Merge pull request #1872 from shlevy/macOS-poll-fix

monitor-fds: Fix on macOS.

src/libutil/monitor-fd.hh

@@ -21,13 +21,29 @@ public:
     MonitorFdHup(int fd)
     {
         thread = std::thread([fd]() {
+            while (true) {
               /* Wait indefinitely until a POLLHUP occurs. */
               struct pollfd fds[1];
               fds[0].fd = fd;
-            fds[0].events = 0;
+              /* This shouldn't be necessary, but macOS doesn't seem to
-            if (poll(fds, 1, -1) == -1) abort(); // can't happen
+                 like a zeroed out events field.
+                 See rdar://37537852.
+              */
+              fds[0].events = POLLHUP;
+              auto count = poll(fds, 1, -1);
+              if (count == -1) abort(); // can't happen
+              /* This shouldn't happen, but can on macOS due to a bug.
+                 See rdar://37550628.
+
+                 This may eventually need a delay or further
+                 coordination with the main thread if spinning proves
+                 too harmful.
+               */
+              if (count == 0) continue;
               assert(fds[0].revents & POLLHUP);
               triggerInterrupt();
+              break;
+            }
         });
     };