src: remap invalid file descriptors using dup2

When checking for the validity of the stdio file descriptors (nodejs#875),
ones which don't exist are intended to be remapped to /dev/null (and, if
that doesn't work, we abort).

This however doesn't work on all platforms and in all cases (e.g.
/dev/null could already have been opened by the acting process and not
actually be mapped to the expected file descriptor); instead, use the
`dup2` syscall as a more robust solution (conforms to POSIX.1).

Refs: nodejs#875
Fixes: nodejs/help#2411

src/node.cc

@@ -607,12 +607,24 @@ static void PlatformInit(ProcessInitializationFlags::Flags flags) {
     // anything.
     for (auto& s : stdio) {
       const int fd = &s - stdio;
-      if (fstat(fd, &s.stat) == 0) continue;
+      if (fstat(fd, &s.stat) == 0)
+        continue;
+
       // Anything but EBADF means something is seriously wrong.  We don't
       // have to special-case EINTR, fstat() is not interruptible.
-      if (errno != EBADF) ABORT();
-      if (fd != open("/dev/null", O_RDWR)) ABORT();
-      if (fstat(fd, &s.stat) != 0) ABORT();
+      if (errno != EBADF)
+        ABORT();
+
+      // If EBADF (file descriptor doesn't exist), open /dev/null and duplicate
+      // its file descriptor to the invalid file descriptor.  Make sure *that*
+      // file descriptor is valid.
+      const int null_fd = open("/dev/null", O_RDWR);
+      if (dup2(null_fd, fd) != 0)
+        ABORT();
+
+      if (fstat(fd, &s.stat) == 0)
+        continue;
+      ABORT();
     }
   }