coder
diff --git a/‎agent/immortalstreams/backedpipe/backed_pipe.go
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+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")
+)
+
+// connectionState represents the current state of the BackedPipe connection.
+type connectionState int
+
+const (
+	// connected indicates the pipe is connected and operational.
+	connected connectionState = iota
+	// disconnected indicates the pipe is not connected but not closed.
+	disconnected
+	// reconnecting indicates a reconnection attempt is in progress.
+	reconnecting
+	// closed indicates the pipe is permanently closed.
+	closed
+)
+
+// ErrorEvent represents an error from a reader or writer with connection generation info.
+type ErrorEvent struct {
+	Err        error
+	Component  string // "reader" or "writer"
+	Generation uint64 // connection generation when error occurred
+}
+
+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 reader sequence number
+//
+// The readerSeqNum parameter is the local reader's current sequence number
+// (total bytes successfully read from the remote). This must be sent to the
+// remote so it can replay its data to us starting from this number.
+//
+// The returned remoteReaderSeqNum should be the remote side's reader sequence
+// number (how many bytes of our outbound data it has successfully read). This
+// informs our writer where to resume (i.e., which bytes to replay to the remote).
+type Reconnector interface {
+	Reconnect(ctx context.Context, readerSeqNum uint64) (conn io.ReadWriteCloser, remoteReaderSeqNum 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
+
+	// State machine
+	state   connectionState
+	connGen uint64 // Increments on each successful reconnection
+
+	// Unified error handling with generation filtering
+	errChan chan ErrorEvent
+
+	// singleflight group to dedupe concurrent ForceReconnect calls
+	sf singleflight.Group
+
+	// Track first error per generation to avoid duplicate reconnections
+	lastErrorGen uint64
+}
+
+// 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)
+
+	errChan := make(chan ErrorEvent, 1)
+
+	bp := &BackedPipe{
+		ctx:         pipeCtx,
+		cancel:      cancel,
+		reconnector: reconnector,
+		state:       disconnected,
+		connGen:     0, // Start with generation 0
+		errChan:     errChan,
+	}
+
+	// Create reader and writer with typed error channel for generation-aware error reporting
+	bp.reader = NewBackedReader(errChan)
+	bp.writer = NewBackedWriter(DefaultBufferSize, errChan)
+
+	// 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.state == closed {
+		return ErrPipeClosed
+	}
+
+	if bp.state == 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
+	state := bp.state
+	bp.mu.RUnlock()
+
+	if state == 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.state == closed {
+		return nil
+	}
+
+	bp.state = closed
+	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()
+		})
+	}
+
+	// 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.state == connected && bp.reader.Connected() && bp.writer.Connected()
+}
+
+// reconnectLocked handles the reconnection logic. Must be called with write lock held.
+func (bp *BackedPipe) reconnectLocked() error {
+	if bp.state == reconnecting {
+		return ErrReconnectionInProgress
+	}
+
+	bp.state = reconnecting
+	defer func() {
+		// Only reset to disconnected if we're still in reconnecting state
+		// (successful reconnection will set state to connected)
+		if bp.state == reconnecting {
+			bp.state = disconnected
+		}
+	}()
+
+	// Close existing connection if any
+	if bp.conn != nil {
+		_ = bp.conn.Close()
+		bp.conn = nil
+	}
+
+	// Increment the generation and update both reader and writer.
+	// We do it now to track even the connections that fail during
+	// Reconnect.
+	bp.connGen++
+	bp.reader.SetGeneration(bp.connGen)
+	bp.writer.SetGeneration(bp.connGen)
+
+	// Reconnect reader and writer
+	seqNum := make(chan uint64, 1)
+	newR := make(chan io.Reader, 1)
+
+	go bp.reader.Reconnect(seqNum, newR)
+
+	// Get the precise reader sequence number from the reader while it holds its lock
+	readerSeqNum, ok := <-seqNum
+	if !ok {
+		// Reader was closed during reconnection
+		return ErrReconnectFailed
+	}
+
+	// Perform reconnect using the exact sequence number we just received
+	conn, remoteReaderSeqNum, err := bp.reconnector.Reconnect(bp.ctx, readerSeqNum)
+	if err != nil {
+		// Unblock reader reconnect
+		newR <- nil
+		return ErrReconnectFailed
+	}
+
+	// Provide the new connection to the reader (reader still holds its lock)
+	newR <- conn
+
+	// Replay our outbound data from the remote's reader sequence number
+	writerReconnectErr := bp.writer.Reconnect(remoteReaderSeqNum, conn)
+	if writerReconnectErr != nil {
+		return ErrReconnectWriterFailed
+	}
+
+	// Success - update state
+	bp.conn = conn
+	bp.state = connected
+
+	return nil
+}
+
+// handleErrors listens for connection errors from reader/writer and triggers reconnection.
+// It filters errors from old connections and ensures only the first error per generation
+// triggers reconnection.
+func (bp *BackedPipe) handleErrors() {
+	for {
+		select {
+		case <-bp.ctx.Done():
+			return
+		case errorEvt := <-bp.errChan:
+			bp.handleConnectionError(errorEvt)
+		}
+	}
+}
+
+// handleConnectionError handles errors from either reader or writer components.
+// It filters errors from old connections and ensures only one reconnection per generation.
+func (bp *BackedPipe) handleConnectionError(errorEvt ErrorEvent) {
+	bp.mu.Lock()
+	defer bp.mu.Unlock()
+
+	// Skip if already closed
+	if bp.state == closed {
+		return
+	}
+
+	// Filter errors from old connections (lower generation)
+	if errorEvt.Generation < bp.connGen {
+		return
+	}
+
+	// Skip if not connected (already disconnected or reconnecting)
+	if bp.state != connected {
+		return
+	}
+
+	// Skip if we've already seen an error for this generation
+	if bp.lastErrorGen >= errorEvt.Generation {
+		return
+	}
+
+	// This is the first error for this generation
+	bp.lastErrorGen = errorEvt.Generation
+
+	// Mark as disconnected
+	bp.state = disconnected
+
+	// 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
+		_ = errorEvt.Err       // Use the original error from the component
+		_ = errorEvt.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.
+// It prevents duplicate reconnections when called concurrently.
+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("force-reconnect", func() (interface{}, error) {
+		bp.mu.Lock()
+		defer bp.mu.Unlock()
+
+		if bp.state == closed {
+			return nil, io.EOF
+		}
+
+		// Don't force reconnect if already reconnecting
+		if bp.state == reconnecting {
+			return nil, ErrReconnectionInProgress
+		}
+
+		return nil, bp.reconnectLocked()
+	})
+	return err
+}