patch 9.2.0176: external diff is allowed in restricted mode

Problem:  When 'diffopt' does not include "internal", Vim attempts to
          execute an external diff command even in restricted mode.
          This could be used to bypass restricted mode.
Solution: Call check_restricted() in diff_file() before attempting to
          execute an external diff (pyllyukko).

closes: #19696

Signed-off-by: pyllyukko <pyllyukko@maimed.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/version9.txt

@@ -52595,6 +52595,9 @@ between them is small (5 bytes or less) and contains only non-word characters.
 This prevents fragmented highlighting when only whitespace or punctuation
 separates changes.
 
+Using external diff mode is no longer allowed when Vim is running in
+|restricted-mode|.
+
 Other ~
 -----
 - The new |xdg.vim| script for full XDG compatibility is included.

src/diff.c

@@ -1372,6 +1372,9 @@ diff_file(diffio_T *dio)
     if (dio->dio_internal)
 	return diff_file_internal(dio);
 
+    if (check_restricted())
+	return FAIL;
+
     len = STRLEN(tmp_orig) + STRLEN(tmp_new)
 				+ STRLEN(tmp_diff) + STRLEN(p_srr) + 27;
     cmd = alloc(len);

src/testdir/test_restricted.vim

@@ -115,4 +115,27 @@ func Test_restricted_mode()
   call delete('Xresult')
 endfunc
 
+" Test that external diff is blocked in restricted mode.
+" Using :diffupdate with 'diffopt' excluding "internal" would call an external
+" diff program via call_shell(), which must be blocked.
+func Test_restricted_diff()
+  let lines =<< trim END
+    set diffopt=filler
+    call writefile(['line1', 'line2'], 'Xrfile1', 'D')
+    call writefile(['line1', 'line3'], 'Xrfile2', 'D')
+    edit Xrfile1
+    diffthis
+    split Xrfile2
+    diffthis
+    call assert_fails('diffupdate', 'E145:')
+    call writefile(v:errors, 'Xresult')
+    qa!
+  END
+  call writefile(lines, 'Xrestricteddiff', 'D')
+  if RunVim([], [], '-Z --clean -S Xrestricteddiff')
+    call assert_equal([], readfile('Xresult'))
+  endif
+  call delete('Xresult')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    176,
 /**/
     175,
 /**/