runtime: add support for os/signal

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

TODO: also support disabling signals.

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

@@ -390,7 +390,8 @@ 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"
@@ -411,7 +412,8 @@ 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

@@ -75,6 +75,7 @@ func TestBuild(t *testing.T) {
 		"oldgo/",
 		"print.go",
 		"reflect.go",
+		"signal.go",
 		"slice.go",
 		"sort.go",
 		"stdlib.go",
@@ -213,6 +214,7 @@ 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") {
@@ -277,6 +279,13 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 				continue
 			}
 		}
+		if isWebAssembly || isBaremetal || options.GOOS == "windows" {
+			switch name {
+			case "signal/":
+				// 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 +222,22 @@ func nanosecondsToTicks(ns int64) timeUnit {
 }
 
 func sleepTicks(d timeUnit) {
+	// Check for incoming signals.
+	if checkSignals() {
+		// Received a signal, so there's probably at least one goroutine that's
+		// runnable again.
+		return
+	}
+
+	// TODO: 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.
+
 	// timeUnit is in nanoseconds, so need to convert to microseconds here.
-	usleep(uint(d) / 1000)
+	result := usleep(uint(d) / 1000)
+	if result != 0 {
+		checkSignals()
+	}
 }
 
 func getTime(clock int32) uint64 {
@@ -307,3 +326,124 @@ 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
+
+// Simple boolean that's true when any signals have been registered.
+var hasSignals uint32
+
+// Mask of signals that have been received. The signal handler atomically ORs
+// signals into this value.
+var receivedSignals 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")
+	}
+	atomic.StoreUint32(&hasSignals, 1)
+	// It's easier to implement this function in C.
+	tinygo_signal_enable(s)
+}
+
+//export tinygo_signal_enable
+func tinygo_signal_enable(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)
+		}
+	}
+}
+
+func waitForEvents() {
+	if atomic.LoadUint32(&hasSignals) != 0 {
+		// TODO: there is a race condition here. If a signal arrives between
+		// checkSignals() and pause(), pause() will not exit early but instead
+		// be delayed until the next signal arrives.
+		// We should use something like this instead to avoid it:
+		//   - mask all active signals
+		//   - run checkSignals()
+		//   - run sigsuspend() with all active signals
+		//   - unmask all active signals
+		// For a longer explanation of the problem, see:
+		// https://www.cipht.net/2023/11/30/perils-of-pause.html
+		checkSignals()
+		pause()
+		checkSignals()
+	} else {
+		// The program doesn't use signals, so this is a deadlock.
+		runtimePanic("deadlocked: no event source")
+	}
+}

src/runtime/runtime_unix.go

@@ -0,0 +1,18 @@
+//go:build none
+
+// Ignore the //go:build above. This file is manually included on Linux and
+// MacOS to provide os/signal support.
+
+#include <stdint.h>
+#include <signal.h>
+#include <unistd.h>
+
+// Signal handler in the runtime.
+void tinygo_signal_handler(int sig);
+
+// Enable a signal from the runtime.
+void tinygo_signal_enable(uint32_t sig) {
+    struct sigaction act = { 0 };
+    act.sa_handler = &tinygo_signal_handler;
+    sigaction(sig, &act, NULL);
+}

src/runtime/signal.c

@@ -1,4 +1,4 @@
-//go:build !tinygo.riscv && !cortexm
+//go:build !tinygo.riscv && !cortexm && !linux && !darwin
 
 package runtime
 

src/runtime/wait_other.go

@@ -0,0 +1,34 @@
+package main
+
+// Test POSIX signals.
+// TODO: run `tinygo test os/signal` instead, once CGo errno return values are
+// supported.
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+func main() {
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, 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())
+			}
+		}
+	}()
+
+	// Send the signal.
+	syscall.Kill(syscall.Getpid(), syscall.SIGUSR1)
+
+	time.Sleep(time.Millisecond * 100)
+	println("exiting signal program")
+}

testdata/signal.go

@@ -0,0 +1,2 @@
+got expected signal
+exiting signal program