader, 1)
+
+	go bp.reader.Reconnect(seqNum, newR)
+
+	// Get sequence number and send new reader
+	<-seqNum
+	newR <- conn
+
+	err = bp.writer.Reconnect(readerSeqNum, conn)
+	if err != nil {
+		_ = conn.Close()
+		return ErrReconnectWriterFailed
+	}
+
+	// Success - update state
+	bp.conn = conn
+	bp.connected = true
+
+	return nil
+}
+
+// handleErrors listens for connection errors from reader/writer and triggers reconnection.
+func (bp *BackedPipe) handleErrors() {
+	for {
+		select {
+		case <-bp.ctx.Done():
+			return
+		case err := <-bp.readerErrorChan:
+			// Reader connection error occurred
+			bp.handleConnectionError(err, "reader")
+		case err := <-bp.writerErrorChan:
+			// Writer connection error occurred
+			bp.handleConnectionError(err, "writer")
+		}
+	}
+}
+
+// handleConnectionError handles errors from either reader or writer components.
+func (bp *BackedPipe) handleConnectionError(err error, component string) {
+	bp.mu.Lock()
+	defer bp.mu.Unlock()
+
+	// Skip if already closed or not connected
+	if bp.closed || !bp.connected {
+		return
+	}
+
+	// Mark as disconnected
+	bp.connected = false
+
+	// Try to reconnect using internal context
+	reconnectErr := bp.reconnectLocked()
+
+	if reconnectErr != nil {
+		// Reconnection failed - log or handle as needed
+		// For now, we'll just continue and wait for manual reconnection
+		_ = err       // Use the original error from the component
+		_ = component // Component info available for potential logging by higher layers
+	}
+}
+
+// ForceReconnect forces a reconnection attempt immediately.
+// This can be used to force a reconnection if a new connection is established.
+func (bp *BackedPipe) ForceReconnect() error {
+	// Deduplicate concurrent ForceReconnect calls so only one reconnection
+	// attempt runs at a time from this API. Use the pipe's internal context
+	// to ensure Close() cancels any in-flight attempt.
+	_, err, _ := bp.sf.Do("backedpipe-reconnect", func() (interface{}, error) {
+		bp.mu.Lock()
+		defer bp.mu.Unlock()
+
+		if bp.closed {
+			return nil, io.EOF
+		}
+
+		return nil, bp.reconnectLocked()
+	})
+	return err
+}