job-control: set CLOEXEC on pty processes. #5986

Before this change, new processes started with libuv prevented SIGHUP
from reaching pty processes (by keeping the ptmx file descriptor open).

src/nvim/os/fs.c

@@ -391,6 +391,35 @@ int os_open(const char* path, int flags, int mode)
   return r;
 }
 
+/// Sets file descriptor `fd` to close-on-exec.
+//
+// @return -1 if failed to set, 0 otherwise.
+int os_set_cloexec(const int fd)
+{
+#ifdef HAVE_FD_CLOEXEC
+  int e;
+  int fdflags = fcntl(fd, F_GETFD);
+  if (fdflags < 0) {
+    e = errno;
+    ELOG("Failed to get flags on descriptor %d: %s", fd, strerror(e));
+    errno = e;
+    return -1;
+  }
+  if ((fdflags & FD_CLOEXEC) == 0
+      && fcntl(fd, F_SETFD, fdflags | FD_CLOEXEC) < 0) {
+    e = errno;
+    ELOG("Failed to set CLOEXEC on descriptor %d: %s", fd, strerror(e));
+    errno = e;
+    return -1;
+  }
+  return 0;
+#endif
+
+  // No FD_CLOEXEC flag. On Windows, the file should have been opened with
+  // O_NOINHERIT anyway.
+  return -1;
+}
+
 /// Close a file
 ///
 /// @return 0 or libuv error code on failure.