app/shim: implement signal handler outcome in Go

This needs to be done from the Go side eventually anyway to integrate the signal forwarding behaviour now supported by the container package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/shim-signal.c

@@ -6,27 +6,43 @@
 #include <unistd.h>
 
 static pid_t hakurei_shim_param_ppid = -1;
+static int hakurei_shim_fd = -1;
 
-// this cannot unblock hlog since Go code is not async-signal-safe
+static ssize_t hakurei_shim_write(const void *buf, size_t count) {
+  int savedErrno = errno;
+  ssize_t ret = write(hakurei_shim_fd, buf, count);
+  if (ret == -1 && errno != EAGAIN)
+    exit(EXIT_FAILURE);
+  errno = savedErrno;
+  return ret;
+}
+
+/* see shim_linux.go for handling of the value */
 static void hakurei_shim_sigaction(int sig, siginfo_t *si, void *ucontext) {
   if (sig != SIGCONT || si == NULL) {
-    // unreachable
-    fprintf(stderr, "sigaction: sa_sigaction got invalid siginfo\n");
+    /* unreachable */
+    hakurei_shim_write("\2", 1);
     return;
   }
 
-  // monitor requests shim exit
-  if (si->si_pid == hakurei_shim_param_ppid)
-    exit(254);
+  if (si->si_pid == hakurei_shim_param_ppid) {
+    /* monitor requests shim exit */
+    hakurei_shim_write("\0", 1);
+    return;
+  }
 
-  fprintf(stderr, "sigaction: got SIGCONT from process %d\n", si->si_pid);
+  /* unexpected si_pid */
+  hakurei_shim_write("\3", 1);
 
-  // shim orphaned before monitor delivers a signal
   if (getppid() != hakurei_shim_param_ppid)
-    exit(3);
+    /* shim orphaned before monitor delivers a signal */
+    hakurei_shim_write("\1", 1);
 }
 
-void hakurei_shim_setup_cont_signal(pid_t ppid) {
+void hakurei_shim_setup_cont_signal(pid_t ppid, int fd) {
+  if (hakurei_shim_param_ppid != -1 || hakurei_shim_fd != -1)
+    *(int *)NULL = 0; /* unreachable */
+
   struct sigaction new_action = {0}, old_action = {0};
   if (sigaction(SIGCONT, NULL, &old_action) != 0)
     return;
@@ -45,4 +61,5 @@ void hakurei_shim_setup_cont_signal(pid_t ppid) {
 
   errno = 0;
   hakurei_shim_param_ppid = ppid;
+  hakurei_shim_fd = fd;
 }