diff: output symlink contents when they differ

* bootstrap.conf (gnulib_modules): Add quote.
* src/diff.c: Include quote.h.
(compare_files): Print contents of symlinks that differ,
and quote their names and contents.
* src/system.h (symlink_size_ok): Remove.
(stat_size): Don’t worry about symlink sizes.
* tests/no-dereference: Adjust tests to match new behavior.

NEWS

@@ -4,6 +4,11 @@ GNU diffutils NEWS                                    -*- outline -*-
 
 ** Improvements
 
+  diff now outputs more information when symbolic links differ, and
+  quotes their names to avoid ambiguity.  For example, instead of
+  "Symbolic links d/x do and e/x do differ", diff now outputs
+  "Symbolic links 'd/x do' -> 'a' and 'e/x do' -> 'b' differ".
+
   diff's --ignore-case (-i) and --ignore-file-name-case options now
   support multi-byte characters.  For example, they treat Greek
   capital Δ like small δ when input uses UTF-8.

bootstrap.conf

@@ -86,6 +86,7 @@ perl
 popen
 progname
 propername-lite
+quote
 raise
 rawmemchr
 readdir

src/diff.c

@@ -34,6 +34,7 @@
 #include <getopt.h>
 #include <hard-locale.h>
 #include <progname.h>
+#include <quote.h>
 #include <sh-quote.h>
 #include <stat-time.h>
 #include <version-etc.h>
@@ -1493,45 +1494,39 @@ compare_files (struct comparison const *parent,
       dassert (no_dereference_symlinks);
 
       /* Compare the values of the symbolic links.  */
-      if (cmp.file[0].stat.st_size != cmp.file[1].stat.st_size
-	  && 0 <= cmp.file[0].stat.st_size
-	  && 0 <= cmp.file[1].stat.st_size)
-	status = EXIT_FAILURE;
-      else
+      char *link_value[2]; link_value[1] = nullptr;
+      char linkbuf[2][128];
+
+      for (bool f = false; ; f = true)
 	{
-	  char *link_value[2]; link_value[1] = nullptr;
-	  char linkbuf[2][128];
-
-	  for (bool f = false; ; f = true)
+	  int dirfd = parent->file[f].desc;
+	  char const *name = cmp.file[f].name;
+	  char const *nm = dirfd < 0 ? name : last_component (name);
+	  link_value[f] = careadlinkat (dirfd, nm,
+					linkbuf[f], sizeof linkbuf[f],
+					nullptr, readlinkat);
+	  if (!link_value[f])
 	    {
-	      int dirfd = parent->file[f].desc;
-	      char const *name = cmp.file[f].name;
-	      char const *nm = dirfd < 0 ? name : last_component (name);
-	      link_value[f] = careadlinkat (dirfd, nm,
-					    linkbuf[f], sizeof linkbuf[f],
-					    nullptr, readlinkat);
-	      if (!link_value[f])
-		{
-		  perror_with_name (cmp.file[f].name);
-		  status = EXIT_TROUBLE;
-		  break;
-		}
-	      if (f)
-		{
-		  status = (STREQ (link_value[0], link_value[f])
-			    ? EXIT_SUCCESS : EXIT_FAILURE);
-		  break;
-		}
+	      perror_with_name (cmp.file[f].name);
+	      status = EXIT_TROUBLE;
+	      break;
+	    }
+	  if (f)
+	    {
+	      status = (STREQ (link_value[0], link_value[f])
+			? EXIT_SUCCESS : EXIT_FAILURE);
+	      break;
 	    }
-
-	  for (int f = 0; f < 2; f++)
-	    if (link_value[f] != linkbuf[f])
-	      free (link_value[f]);
 	}
 
       if (status == EXIT_FAILURE)
-	message ("Symbolic links %s and %s differ\n",
-		 cmp.file[0].name, cmp.file[1].name);
+	message ("Symbolic links %s -> %s and %s -> %s differ\n",
+		 quote_n (0, cmp.file[0].name), quote_n (1, link_value[0]),
+		 quote_n (2, cmp.file[1].name), quote_n (3, link_value[1]));
+
+      for (int f = 0; f < 2; f++)
+	if (link_value[f] != linkbuf[f])
+	  free (link_value[f]);
     }
   else if (files_can_be_treated_as_binary
 	   && cmp.file[0].detype == DE_REG

src/system.h

@@ -114,7 +114,7 @@
 SYSTEM_EXTERN dev_t proc_dev;
 #endif
 
-#if defined __linux__ || defined __ANDROID__
+#if false && (defined __linux__ || defined __ANDROID__)
 # include <sys/utsname.h>
 /* 1 if symlink st_size is OK, -1 if not, 0 if unknown yet.  */
 SYSTEM_EXTERN signed char symlink_size_ok;
@@ -197,9 +197,10 @@ SYSTEM_INLINE off_t stat_size (struct stat *s)
 	return -1;
     }
 #endif
-#if defined __linux__ || defined __ANDROID__
+#if false && (defined __linux__ || defined __ANDROID__)
   /* Symlinks have suspect sizes on Linux kernels before 5.15,
-     due to bugs in fscrypt.  */
+     due to bugs in fscrypt.  However, diffutils never looks
+     at symlink sizes so this code is not needed.  */
   if (S_ISLNK (mode))
     {
       if (! symlink_size_ok)

tests/no-dereference

@@ -43,17 +43,17 @@ compare /dev/null out || fail=1
 
 # Test case 7: Compare symbolic links with different value and different target
 # contents.
-returns_ 1 diff --no-dereference symlink1 symlink2 > out || fail=1
+LC_ALL=C returns_ 1 diff --no-dereference symlink1 symlink2 > out || fail=1
 cat <<EOF > expected || framework_failure_
-Symbolic links symlink1 and symlink2 differ
+Symbolic links 'symlink1' -> 'regular1' and 'symlink2' -> 'regular2' differ
 EOF
 compare expected out || fail=1
 
 # Test case 8: Compare symbolic links with different value and same target
 # contents.
-returns_ 1 diff --no-dereference symlink2 symlink3 > out || fail=1
+LC_ALL=C returns_ 1 diff --no-dereference symlink2 symlink3 > out || fail=1
 cat <<EOF > expected || framework_failure_
-Symbolic links symlink2 and symlink3 differ
+Symbolic links 'symlink2' -> 'regular2' and 'symlink3' -> 'regular3' differ
 EOF
 compare expected out || fail=1
 
@@ -134,9 +134,9 @@ mkdir subdir7a
 mkdir subdir7b
 ln -s ../regular1 subdir7a/foo
 ln -s ../regular2 subdir7b/foo
-returns_ 1 diff -r --no-dereference subdir7a subdir7b > out || fail=1
+LC_ALL=C returns_ 1 diff -r --no-dereference subdir7a subdir7b > out || fail=1
 cat <<EOF > expected || framework_failure_
-Symbolic links subdir7a/foo and subdir7b/foo differ
+Symbolic links 'subdir7a/foo' -> '../regular1' and 'subdir7b/foo' -> '../regular2' differ
 EOF
 compare expected out || fail=1
 
@@ -146,9 +146,9 @@ mkdir subdir8a
 mkdir subdir8b
 ln -s ../regular2 subdir8a/foo
 ln -s ../regular3 subdir8b/foo
-returns_ 1 diff -r --no-dereference subdir8a subdir8b > out || fail=1
+LC_ALL=C returns_ 1 diff -r --no-dereference subdir8a subdir8b > out || fail=1
 cat <<EOF > expected || framework_failure_
-Symbolic links subdir8a/foo and subdir8b/foo differ
+Symbolic links 'subdir8a/foo' -> '../regular2' and 'subdir8b/foo' -> '../regular3' differ
 EOF
 compare expected out || fail=1