Stop the stdio channel holding its lock across a pipe write, so a refused subscribe cannot deadlock it (issue 1286)

go/stdio.go

@@ -57,7 +57,11 @@ type message struct {
 // channel is the one conversation with the runtime: what this bundle writes, serialised, and what
 // it asked and waits for.
 type channel struct {
-	mu        sync.Mutex
+	mu sync.Mutex
+	// wmu orders the writes to the runtime and nothing else: a write waits on the pipe for as long as
+	// the runtime does not read, and mu must never wait with it, or the loop that reads the runtime's
+	// answers stops (issue 1286).
+	wmu       sync.Mutex
 	enc       *json.Encoder
 	next      int
 	waiting   map[string]chan message
@@ -90,8 +94,8 @@ type Envelope struct {
 
 func (c *channel) say(m map[string]any) {
 	m["jsonrpc"] = "2.0"
-	c.mu.Lock()
-	defer c.mu.Unlock()
+	c.wmu.Lock()
+	defer c.wmu.Unlock()
 	_ = c.enc.Encode(m)
 }