mesh-sdk: a bus function called before Serve waits for Serve instead of refusing (issue 1966)

A module that starts a goroutine before stdio.Serve and calls the bus there was refused and retried on its own tick, a minute later. Refused now only when the process never serves.

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/bus_waits_for_serve_test.go

@@ -0,0 +1,58 @@
+package stdio
+
+import (
+	"io"
+	"testing"
+	"time"
+)
+
+// A bus function called before Serve waits for Serve and proceeds (issue 1966): a module that starts a goroutine
+// before stdio.Serve and calls the bus there is no longer refused. It is refused only when the process never serves.
+func TestBusFunctionsWaitForServe(t *testing.T) {
+	for _, tc := range []struct {
+		name       string
+		wait       time.Duration
+		serveAt    time.Duration // 0: Serve never runs
+		serveFirst bool
+		wantOK     bool
+	}{
+		{"a call before Serve completes after Serve", 5 * time.Second, 50 * time.Millisecond, false, true},
+		{"a call whose wait ends before Serve is refused", 30 * time.Millisecond, 0, false, false},
+		{"a call after Serve is as before", 5 * time.Second, 0, true, true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			servesAlone(t)
+			old := serveWait
+			serveWait = tc.wait
+			t.Cleanup(func() { serveWait = old })
+			inR, inW := io.Pipe()
+			outR, outW := io.Pipe()
+			stopped := make(chan struct{})
+			started := false
+			t.Cleanup(func() {
+				_ = inW.Close()
+				go func() { _, _ = io.Copy(io.Discard, outR) }()
+				if started {
+					<-stopped
+				}
+			})
+			serve := func() {
+				started = true
+				go func() { _ = ServeOn("m", nil, inR, outW); close(stopped) }()
+			}
+			if tc.serveFirst {
+				serve()
+				<-Served()
+			} else if tc.serveAt > 0 {
+				time.AfterFunc(tc.serveAt, serve)
+			}
+			c, err := live()
+			if tc.wantOK != (err == nil && c != nil) {
+				t.Fatalf("live() = %v, %v; want ok=%v", c, err, tc.wantOK)
+			}
+			if !tc.wantOK && err.Error() != "before Serve: a bundle reaches the bus through the runtime that launched it" {
+				t.Fatalf("refusal words changed: %v", err)
+			}
+		})
+	}
+}

go/live_before_serve_test.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"os/exec"
 	"testing"
+	"time"
 )
 
 // 1171/AC4: before Serve, a bundle's bus functions still refuse exactly as on main (stdio.go:391-398,
@@ -13,6 +14,7 @@ import (
 func TestBusFunctionsRefuseBeforeServeInAFreshProcess(t *testing.T) {
 	const marker = "MESH_SDK_TEST_FRESH_PROCESS"
 	if os.Getenv(marker) == "1" {
+		serveWait = 10 * time.Millisecond
 		if err := Emit("x", map[string]any{}); err == nil ||
 			err.Error() != "emit before Serve: a bundle emits through the runtime that launched it" {
 			t.Fatalf("Emit before Serve: %v", err)

go/stdio.go

@@ -441,13 +441,36 @@ func ServeOn(name string, tools []Tool, in io.Reader, out io.Writer) error {
 	return scanner.Err()
 }
 
-func live() (*channel, error) {
+// serveWait is how long a bus function called before Serve waits for Serve to have the runtime's channel. A bundle
+// that starts a goroutine before stdio.Serve and calls the bus there finds the channel a moment later; only a
+// process that never serves is refused, after this long.
+var serveWait = 30 * time.Second
+
+// awaitChannel is the runtime's channel, waiting up to serveWait for Serve to set it.
+func awaitChannel() *channel {
+	currentMu.Lock()
+	c, ready := current, served
+	currentMu.Unlock()
+	if c != nil {
+		return c
+	}
+	t := time.NewTimer(serveWait)
+	defer t.Stop()
+	select {
+	case <-ready:
+	case <-t.C:
+		return nil
+	}
 	currentMu.Lock()
 	defer currentMu.Unlock()
-	if current == nil {
-		return nil, errors.New("before Serve: a bundle reaches the bus through the runtime that launched it")
+	return current
+}
+
+func live() (*channel, error) {
+	if c := awaitChannel(); c != nil {
+		return c, nil
 	}
-	return current, nil
+	return nil, errors.New("before Serve: a bundle reaches the bus through the runtime that launched it")
 }
 
 // Subscribe asks the runtime for this module's events whose type matches the pattern (`*` one
@@ -571,9 +594,7 @@ type EmitOptions struct {
 // accepted it (ADR 0193). The envelope and headers are the TypeScript SDK's exactly: the source and
 // node come from MESH_MODULE and MESH_NODE, which the runtime sets.
 func Emit(eventType string, body any, opts ...EmitOptions) error {
-	currentMu.Lock()
-	c := current
-	currentMu.Unlock()
+	c := awaitChannel()
 	if c == nil {
 		return errors.New("emit before Serve: a bundle emits through the runtime that launched it")
 	}