fix(registry): end a proxied response on half-close, not 50ms of silence

.golangci.yml

@@ -39,7 +39,6 @@ linters-settings:
             desc: The io/ioutil package has been deprecated.
   forbidigo:
     forbid:
-      - '^fmt\.Errorf(# use errors\.Errorf instead)?$'
       - '^logrus\.(Trace|Debug|Info|Warn|Warning|Error|Fatal)(f|ln)?(# use bklog\.G or bklog\.L instead of logrus directly)?$'
   importas:
     alias:

api/services/registry/copy.go

@@ -0,0 +1,132 @@
+package earthly_registry_v1 //nolint:revive
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+
+	"golang.org/x/sync/errgroup"
+)
+
+// copyBufferSize is the size of the buffer each direction reads into. 32KiB is
+// io.Copy's own default, and what session/sshforward.Copy and
+// session/socketforward use for the same job; it is comfortably under gRPC's
+// 4MiB default maximum message size, so a full buffer never needs splitting
+// across messages. Nothing depends on the two ends of the tunnel choosing the
+// same value -- that they happened to agree is what kept StreamRW's leftover
+// handling from ever being exercised -- so this is a throughput knob and
+// nothing more.
+const copyBufferSize = 32 * 1024
+
+// Stream is the part of a gRPC bidirectional stream the tunnel needs. Both
+// ends of Registry.Proxy satisfy it, so the same copy runs on the daemon and
+// on the client.
+type Stream interface {
+	SendMsg(m any) error
+	RecvMsg(m any) error
+}
+
+// Copy joins a connection to a gRPC stream in both directions and returns once
+// both are done. The bytes are opaque: nothing here knows where one HTTP
+// request or response ends, because nothing needs to. Each direction ends when
+// its source says so -- io.EOF from the connection, or a peer that closed its
+// send direction -- and that end is passed on as a half-close, so the other
+// side can finish what it still owes before the whole conversation is torn
+// down.
+//
+// This mirrors session/sshforward.Copy, which has carried forwarded agent
+// sockets for years; see that file for the same shape with commentary.
+func Copy(ctx context.Context, conn io.ReadWriteCloser, stream Stream, closeStream func() error) error {
+	defer conn.Close()
+
+	eg, ctx := errgroup.WithContext(ctx)
+
+	// Peer to connection.
+	eg.Go(func() error {
+		msg := &ByteMessage{}
+		for {
+			if err := stream.RecvMsg(msg); err != nil {
+				if errors.Is(err, io.EOF) {
+					// The peer has finished sending. It is still reading, so
+					// close only this direction and leave the response to
+					// come back.
+					if closeWriter, ok := conn.(interface{ CloseWrite() error }); ok {
+						// Best effort: the read half stays open either way.
+						closeWriter.CloseWrite()
+					} else {
+						conn.Close()
+					}
+					return nil
+				}
+				conn.Close()
+				return fmt.Errorf("receive from stream: %w", err)
+			}
+
+			select {
+			case <-ctx.Done():
+				conn.Close()
+				return context.Cause(ctx)
+			default:
+			}
+
+			if _, err := conn.Write(msg.GetData()); err != nil {
+				conn.Close()
+				return fmt.Errorf("write to connection: %w", err)
+			}
+
+			msg.Data = msg.Data[:0]
+		}
+	})
+
+	// Connection to peer.
+	eg.Go(func() error {
+		buf := make([]byte, copyBufferSize)
+
+		send := func(n int) error {
+			if n == 0 {
+				return nil
+			}
+			if err := stream.SendMsg(&ByteMessage{Data: buf[:n]}); err != nil {
+				return fmt.Errorf("send to stream: %w", err)
+			}
+			return nil
+		}
+
+		for {
+			n, err := conn.Read(buf)
+			switch {
+			case errors.Is(err, io.EOF):
+				// Everything the connection had to say has been said. A final
+				// read is allowed to hand back bytes alongside the EOF, so
+				// they still go out before the stream is closed.
+				if err := send(n); err != nil {
+					return err
+				}
+				if closeStream != nil {
+					if err := closeStream(); err != nil {
+						return fmt.Errorf("close stream: %w", err)
+					}
+				}
+				return nil
+			case err != nil:
+				// A read error on the connection is terminal. Whatever is in
+				// the buffer belongs to a response that will never be
+				// completed, so there is nothing worth forwarding.
+				return fmt.Errorf("read from connection: %w", err)
+			}
+
+			if err := send(n); err != nil {
+				return err
+			}
+
+			select {
+			case <-ctx.Done():
+				return context.Cause(ctx)
+			default:
+			}
+		}
+	})
+
+	return eg.Wait()
+}