runtime: deliver os/signal notifications under the threads scheduler

src/runtime/runtime_unix.go

@@ -4,7 +4,6 @@ package runtime
 
 import (
 	"internal/futex"
-	"internal/task"
 	"math/bits"
 	"sync/atomic"
 	"tinygo"
@@ -414,17 +413,6 @@ func signal_disable(s uint32) {
 	tinygo_signal_disable(s)
 }
 
-//go:linkname signal_waitUntilIdle os/signal.signalWaitUntilIdle
-func signal_waitUntilIdle() {
-	// Wait until signal_recv has processed all signals.
-	for receivedSignals.Load() != 0 {
-		// TODO: this becomes a busy loop when using threads.
-		// We might want to pause until signal_recv has no more incoming signals
-		// to process.
-		Gosched()
-	}
-}
-
 //export tinygo_signal_enable
 func tinygo_signal_enable(s uint32)
 
@@ -448,48 +436,28 @@ func tinygo_signal_handler(s int32) {
 		// goroutines.
 		signalFutex.WakeAll()
 	}
-}
 
-// Task waiting for a signal to arrive, or nil if it is running or there are no
-// signals.
-var signalRecvWaiter atomic.Pointer[task.Task]
-
-//go:linkname signal_recv os/signal.signal_recv
-func signal_recv() uint32 {
-	// Function called from os/signal to get the next received signal.
-	for {
-		val := receivedSignals.Load()
-		if val == 0 {
-			// There are no signals to receive. Sleep until there are.
-			if signalRecvWaiter.Swap(task.Current()) != nil {
-				// We expect only a single goroutine to call signal_recv.
-				runtimeFatal("signal_recv called concurrently")
-			}
-			task.Pause()
-			continue
-		}
+	// Wake the goroutine inside os/signal that gives signals to the channels
+	// of signal.Notify. This is a no-op with the cooperative scheduler, which
+	// resumes that goroutine from checkSignals instead.
+	signalRecvWake()
+}
 
-		// Extract the lowest numbered signal number from receivedSignals.
-		num := uint32(bits.TrailingZeros32(val))
+// nextReceivedSignal takes the lowest numbered signal out of receivedSignals,
+// or returns 0, false when there are none pending.
+func nextReceivedSignal() (uint32, bool) {
+	val := receivedSignals.Load()
+	if val == 0 {
+		return 0, false
+	}
 
-		// Atomically clear the signal number from receivedSignals.
-		receivedSignals.And(^(uint32(1) << num))
+	// Extract the lowest numbered signal number from receivedSignals.
+	num := uint32(bits.TrailingZeros32(val))
 
-		return num
-	}
-}
+	// Atomically clear the signal number from receivedSignals.
+	receivedSignals.And(^(uint32(1) << num))
 
-// Reactivate the goroutine waiting for signals, if there are any.
-// Return true if it was reactivated (and therefore the scheduler should run
-// again), and false otherwise.
-func checkSignals() bool {
-	if receivedSignals.Load() != 0 {
-		if waiter := signalRecvWaiter.Swap(nil); waiter != nil {
-			scheduleTask(waiter)
-			return true
-		}
-	}
-	return false
+	return num, true
 }
 
 func waitForEvents() {

src/runtime/signal_cooperative.go

@@ -0,0 +1,60 @@
+//go:build (darwin || (linux && !baremetal && !wasip1 && !wasm_unknown && !wasip2 && !nintendoswitch)) && !scheduler.threads
+
+package runtime
+
+import (
+	"internal/task"
+	"sync/atomic"
+)
+
+// The goroutine inside os/signal that reads signals is an ordinary task here.
+// It is parked with task.Pause and resumed from checkSignals below.
+
+// Task waiting for a signal to arrive, or nil if it is running or there are no
+// signals.
+var signalRecvWaiter atomic.Pointer[task.Task]
+
+//go:linkname signal_recv os/signal.signal_recv
+func signal_recv() uint32 {
+	// Function called from os/signal to get the next received signal.
+	for {
+		if num, ok := nextReceivedSignal(); ok {
+			return num
+		}
+
+		// There are no signals to receive. Sleep until there are.
+		if signalRecvWaiter.Swap(task.Current()) != nil {
+			// We expect only a single goroutine to call signal_recv.
+			runtimeFatal("signal_recv called concurrently")
+		}
+		task.Pause()
+	}
+}
+
+//go:linkname signal_waitUntilIdle os/signal.signalWaitUntilIdle
+func signal_waitUntilIdle() {
+	// Wait until signal_recv has processed all signals. Yielding is enough:
+	// the scheduler runs signal_recv, which is the only thing that empties
+	// receivedSignals.
+	for receivedSignals.Load() != 0 {
+		Gosched()
+	}
+}
+
+// Called from the signal handler. The waiting task is resumed by checkSignals
+// instead, from the scheduler, so there is nothing to do here.
+func signalRecvWake() {
+}
+
+// Reactivate the goroutine waiting for signals, if there are any.
+// Return true if it was reactivated (and therefore the scheduler should run
+// again), and false otherwise.
+func checkSignals() bool {
+	if receivedSignals.Load() != 0 {
+		if waiter := signalRecvWaiter.Swap(nil); waiter != nil {
+			scheduleTask(waiter)
+			return true
+		}
+	}
+	return false
+}

src/runtime/signal_threads.go

@@ -0,0 +1,69 @@
+//go:build (darwin || (linux && !baremetal && !wasip1 && !wasm_unknown && !wasip2 && !nintendoswitch)) && scheduler.threads
+
+package runtime
+
+import (
+	"internal/futex"
+)
+
+// Futex the receiver in os/signal waits on. Its value is 1 when the handler
+// has stored a signal that the receiver did not read yet.
+//
+// It cannot wait on signalFutex, because sleepTicks waits on that one too and
+// swaps it back to zero, which would lose the wakeup of the receiver.
+var signalRecvFutex futex.Futex
+
+// Futex signalWaitUntilIdle waits on. Its value is always zero, so it is only
+// a wakeup address. The wait has a timeout because a wake that arrives before
+// the wait starts is not remembered.
+var signalIdleFutex futex.Futex
+
+// How long signalWaitUntilIdle blocks before rechecking on its own.
+const signalIdlePoll = 1e6 // 1ms, in nanoseconds
+
+//go:linkname signal_recv os/signal.signal_recv
+func signal_recv() uint32 {
+	// Function called from os/signal to get the next received signal.
+	for {
+		if num, ok := nextReceivedSignal(); ok {
+			if receivedSignals.Load() == 0 {
+				// That was the last pending signal, so signalWaitUntilIdle can
+				// return now.
+				signalIdleFutex.WakeAll()
+			}
+			return num
+		}
+
+		// Clear the flag and then read receivedSignals again. The handler
+		// stores the signal before the flag, so no wakeup is lost.
+		signalRecvFutex.Store(0)
+		if receivedSignals.Load() != 0 {
+			continue
+		}
+		signalRecvFutex.Wait(0)
+	}
+}
+
+//go:linkname signal_waitUntilIdle os/signal.signalWaitUntilIdle
+func signal_waitUntilIdle() {
+	// Wait until signal_recv has processed all signals. Gosched is a no-op
+	// with threads, so this must block.
+	for receivedSignals.Load() != 0 {
+		signalIdleFutex.WaitUntil(0, signalIdlePoll)
+	}
+}
+
+// Called from the signal handler to wake signal_recv. An atomic store and a
+// futex wake syscall are both safe in a signal handler.
+func signalRecvWake() {
+	if signalRecvFutex.Swap(1) == 0 {
+		// Changed from 0 to 1, so signal_recv may be waiting on it.
+		signalRecvFutex.WakeAll()
+	}
+}
+
+// Reactivate the goroutine waiting for signals, if there are any. There is no
+// such goroutine here, because the handler wakes the receiver directly.
+func checkSignals() bool {
+	return false
+}

testdata/signal.go

@@ -12,17 +12,24 @@ import (
 )
 
 func main() {
+	// A signal must reach the channel while the program does nothing else. The
+	// receive below is the only thing that runs, so no timer and no sleep can
+	// carry the delivery.
 	c := make(chan os.Signal, 1)
 	signal.Notify(c, syscall.SIGUSR1)
+	syscall.Kill(syscall.Getpid(), syscall.SIGUSR1)
+	report(<-c)
+	signal.Stop(c)
+
+	// The same again, with a goroutine that reads the channel while the main
+	// goroutine sleeps.
+	c2 := make(chan os.Signal, 1)
+	signal.Notify(c2, syscall.SIGUSR1)
 
 	// Wait for signals to arrive.
 	go func() {
-		for sig := range c {
-			if sig == syscall.SIGUSR1 {
-				println("got expected signal")
-			} else {
-				println("got signal:", sig.String())
-			}
+		for sig := range c2 {
+			report(sig)
 		}
 	}()
 
@@ -36,7 +43,15 @@ func main() {
 	// in a unit test).
 	signal.Ignore(syscall.SIGUSR1)
 
-	signal.Stop(c)
+	signal.Stop(c2)
 
 	println("exiting signal program")
 }
+
+func report(sig os.Signal) {
+	if sig == syscall.SIGUSR1 {
+		println("got expected signal")
+	} else {
+		println("got signal:", sig.String())
+	}
+}

testdata/signal.txt

@@ -1,2 +1,3 @@
 got expected signal
+got expected signal
 exiting signal program