runtime: block signal_recv on targets without signal delivery (#5619)

main_test.go

@@ -81,6 +81,7 @@ func TestBuild(t *testing.T) {
 		"print.go",
 		"reflect.go",
 		"signal.go",
+		"signalnotify.go",
 		"slice.go",
 		"sort.go",
 		"stdlib.go",

src/runtime/signalstub.go

@@ -18,4 +18,12 @@ func signal_ignore(uint32) {}
 func signal_waitUntilIdle() {}
 
 //go:linkname signal_recv os/signal.signal_recv
-func signal_recv() uint32 { return ^uint32(0) }
+func signal_recv() uint32 {
+	// Signals can never arrive on these platforms, so block forever, like the
+	// real implementation does while no signal is pending. os/signal.loop
+	// calls this function in a tight loop; returning immediately would make
+	// that goroutine spin, which starves cooperative schedulers: on wasm the
+	// program never yields back to the host again after signal.Notify.
+	deadlock()
+	return 0
+}

testdata/signalnotify.go

@@ -0,0 +1,22 @@
+package main
+
+// Verify that signal.Notify does not wedge the scheduler on platforms where
+// signals can never arrive (wasm and baremetal, see runtime/signalstub.go).
+// The signal watcher goroutine that Notify starts must block forever instead
+// of spinning: on a cooperative scheduler a spinning goroutine starves every
+// other goroutine, so the sleep below would never return.
+
+import (
+	"os"
+	"os/signal"
+	"time"
+)
+
+func main() {
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, os.Interrupt)
+	// Yield so the signal watcher goroutine runs (and, before the fix, takes
+	// over the scheduler forever).
+	time.Sleep(10 * time.Millisecond)
+	println("done")
+}

testdata/signalnotify.txt

@@ -0,0 +1 @@
+done