nix-daemon: Use a thread instead of SIGPOLL to catch client disconnects

The thread calls poll() to wait until a HUP (or other error event)
happens on the client connection. If so, it sends SIGINT to the main
thread, which is then cleaned up normally. This is much nicer than
messing around with SIGPOLL.

src/libutil/monitor-fd.hh

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <thread>
+
+#include <poll.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <signal.h>
+
+namespace nix {
+
+
+class MonitorFdHup
+{
+private:
+    std::thread thread;
+
+public:
+    MonitorFdHup(int fd)
+    {
+        thread = std::thread([&]() {
+            /* Wait indefinitely until a POLLHUP occurs. */
+            struct pollfd fds[1];
+            fds[0].fd = fd;
+            fds[0].events = 0;
+            if (poll(fds, 1, -1) == -1) {
+                if (errno != EINTR) abort(); // can't happen
+                return; // destructor is asking us to exit
+            }
+            /* We got POLLHUP, so send an INT signal to the main thread. */
+            kill(getpid(), SIGINT);
+        });
+    };
+
+    ~MonitorFdHup()
+    {
+        pthread_kill(thread.native_handle(), SIGINT);
+        thread.join();
+    }
+};
+
+
+}