From cccac9047689fd25acfbab52c3ae3d7b09aca5a2 Mon Sep 17 00:00:00 2001
From: Moses Narrow <36607567+0pcom@users.noreply.github.com>
Date: Tue, 14 Jul 2026 18:39:31 -0500
Subject: [PATCH 1/2] runtime: deliver signals under the threads scheduler when
 no goroutine sleeps
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Under the threads scheduler there is no cooperative idle loop, so checkSignals()
— which resumes the parked os/signal signal_recv goroutine — was only ever
reached from sleepTicks(). That means a signal (e.g. SIGINT/Ctrl+C) was only
noticed while some goroutine happened to be inside time.Sleep. A program blocked
purely on I/O, channels, mutexes or timers (time.NewTicker uses the timer queue,
not sleepTicks) would never observe the signal at all.

Start a dedicated signal-watcher thread the first time a signal is enabled,
gated to the threads scheduler (!hasScheduler && hasParallelism). It blocks on
signalFutex and calls checkSignals() on wake, mirroring the signal half of
waitForEvents() that the cooperative scheduler runs from its idle loop. Other
schedulers are unaffected (the start is a compile-time no-op for them).

Verified: a channel/Accept-blocked program with no time.Sleep now receives
SIGINT, and the skycoin daemon (previously unkillable with Ctrl+C under TinyGo)
now shuts down cleanly on SIGINT, both idle and during active block sync.

(cherry picked from commit ada7ab6b47f50936c3736da9c8fba63dbbed7179)
---
 src/runtime/runtime_unix.go | 40 +++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/src/runtime/runtime_unix.go b/src/runtime/runtime_unix.go
index 12a8c265cb..019a959d3e 100644
--- a/src/runtime/runtime_unix.go
+++ b/src/runtime/runtime_unix.go
@@ -390,10 +390,50 @@ func signal_enable(s uint32) {
 	// scheduler (and therefore there is no parallelism).
 	hasSignals = true
 
+	// Under the threads scheduler there is no scheduler idle loop to notice
+	// signals: checkSignals() is only reached from sleepTicks(), i.e. while
+	// some goroutine happens to be inside time.Sleep. A program blocked purely
+	// on I/O or channels would otherwise never observe a signal (Ctrl+C would
+	// be ignored). Start a dedicated watcher thread to cover that case. This is
+	// a no-op for every other scheduler.
+	startSignalWatcher()
+
 	// It's easier to implement this function in C.
 	tinygo_signal_enable(s)
 }
 
+// signalWatcherStarted guards the one-time start of signalWatcher. signal_enable
+// is serialized by os/signal's handlers lock, but this stays defensive.
+var signalWatcherStarted atomic.Uint32
+
+// startSignalWatcher starts the signal watcher thread the first time a signal is
+// enabled, but only under the threads scheduler (!hasScheduler && hasParallelism
+// is true only there). The cooperative and multicore schedulers process signals
+// from their idle loop (waitForEvents), and the "none" scheduler has no
+// goroutines, so none of them need this.
+func startSignalWatcher() {
+	if hasScheduler || !hasParallelism {
+		return
+	}
+	if signalWatcherStarted.Swap(1) == 0 {
+		go signalWatcher()
+	}
+}
+
+// signalWatcher runs on its own thread under the threads scheduler. It blocks on
+// signalFutex and resumes the signal-receiving goroutine (signal_recv) whenever
+// a signal arrives, decoupling signal delivery from sleepTicks(). It mirrors the
+// signal half of waitForEvents(), which the threads scheduler never calls.
+func signalWatcher() {
+	for {
+		// Block until the signal handler bumps the futex from 0 to 1.
+		signalFutex.Wait(0)
+		if signalFutex.Swap(0) != 0 {
+			checkSignals()
+		}
+	}
+}
+
 //go:linkname signal_ignore os/signal.signal_ignore
 func signal_ignore(s uint32) {
 	if s >= 32 {

From e62620236cc7dac71dadc2dfde9ab9cb8fe1eb8c Mon Sep 17 00:00:00 2001
From: Moses Narrow <36607567+0pcom@users.noreply.github.com>
Date: Thu, 13 Aug 2026 20:33:38 -0500
Subject: [PATCH 2/2] runtime: let the signal watcher thread exit
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Review feedback: the watcher was started as a goroutine with no exit
condition. It blocked on a futex forever, so a program that finished with
signals kept a thread parked on one for the rest of its life. Nothing
observable broke — the thread is idle and process exit tears it down —
but a loop with no way out is a property worth not having.

The watcher exists only to serve enabled signals, so that is now its
lifetime: enabledSignals tracks the set os/signal wants delivered, the
last signal_disable/signal_ignore stops the thread, and a later
signal_enable starts a fresh one.

Stopping sets the flag, bumps the futex value and wakes it. The bump
matters as much as the wake: Wait(0) returns immediately when the futex
is already non-zero, which closes the window between the store and a
watcher that is about to sleep. On the way out the watcher resets the
futex to 0 so the next one can block on it.

Verified with a program that blocks on a channel (never time.Sleep, so
delivery can only come from the watcher): the signal arrives, signal.Stop
lets the thread exit, and a later signal.Notify starts a new watcher that
delivers again.
---
 src/runtime/runtime_unix.go | 61 ++++++++++++++++++++++++++++++++++---
 1 file changed, 56 insertions(+), 5 deletions(-)

diff --git a/src/runtime/runtime_unix.go b/src/runtime/runtime_unix.go
index 019a959d3e..938a6e057f 100644
--- a/src/runtime/runtime_unix.go
+++ b/src/runtime/runtime_unix.go
@@ -396,38 +396,87 @@ func signal_enable(s uint32) {
 	// on I/O or channels would otherwise never observe a signal (Ctrl+C would
 	// be ignored). Start a dedicated watcher thread to cover that case. This is
 	// a no-op for every other scheduler.
-	startSignalWatcher()
+	startSignalWatcher(s)
 
 	// It's easier to implement this function in C.
 	tinygo_signal_enable(s)
 }
 
-// signalWatcherStarted guards the one-time start of signalWatcher. signal_enable
-// is serialized by os/signal's handlers lock, but this stays defensive.
+// signalWatcherStarted guards the start of signalWatcher. signal_enable is
+// serialized by os/signal's handlers lock, but this stays defensive.
 var signalWatcherStarted atomic.Uint32
 
-// startSignalWatcher starts the signal watcher thread the first time a signal is
+// signalWatcherStop asks signalWatcher to return. The watcher reads it after
+// waking, so stopping it means setting this and then waking the futex.
+var signalWatcherStop atomic.Uint32
+
+// enabledSignals is the set of signals os/signal currently wants delivered. The
+// watcher thread exists only to serve them, so it runs exactly while this is
+// non-zero: the last disable/ignore stops it, and a later enable starts a fresh
+// one.
+var enabledSignals atomic.Uint32
+
+// startSignalWatcher starts the signal watcher thread when the first signal is
 // enabled, but only under the threads scheduler (!hasScheduler && hasParallelism
 // is true only there). The cooperative and multicore schedulers process signals
 // from their idle loop (waitForEvents), and the "none" scheduler has no
 // goroutines, so none of them need this.
-func startSignalWatcher() {
+func startSignalWatcher(s uint32) {
 	if hasScheduler || !hasParallelism {
 		return
 	}
+	enabledSignals.Or(uint32(1) << s)
+	signalWatcherStop.Store(0)
 	if signalWatcherStarted.Swap(1) == 0 {
 		go signalWatcher()
 	}
 }
 
+// stopSignalWatcher lets the watcher thread return once the signal it was
+// serving is the last one to go away.
+//
+// Without this the thread is unstoppable by construction: it blocks on a futex
+// forever, so a program that finishes with signals keeps a thread parked on one
+// for the rest of its life. Nothing observable breaks — the thread is idle and
+// the process exit tears it down — but "no exit condition" is a property worth
+// not having, and it costs a flag and a wake to avoid.
+func stopSignalWatcher(s uint32) {
+	if hasScheduler || !hasParallelism {
+		return
+	}
+	// And returns the value BEFORE the mask was applied, so clear the bit from
+	// it to get what is left enabled.
+	bit := uint32(1) << s
+	if enabledSignals.And(^bit)&^bit != 0 {
+		return // still serving other signals
+	}
+	if signalWatcherStarted.Swap(0) == 0 {
+		return // not running
+	}
+	signalWatcherStop.Store(1)
+	// Wake it so it can observe the flag. The value bump matters as much as the
+	// wake: Wait(0) returns immediately if the futex is already non-zero, which
+	// closes the window between the store above and a watcher about to sleep.
+	signalFutex.Store(1)
+	signalFutex.Wake()
+}
+
 // signalWatcher runs on its own thread under the threads scheduler. It blocks on
 // signalFutex and resumes the signal-receiving goroutine (signal_recv) whenever
 // a signal arrives, decoupling signal delivery from sleepTicks(). It mirrors the
 // signal half of waitForEvents(), which the threads scheduler never calls.
+//
+// It returns when stopSignalWatcher says the last enabled signal has gone away.
 func signalWatcher() {
 	for {
 		// Block until the signal handler bumps the futex from 0 to 1.
 		signalFutex.Wait(0)
+		if signalWatcherStop.Load() != 0 {
+			// Leave the futex as we found it for whoever runs next: a later
+			// signal_enable starts a new watcher, and it must be able to sleep.
+			signalFutex.Store(0)
+			return
+		}
 		if signalFutex.Swap(0) != 0 {
 			checkSignals()
 		}
@@ -441,6 +490,7 @@ func signal_ignore(s uint32) {
 		// receivedSignals into a uint32 array.
 		runtimePanicAt(returnAddress(0), "unsupported signal number")
 	}
+	stopSignalWatcher(s)
 	tinygo_signal_ignore(s)
 }
 
@@ -451,6 +501,7 @@ func signal_disable(s uint32) {
 		// receivedSignals into a uint32 array.
 		runtimePanicAt(returnAddress(0), "unsupported signal number")
 	}
+	stopSignalWatcher(s)
 	tinygo_signal_disable(s)
 }