Cygwin: implement /proc/PID/fd/DESCRIPTOR reopening by handle

Allows expressions along the lines of `cat /proc/self/fd/0 <<EOF'.
The problem here is that the temporary file used for the here script
has already been deleted by the shell.  Opening by filename, as
implemented so far, doesn't work because the file has been moved
to the bin.

Allow reopening files by handle the same way from another process
as long as we have sufficient permissions on the foreign process.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_process.cc

@@ -321,6 +321,84 @@ out:
   return res;
 }
 
+fhandler_base *
+fhandler_process::fd_reopen (int flags)
+{
+  const char *path;
+  char *e;
+  int fd;
+  HANDLE proc = NULL;
+  HANDLE hdl = NULL;
+  fhandler_base *fh = NULL;
+
+  path = get_name () + proc_len + 1;
+  pid = strtoul (path, &e, 10);
+  path = e + 4;
+  fd = strtoul (path, &e, 10);
+  if (e == path || *e != '\0')
+    {
+      set_errno (ENOENT);
+      return NULL;
+    }
+
+  if (pid == myself->pid)
+    {
+      cygheap_fdget cfd (fd);
+      if (cfd < 0)
+	return NULL;
+      fh = build_fh_pc (cfd->pc);
+      if (!fh)
+	goto err_out;
+      fh->set_io_handle (cfd->get_handle ());
+    }
+  else
+    {
+      size_t size;
+      path_conv pc;
+
+      pinfo p (pid);
+      if (!p)
+	{
+	  set_errno (ENOENT);
+	  return NULL;
+	}
+      if (!(proc = OpenProcess (PROCESS_DUP_HANDLE, false, p->dwProcessId)))
+	{
+	  __seterrno ();
+	  return NULL;
+	}
+      void *buf = p->file_pathconv (fd, size);
+      if (size == 0)
+	{
+	  set_errno (EPERM);
+	  goto err_out;
+	}
+      hdl = pc.deserialize (buf);
+      if (!DuplicateHandle (proc, hdl, GetCurrentProcess (), &hdl,
+			    0, FALSE, DUPLICATE_SAME_ACCESS))
+	{
+	  __seterrno ();
+	  hdl = NULL;
+	  goto err_out;
+	}
+      fh = build_fh_pc (pc);
+      if (!fh)
+	goto err_out;
+      fh->set_io_handle (hdl);
+    }
+  if (!fh->open_with_arch (flags, 0))
+    {
+      delete fh;
+      fh = NULL;
+    }
+err_out:
+  if (hdl)
+    CloseHandle (hdl);
+  if (proc)
+    CloseHandle (proc);
+  return fh;
+}
+
 struct process_fd_t {
   const char *path;
   _pinfo *p;