gnulib-tool: In sh+py mode, don't fail because of dangling symlinks

[Bruno Haible <bruno@clisp.org>]

Paul Eggert reported:
> I tried this on my well-worn copy of GNU diffutils, and got the 
> following diagnostics. These were all diagnostics about backup files, or 
> files I manually deleted by moving them into a '.del' subdirectory (an 
> old habit of mine), or symlink loops that I had created to test.
> 
> Perhaps the advice should start with, "Start with a fresh checkout from 
> Git."
> 
> -----
> diff: ./.del/gnulib-tests/test-fnmatch-1.sh: No such file or directory
> diff: /home/eggert/src/gnu/glpyXeRWfy/.del/gnulib-tests/test-fnmatch-1.sh: No such file or directory
> ...

These old files most likely were not backup files, but dangling symlinks.
With dangling symlinks, I can reproduce the error, and this patch fixes it:


2024-04-20  Bruno Haible  <bruno@clisp.org>

	gnulib-tool: In sh+py mode, don't fail because of dangling symlinks.
	Reported by Paul Eggert in
	<https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00318.html>
	and Pádraig Brady in
	<https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00321.html>.
	* gnulib-tool: In sh+py mode, use diff option '--no-dereference' if
	available.

diff --git a/gnulib-tool b/gnulib-tool
index d7b6d33912..6d430e56e6 100755
--- a/gnulib-tool
+++ b/gnulib-tool
@@ -192,7 +192,14 @@ case "$GNULIB_TOOL_IMPL" in
           func_fatal_error "gnulib-tool.sh succeeded but gnulib-tool.py failed! Inspect $tmp/ and $tmp-py-err."
         fi
         # Compare the two results on the file system.
-        diff -r -q . "$tmp" >/dev/null ||
+        # GNU diffutils 3.3 or newer support option --no-dereference. This
+        # option avoids errors on dangling links.
+        if LC_ALL=C diff --help 2>/dev/null | grep no-dereference >/dev/null; then
+          diff_options='--no-dereference'
+        else
+          diff_options=
+        fi
+        diff -r $diff_options -q . "$tmp" >/dev/null ||
           func_fatal_error "gnulib-tool.py produced different files than gnulib-tool.sh! Compare `pwd` and $tmp."
         # Compare the two outputs.
         diff -q "$tmp-sh-out" "$tmp-py-out" >/dev/null ||
@@ -235,7 +242,14 @@ case "$GNULIB_TOOL_IMPL" in
           func_fatal_error "gnulib-tool.sh succeeded but gnulib-tool.py failed! Inspect $tmp/ and $tmp-py-err."
         fi
         # Compare the two results on the file system.
-        diff -r -q "$dir" "$tmp" >/dev/null ||
+        # GNU diffutils 3.3 or newer support option --no-dereference. This
+        # option avoids errors on dangling links.
+        if LC_ALL=C diff --help 2>/dev/null | grep no-dereference >/dev/null; then
+          diff_options='--no-dereference'
+        else
+          diff_options=
+        fi
+        diff -r $diff_options -q "$dir" "$tmp" >/dev/null ||
           func_fatal_error "gnulib-tool.py produced different files than gnulib-tool.sh! Compare $dir and $tmp."
         # Compare the two outputs.
         diff -q "$tmp-sh-out" "$tmp-py-out" >/dev/null ||