Handle SIGCHLD for exec/forkExec

When forkExec is called it begins to ignore all SIGCHLD signals for
the rest of the progam's execution so that they are automatically
reaped. However, this means that subsequent waitpid calls in the exec
function will always fail. So instead handle SIGCHLD by reaping any
processes created by forkExec and ignoring all others so that they can be
handled directly by the exec function.

src/main.cpp

@@ -1,7 +1,32 @@
 #include <csignal>
+#include <list>
+#include <sys/types.h>
+#include <sys/wait.h>
 #include <spdlog/spdlog.h>
 #include "client.hpp"
 
+sig_atomic_t is_inserting_pid = false;
+std::list<pid_t> reap;
+
+static void handler(int sig) {
+  int saved_errno = errno;
+  if (!is_inserting_pid) {
+    for (auto it = reap.begin(); it != reap.end(); ++it) {
+      if (waitpid(*it, nullptr, WNOHANG) == *it) {
+        it = reap.erase(it);
+      }
+    }
+  }
+  errno = saved_errno;
+}
+
+inline void installSigChldHandler(void) {
+  struct sigaction sa;
+  sigemptyset(&sa.sa_mask);
+  sa.sa_handler = handler;
+  sigaction(SIGCHLD, &sa, nullptr);
+}
+
 int main(int argc, char* argv[]) {
   try {
     auto client = waybar::Client::inst();
@@ -18,6 +43,7 @@ int main(int argc, char* argv[]) {
         }
       });
     }
+    installSigChldHandler();
 
     auto ret = client->main(argc, argv);
     delete client;