Run an apply in a context the bus's loss cannot end, and say why a killed command was killed (issue 1945)

cmd/mesh-host/apply_context_test.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"context"
+	"testing"
+)
+
+// An apply runs in a context the bus's loss cannot end: the link's context ends when the bus goes, and the bus may go
+// because of the apply itself (it replaces the bus's own container), which then must finish (issue 1945, live on
+// 2026-10-25 at 11:23: novox's node-engine removed the bus container, its link's context ended, and the docker run that
+// would have made the new one was never made).
+func TestAnApplyOutlivesTheLinksContext(t *testing.T) {
+	type key struct{}
+	link, cancel := context.WithCancel(context.WithValue(context.Background(), key{}, "v"))
+	ctx := applyContext(link)
+	cancel()
+	if ctx.Err() != nil {
+		t.Fatalf("the apply's context ended with the link's: %v", ctx.Err())
+	}
+	if ctx.Value(key{}) != "v" {
+		t.Fatal("the apply's context lost the link's values")
+	}
+}

cmd/mesh-host/nodeagent.go

@@ -298,13 +298,13 @@ func nodeAgent(ctx context.Context, opts options, signer ed25519.PublicKey, sess
 			Backup:   backupNode(session),
 			Say:      say,
 			Apply: func(ctx context.Context, c delivery.Composed, others []delivery.Composed) agent.Result {
-				res := applyComposition(session.WithLink(ctx), opts, c, others, sched, say)
+				res := applyComposition(applyContext(session.WithLink(ctx)), opts, c, others, sched, say)
 				afterApply()
 				sayFacts(ctx, ag)
 				return res
 			},
 			Remove: func(ctx context.Context, module string, others []delivery.Composed) agent.Result {
-				res := removeComposition(session.WithLink(ctx), opts, module, others, sched, say)
+				res := removeComposition(applyContext(session.WithLink(ctx)), opts, module, others, sched, say)
 				afterApply()
 				sayFacts(ctx, ag)
 				return res
@@ -567,3 +567,9 @@ func declaresNothingToBackUp(said json.RawMessage, module string) (nothing bool,
 	}
 	return true, true
 }
+
+// applyContext is the context an apply or a removal runs in: the link's values, never its end. The link's context ends
+// when the bus goes, and the bus may go because of the apply itself (it replaces the bus's own container), which must
+// then finish: a container removed and never made again took the mesh down (issue 1945, live on 2026-10-25 at 11:23).
+// An apply is bounded by its own steps' bounds, not by the link.
+func applyContext(ctx context.Context) context.Context { return context.WithoutCancel(ctx) }

internal/apply/apply.go

@@ -2464,12 +2464,18 @@ func ExecRunner(ctx context.Context, name string, args ...string) (string, error
 	if err != nil {
 		var exit *exec.ExitError
 		if errors.As(err, &exit) {
-			// The words as they always were, and the exit underneath them, so a caller asks the
-			// code (system.ExitCode) rather than matching text that this line is free to reword.
-			return string(out), &exited{
-				words: fmt.Sprintf("%s exited %d: %s", name, exit.ExitCode(), strings.TrimSpace(string(exit.Stderr))),
-				exit:  exit,
+			words := fmt.Sprintf("%s exited %d: %s", name, exit.ExitCode(), strings.TrimSpace(string(exit.Stderr)))
+			if exit.ExitCode() == -1 {
+				// Killed, not exited: say by what, and the context's end when that is what killed it (issue 1945:
+				// "docker exited -1:" said nothing while a bus container was half replaced).
+				words = fmt.Sprintf("%s was %s", name, exit.String())
+				if ctx.Err() != nil {
+					words += " because its context ended: " + ctx.Err().Error()
+				}
 			}
+			// The words, and the exit underneath them, so a caller asks the code (system.ExitCode) rather than
+			// matching text that this line is free to reword.
+			return string(out), &exited{words: words, exit: exit}
 		}
 		return string(out), fmt.Errorf("%s: %w", name, err)
 	}

internal/apply/exec_killed_test.go

@@ -0,0 +1,19 @@
+package apply
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+// A command killed because its context ended says so, and why: "docker exited -1:" said nothing while novox's bus
+// container was half replaced (issue 1945, live on 2026-10-25 at 11:23).
+func TestACommandKilledByItsContextSaysWhy(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() { time.Sleep(50 * time.Millisecond); cancel() }()
+	_, err := ExecRunner(ctx, "sleep", "5")
+	if err == nil || !strings.Contains(err.Error(), "killed") || !strings.Contains(err.Error(), "context canceled") {
+		t.Fatalf("a command its context killed said: %v", err)
+	}
+}