runtime: add support for os/signal

This adds support for enabling and listening to signals on Linux and
MacOS.

Author: aykevl

builder/musl.go

@@ -128,6 +128,7 @@ var libMusl = Library{
 			"mman/*.c",
 			"math/*.c",
 			"multibyte/*.c",
+			"signal/" + arch + "/*.s",
 			"signal/*.c",
 			"stdio/*.c",
 			"string/*.c",

compileopts/target.go

@@ -391,7 +391,8 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		)
 		spec.ExtraFiles = append(spec.ExtraFiles,
 			"src/runtime/os_darwin.c",
-			"src/runtime/runtime_unix.c")
+			"src/runtime/runtime_unix.c",
+			"src/runtime/signal.c")
 	case "linux":
 		spec.Linker = "ld.lld"
 		spec.RTLib = "compiler-rt"
@@ -412,7 +413,8 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 			spec.CFlags = append(spec.CFlags, "-mno-outline-atomics")
 		}
 		spec.ExtraFiles = append(spec.ExtraFiles,
-			"src/runtime/runtime_unix.c")
+			"src/runtime/runtime_unix.c",
+			"src/runtime/signal.c")
 	case "windows":
 		spec.Linker = "ld.lld"
 		spec.Libc = "mingw-w64"

main_test.go

@@ -79,6 +79,7 @@ func TestBuild(t *testing.T) {
 		"oldgo/",
 		"print.go",
 		"reflect.go",
+		"signal.go",
 		"slice.go",
 		"sort.go",
 		"stdlib.go",
@@ -217,6 +218,7 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 	// isWebAssembly := strings.HasPrefix(spec.Triple, "wasm")
 	isWASI := strings.HasPrefix(options.Target, "wasi")
 	isWebAssembly := isWASI || strings.HasPrefix(options.Target, "wasm") || (options.Target == "" && strings.HasPrefix(options.GOARCH, "wasm"))
+	isBaremetal := options.Target == "simavr" || options.Target == "cortex-m-qemu" || options.Target == "riscv-qemu"
 
 	for _, name := range tests {
 		if options.GOOS == "linux" && (options.GOARCH == "arm" || options.GOARCH == "386") {
@@ -281,6 +283,13 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 				continue
 			}
 		}
+		if isWebAssembly || isBaremetal || options.GOOS == "windows" {
+			switch name {
+			case "signal.go":
+				// Signals only work on POSIX-like systems.
+				continue
+			}
+		}
 
 		name := name // redefine to avoid race condition
 		t.Run(name, func(t *testing.T) {

src/os/signal/signal.go

@@ -3,6 +3,8 @@
 package runtime
 
 import (
+	"math/bits"
+	"sync/atomic"
 	"unsafe"
 )
 
@@ -12,6 +14,9 @@ func libc_write(fd int32, buf unsafe.Pointer, count uint) int
 //export usleep
 func usleep(usec uint) int
 
+//export pause
+func pause() int32
+
 // void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
 // Note: off_t is defined as int64 because:
 //   - musl (used on Linux) always defines it as int64
@@ -217,8 +222,47 @@ func nanosecondsToTicks(ns int64) timeUnit {
 }
 
 func sleepTicks(d timeUnit) {
-	// timeUnit is in nanoseconds, so need to convert to microseconds here.
-	usleep(uint(d) / 1000)
+	// When there are no signal handlers present, we can simply go to sleep.
+	if !hasSignals {
+		// timeUnit is in nanoseconds, so need to convert to microseconds here.
+		usleep(uint(d) / 1000)
+		return
+	}
+
+	if GOOS == "darwin" {
+		// Check for incoming signals.
+		if checkSignals() {
+			// Received a signal, so there's probably at least one goroutine
+			// that's runnable again.
+			return
+		}
+
+		// WARNING: there is a race condition here. If a signal arrives between
+		// checkSignals() and usleep(), the usleep() call will not exit early so
+		// the signal is delayed until usleep finishes or another signal
+		// arrives.
+		// There doesn't appear to be a simple way to fix this on MacOS.
+
+		// timeUnit is in nanoseconds, so need to convert to microseconds here.
+		result := usleep(uint(d) / 1000)
+		if result != 0 {
+			checkSignals()
+		}
+	} else {
+		// Linux (and various other POSIX systems) implement sigtimedwait so we
+		// can do this in a non-racy way.
+		tinygo_wfi_mask(activeSignals)
+		if checkSignals() {
+			tinygo_wfi_unmask()
+			return
+		}
+		signal := tinygo_wfi_sleep(activeSignals, uint64(d))
+		if signal >= 0 {
+			tinygo_signal_handler(signal)
+			checkSignals()
+		}
+		tinygo_wfi_unmask()
+	}
 }
 
 func getTime(clock int32) uint64 {
@@ -307,3 +351,183 @@ func growHeap() bool {
 	setHeapEnd(heapStart + heapSize)
 	return true
 }
+
+func init() {
+	// Set up a channel to receive signals into.
+	signalChan = make(chan uint32, 1)
+}
+
+var signalChan chan uint32
+
+// Indicate whether signals have been registered.
+var hasSignals bool
+
+// Mask of signals that have been received. The signal handler atomically ORs
+// signals into this value.
+var receivedSignals uint32
+
+var activeSignals uint32
+
+//go:linkname signal_enable os/signal.signal_enable
+func signal_enable(s uint32) {
+	if s >= 32 {
+		// TODO: to support higher signal numbers, we need to turn
+		// receivedSignals into a uint32 array.
+		runtimePanicAt(returnAddress(0), "unsupported signal number")
+	}
+	hasSignals = true
+	activeSignals |= 1 << s
+	// It's easier to implement this function in C.
+	tinygo_signal_enable(s)
+}
+
+//go:linkname signal_ignore os/signal.signal_ignore
+func signal_ignore(s uint32) {
+	if s >= 32 {
+		// TODO: to support higher signal numbers, we need to turn
+		// receivedSignals into a uint32 array.
+		runtimePanicAt(returnAddress(0), "unsupported signal number")
+	}
+	activeSignals &^= 1 << s
+	tinygo_signal_ignore(s)
+}
+
+//go:linkname signal_disable os/signal.signal_disable
+func signal_disable(s uint32) {
+	if s >= 32 {
+		// TODO: to support higher signal numbers, we need to turn
+		// receivedSignals into a uint32 array.
+		runtimePanicAt(returnAddress(0), "unsupported signal number")
+	}
+	activeSignals &^= 1 << s
+	tinygo_signal_disable(s)
+}
+
+//go:linkname signal_waitUntilIdle os/signal.signalWaitUntilIdle
+func signal_waitUntilIdle() {
+	// Make sure all signals are sent on the channel.
+	for atomic.LoadUint32(&receivedSignals) != 0 {
+		checkSignals()
+		Gosched()
+	}
+
+	// Make sure all signals are processed.
+	for len(signalChan) != 0 {
+		Gosched()
+	}
+}
+
+//export tinygo_signal_enable
+func tinygo_signal_enable(s uint32)
+
+//export tinygo_signal_ignore
+func tinygo_signal_ignore(s uint32)
+
+//export tinygo_signal_disable
+func tinygo_signal_disable(s uint32)
+
+// void tinygo_signal_handler(int sig);
+//
+//export tinygo_signal_handler
+func tinygo_signal_handler(s int32) {
+	// This loop is essentially the atomic equivalent of the following:
+	//
+	//   receivedSignals |= 1 << s
+	//
+	// TODO: use atomic.Uint32.And once we drop support for Go 1.22 instead of
+	// this loop.
+	for {
+		mask := uint32(1) << uint32(s)
+		val := atomic.LoadUint32(&receivedSignals)
+		swapped := atomic.CompareAndSwapUint32(&receivedSignals, val, val|mask)
+		if swapped {
+			break
+		}
+	}
+}
+
+//go:linkname signal_recv os/signal.signal_recv
+func signal_recv() uint32 {
+	// Function called from os/signal to get the next received signal.
+	val := <-signalChan
+	checkSignals()
+	return val
+}
+
+// Atomically find a signal that previously occured and send it into the
+// signalChan channel. Return true if at least one signal was delivered this
+// way, false otherwise.
+func checkSignals() bool {
+	gotSignals := false
+	for {
+		// Extract the lowest numbered signal number from receivedSignals.
+		val := atomic.LoadUint32(&receivedSignals)
+		if val == 0 {
+			// There is no signal ready to be received by the program (common
+			// case).
+			return gotSignals
+		}
+		num := uint32(bits.TrailingZeros32(val))
+
+		// Do a non-blocking send on signalChan.
+		select {
+		case signalChan <- num:
+			// There was room free in the channel, so remove the signal number
+			// from the receivedSignals mask.
+			gotSignals = true
+		default:
+			// Could not send the signal number on the channel. This means
+			// there's still a signal pending. In that case, let it be received
+			// at which point checkSignals is called again to put the next one
+			// in the channel buffer.
+			return gotSignals
+		}
+
+		// Atomically clear the signal number from receivedSignals.
+		// TODO: use atomic.Uint32.Or once we drop support for Go 1.22 instead
+		// of this loop.
+		for {
+			newVal := val &^ (1 << num)
+			swapped := atomic.CompareAndSwapUint32(&receivedSignals, val, newVal)
+			if swapped {
+				break
+			}
+			val = atomic.LoadUint32(&receivedSignals)
+		}
+	}
+}
+
+//export tinygo_wfi_mask
+func tinygo_wfi_mask(active uint32)
+
+//export tinygo_wfi_sleep
+func tinygo_wfi_sleep(active uint32, timeout uint64) int32
+
+//export tinygo_wfi_wait
+func tinygo_wfi_wait(active uint32) int32
+
+//export tinygo_wfi_unmask
+func tinygo_wfi_unmask()
+
+func waitForEvents() {
+	if hasSignals {
+		// We could have used pause() here, but that function is impossible to
+		// use in a race-free way:
+		// https://www.cipht.net/2023/11/30/perils-of-pause.html
+		// Therefore we need something better.
+		// Note: this is unsafe with multithreading, because sigprocmask is only
+		// defined for single-threaded applictions.
+		tinygo_wfi_mask(activeSignals)
+		if checkSignals() {
+			tinygo_wfi_unmask()
+			return
+		}
+		signal := tinygo_wfi_wait(activeSignals)
+		tinygo_signal_handler(signal)
+		checkSignals()
+		tinygo_wfi_unmask()
+	} else {
+		// The program doesn't use signals, so this is a deadlock.
+		runtimePanic("deadlocked: no event source")
+	}
+}