runtime: add support for os/signal

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

src/runtime/signal.c

@@ -0,0 +1,89 @@
+//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 <time.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);
+}
+
+void tinygo_signal_ignore(uint32_t sig) {
+    struct sigaction act = { 0 };
+    act.sa_handler = SIG_IGN;
+    sigaction(sig, &act, NULL);
+}
+
+void tinygo_signal_disable(uint32_t sig) {
+    struct sigaction act = { 0 };
+    act.sa_handler = SIG_DFL;
+    sigaction(sig, &act, NULL);
+}
+
+// Implement waitForEvents and sleep with signals.
+// Warning: sigprocmask is not defined in a multithreaded program so will need
+// to be replaced with something else once we implement threading on POSIX.
+
+// Signals active before a call to tinygo_wfi_mask.
+static sigset_t active_signals;
+
+static void tinygo_set_signals(sigset_t *mask, uint32_t signals) {
+    sigemptyset(mask);
+    for (int i=0; i<32; i++) {
+        if ((signals & (1<<i)) != 0) {
+            sigaddset(mask, i);
+        }
+    }
+}
+
+// Mask the given signals.
+// This function must always restore the previous signals using
+// tinygo_wfi_unmask, to create a critical section.
+void tinygo_wfi_mask(uint32_t active) {
+    sigset_t mask;
+    tinygo_set_signals(&mask, active);
+
+    sigprocmask(SIG_BLOCK, &mask, &active_signals);
+}
+
+// Wait until a signal becomes pending (or is already pending), and return the
+// signal.
+#if !defined(__APPLE__)
+int tinygo_wfi_sleep(uint32_t active, uint64_t timeout) {
+    sigset_t active_set;
+    tinygo_set_signals(&active_set, active);
+
+    struct timespec ts = {0};
+    ts.tv_sec  = timeout / 1000000000;
+    ts.tv_nsec = timeout % 1000000000;
+
+    int result = sigtimedwait(&active_set, NULL, &ts);
+    return result;
+}
+#endif
+
+// Wait until any of the active signals becomes pending (or returns immediately
+// if one is already pending).
+int tinygo_wfi_wait(uint32_t active) {
+    sigset_t active_set;
+    tinygo_set_signals(&active_set, active);
+
+    int sig = 0;
+    sigwait(&active_set, &sig);
+    return sig;
+}
+
+// Restore previous signal mask.
+void tinygo_wfi_unmask(void) {
+    sigprocmask(SIG_SETMASK, &active_signals, NULL);
+}