diff3: fix leaks, for real

* src/diff3.c (struct diff_block)[lint]: Add member, n2.
(free_diff_block, next_to_n2): New functions.
* tests/diff3: Add more test coverage.

src/diff3.c

@@ -78,6 +78,9 @@ struct diff_block {
   char **lines[2];		/* The actual lines (may contain nulls) */
   size_t *lengths[2];		/* Line lengths (including newlines, if any) */
   struct diff_block *next;
+#ifdef lint
+  struct diff_block *n2;	/* Used only when freeing.  */
+#endif
 };
 
 /* Three way diff */
@@ -176,7 +179,7 @@ static struct diff3_block *create_diff3_block (lin, lin, lin, lin, lin, lin);
 static struct diff3_block *make_3way_diff (struct diff_block *, struct diff_block *);
 static struct diff3_block *reverse_diff3_blocklist (struct diff3_block *);
 static struct diff3_block *using_to_diff3_block (struct diff_block *[2], struct diff_block *[2], int, int, struct diff3_block const *);
-static struct diff_block *process_diff (char const *, char const *, struct diff_block **);
+static struct diff_block *process_diff (char const *, char const *, struct diff_block **, char **);
 static void check_stdout (void);
 static void fatal (char const *) __attribute__((noreturn));
 static void output_diff3 (FILE *, struct diff3_block *, int const[3], int const[3]);
@@ -212,6 +215,38 @@ static struct option const longopts[] =
   {0, 0, 0, 0}
 };
 
+static void
+free_diff_block (struct diff_block *p)
+{
+#ifndef lint
+  (void)p;
+#else
+  while (p)
+    {
+      free (p->lines[0]);
+      free (p->lines[1]);
+      free (p->lengths[0]);
+      free (p->lengths[1]);
+      struct diff_block *next = p->n2;
+      free (p);
+      p = next;
+    }
+#endif
+}
+
+/* Copy each next pointer to n2, since make_3way_diff would clobber the former,
+   yet we will still need something to free these buffers.  */
+static void
+next_to_n2 (struct diff_block *p)
+{
+#ifndef lint
+  (void)p;
+#else
+  while (p)
+    p = p->n2 = p->next;
+#endif
+}
+
 int
 main (int argc, char **argv)
 {
@@ -377,10 +412,19 @@ main (int argc, char **argv)
   /* Invoke diff twice on two pairs of input files, combine the two
      diffs, and output them.  */
 
+  char *b0, *b1;
   commonname = file[rev_mapping[FILEC]];
-  thread1 = process_diff (file[rev_mapping[FILE1]], commonname, &last_block);
-  thread0 = process_diff (file[rev_mapping[FILE0]], commonname, &last_block);
+  thread1 = process_diff (file[rev_mapping[FILE1]], commonname, &last_block, &b1);
+  thread0 = process_diff (file[rev_mapping[FILE0]], commonname, &last_block, &b0);
+
+  next_to_n2 (thread0);
+  next_to_n2 (thread1);
+
   diff3 = make_3way_diff (thread0, thread1);
+
+  free_diff_block (thread0);
+  free_diff_block (thread1);
+
   if (edscript)
     conflicts_found
       = output_diff3_edscript (stdout, diff3, mapping, rev_mapping,
@@ -400,6 +444,8 @@ main (int argc, char **argv)
       conflicts_found = false;
     }
 
+  free (b0);
+  free (b1);
   check_stdout ();
   exit (conflicts_found);
 }
@@ -938,7 +984,8 @@ compare_line_list (char * const list1[], size_t const lengths1[],
 static struct diff_block *
 process_diff (char const *filea,
 	      char const *fileb,
-	      struct diff_block **last_block)
+	      struct diff_block **last_block,
+	      char **buf_to_free)
 {
   char *diff_contents;
   char *diff_limit;
@@ -953,6 +1000,7 @@ process_diff (char const *filea,
 				  sizeof *bptr->lengths[1]));
 
   diff_limit = read_diff (filea, fileb, &diff_contents);
+  *buf_to_free = diff_contents;
   scan_diff = diff_contents;
 
   while (scan_diff < diff_limit)

tests/diff3

@@ -27,4 +27,70 @@ diff3 a a a > out 2> err || fail=1
 compare /dev/null out || fail=1
 compare /dev/null err || fail=1
 
+# This would have provoked a nontrivial leak prior to diffutils-3.5,
+# due to the nontrivial list of diff_block structs.
+seq 10 40|sed 's/1$/x/' > d || framework_failure_
+seq 10 40|sed 's/5$/y/' > e || framework_failure_
+seq 10 40|sed 's/8$/z/' > f || framework_failure_
+cat <<'EOF' > exp40 || framework_failure_
+====1
+1:2c
+  1x
+2:2c
+3:2c
+  11
+====2
+1:6c
+3:6c
+  15
+2:6c
+  1y
+====3
+1:9c
+2:9c
+  18
+3:9c
+  1z
+====1
+1:12c
+  2x
+2:12c
+3:12c
+  21
+====2
+1:16c
+3:16c
+  25
+2:16c
+  2y
+====3
+1:19c
+2:19c
+  28
+3:19c
+  2z
+====1
+1:22c
+  3x
+2:22c
+3:22c
+  31
+====2
+1:26c
+3:26c
+  35
+2:26c
+  3y
+====3
+1:29c
+2:29c
+  38
+3:29c
+  3z
+EOF
+
+diff3 d e f > out 2> err
+compare exp40 out || fail=1
+compare /dev/null err || fail=1
+
 Exit $fail