Propagate RPC cancellation to daemon requests

Stop long-running daemon operations from running under context.Background by\nthreading a request-scoped context from handleConn into dispatch. The daemon\nnow cancels in-flight handlers when the client socket goes away, and the RPC\nclient closes its Unix connection when the caller context is canceled so that\ninterrupts actually reach the daemon boundary.\n\nAdd regression coverage for both sides of the path: canceled dispatch calls,\nclient disconnects during handleConn, watcher EOF cancellation, and context\ncancellation without an RPC deadline.\n\nValidated with GOCACHE=/tmp/banger-gocache go test ./... and\nGOCACHE=/tmp/banger-gocache make build.

internal/daemon/daemon.go

@@ -25,16 +25,17 @@ import (
 )
 
 type Daemon struct {
-	layout   paths.Layout
-	config   model.DaemonConfig
-	store    *store.Store
-	runner   system.CommandRunner
-	logger   *slog.Logger
-	mu       sync.Mutex
-	closing  chan struct{}
-	once     sync.Once
-	pid      int
-	listener net.Listener
+	layout         paths.Layout
+	config         model.DaemonConfig
+	store          *store.Store
+	runner         system.CommandRunner
+	logger         *slog.Logger
+	mu             sync.Mutex
+	closing        chan struct{}
+	once           sync.Once
+	pid            int
+	listener       net.Listener
+	requestHandler func(context.Context, rpc.Request) rpc.Response
 }
 
 func Open(ctx context.Context) (*Daemon, error) {
@@ -143,23 +144,73 @@ func (d *Daemon) Serve(ctx context.Context) error {
 
 func (d *Daemon) handleConn(conn net.Conn) {
 	defer conn.Close()
+	reader := bufio.NewReader(conn)
 	var req rpc.Request
-	if err := json.NewDecoder(bufio.NewReader(conn)).Decode(&req); err != nil {
+	if err := json.NewDecoder(reader).Decode(&req); err != nil {
 		if d.logger != nil {
 			d.logger.Warn("daemon request decode failed", "remote", conn.RemoteAddr().String(), "error", err.Error())
 		}
 		_ = json.NewEncoder(conn).Encode(rpc.NewError("bad_request", err.Error()))
 		return
 	}
-	resp := d.dispatch(req)
-	_ = json.NewEncoder(conn).Encode(resp)
+	reqCtx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	stopWatch := d.watchRequestDisconnect(conn, reader, req.Method, cancel)
+	defer stopWatch()
+	resp := d.dispatch(reqCtx, req)
+	if reqCtx.Err() != nil {
+		return
+	}
+	if err := json.NewEncoder(conn).Encode(resp); err != nil && d.logger != nil {
+		d.logger.Warn("daemon response encode failed", "method", req.Method, "remote", conn.RemoteAddr().String(), "error", err.Error())
+	}
 }
 
-func (d *Daemon) dispatch(req rpc.Request) rpc.Response {
+func (d *Daemon) watchRequestDisconnect(conn net.Conn, reader *bufio.Reader, method string, cancel context.CancelFunc) func() {
+	if conn == nil || reader == nil {
+		return func() {}
+	}
+	done := make(chan struct{})
+	var once sync.Once
+	go func() {
+		go func() {
+			<-done
+			if deadlineSetter, ok := conn.(interface{ SetReadDeadline(time.Time) error }); ok {
+				_ = deadlineSetter.SetReadDeadline(time.Now())
+			}
+		}()
+		var buf [1]byte
+		for {
+			_, err := reader.Read(buf[:])
+			if err == nil {
+				continue
+			}
+			select {
+			case <-done:
+				return
+			default:
+			}
+			if d.logger != nil {
+				d.logger.Info("daemon request canceled", "method", method, "remote", conn.RemoteAddr().String(), "error", err.Error())
+			}
+			cancel()
+			return
+		}
+	}()
+	return func() {
+		once.Do(func() {
+			close(done)
+		})
+	}
+}
+
+func (d *Daemon) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 	if req.Version != rpc.Version {
 		return rpc.NewError("bad_version", fmt.Sprintf("unsupported version %d", req.Version))
 	}
-	ctx := context.Background()
+	if d.requestHandler != nil {
+		return d.requestHandler(ctx, req)
+	}
 	switch req.Method {
 	case "ping":
 		result, _ := rpc.NewResult(api.PingResult{Status: "ok", PID: d.pid})

internal/daemon/daemon_test.go

@@ -1,13 +1,19 @@
 package daemon
 
 import (
+	"bufio"
 	"context"
+	"encoding/json"
+	"net"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
+	"banger/internal/api"
 	"banger/internal/model"
+	"banger/internal/rpc"
 	"banger/internal/store"
 )
 
@@ -330,3 +336,107 @@ func TestSetDNSUsesMapDNSDefaultsWhenDataFileUnset(t *testing.T) {
 	}
 	runner.assertExhausted()
 }
+
+func TestDispatchUsesPassedContext(t *testing.T) {
+	t.Parallel()
+
+	db := openDefaultImageStore(t, t.TempDir())
+	d := &Daemon{store: db}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	resp := d.dispatch(ctx, rpc.Request{
+		Version: rpc.Version,
+		Method:  "vm.list",
+		Params:  mustJSON(t, api.Empty{}),
+	})
+
+	if resp.OK {
+		t.Fatal("dispatch() succeeded with canceled context")
+	}
+	if resp.Error == nil || !strings.Contains(resp.Error.Message, context.Canceled.Error()) {
+		t.Fatalf("dispatch() error = %+v, want context canceled", resp.Error)
+	}
+}
+
+func TestHandleConnCancelsRequestWhenClientDisconnects(t *testing.T) {
+	t.Parallel()
+
+	server, client := net.Pipe()
+	defer client.Close()
+
+	requestCanceled := make(chan struct{})
+	done := make(chan struct{})
+	d := &Daemon{
+		closing: make(chan struct{}),
+		requestHandler: func(ctx context.Context, req rpc.Request) rpc.Response {
+			if req.Method != "block" {
+				t.Errorf("request method = %q, want block", req.Method)
+			}
+			<-ctx.Done()
+			close(requestCanceled)
+			return rpc.NewError("operation_failed", ctx.Err().Error())
+		},
+	}
+
+	go func() {
+		d.handleConn(server)
+		close(done)
+	}()
+
+	if err := json.NewEncoder(client).Encode(rpc.Request{Version: rpc.Version, Method: "block"}); err != nil {
+		t.Fatalf("encode request: %v", err)
+	}
+	if err := client.Close(); err != nil {
+		t.Fatalf("close client: %v", err)
+	}
+
+	select {
+	case <-requestCanceled:
+	case <-time.After(2 * time.Second):
+		t.Fatal("request context was not canceled after client disconnect")
+	}
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("handleConn did not return after client disconnect")
+	}
+}
+
+func TestWatchRequestDisconnectCancelsContextOnEOF(t *testing.T) {
+	t.Parallel()
+
+	server, client := net.Pipe()
+	defer server.Close()
+
+	reader := bufio.NewReader(server)
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+
+	d := &Daemon{closing: make(chan struct{})}
+	stop := d.watchRequestDisconnect(server, reader, "block", cancel)
+	defer stop()
+
+	if err := client.Close(); err != nil {
+		t.Fatalf("close client: %v", err)
+	}
+
+	select {
+	case <-ctx.Done():
+		if !strings.Contains(ctx.Err().Error(), context.Canceled.Error()) {
+			t.Fatalf("ctx.Err() = %v, want canceled", ctx.Err())
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("watchRequestDisconnect did not cancel context")
+	}
+}
+
+func mustJSON(t *testing.T, v any) []byte {
+	t.Helper()
+	data, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("json.Marshal(%T): %v", v, err)
+	}
+	return data
+}