[PATCH] backupfile: new function to validate backup suffix

[Paul Eggert]

* lib/backupfile.c (set_simple_backup_suffix): New function.
Also, reject suffixes containing "/".
(backupfile_internal): Use it.
---
 ChangeLog        |  4 ++++
 lib/backupfile.c | 19 +++++++++++--------
 lib/backupfile.h |  1 +
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 72d5a77..7aedb71 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2017-07-30  Paul Eggert  <address@hidden>
 
+       backupfile: new function to validate backup suffix
+       * lib/backupfile.c (set_simple_backup_suffix): New function.
+       (backupfile_internal): Use it.
+
        canonicalize: fix EOVERFLOW commentary
        Problem reported by Bruno Haible in:
        http://lists.gnu.org/archive/html/bug-gnulib/2017-07/msg00147.html
diff --git a/lib/backupfile.c b/lib/backupfile.c
index 9773fe2..d7115bd 100644
--- a/lib/backupfile.c
+++ b/lib/backupfile.c
@@ -83,6 +83,16 @@
    to numbered) backup file name. */
 char const *simple_backup_suffix = NULL;
 
+/* Set SIMPLE_BACKUP_SUFFIX to S, or to a default specified by the
+   environment if S is null.  If S or the environment does not specify
+   a valid backup suffix, use "~".  */
+void
+set_simple_backup_suffix (char const *s)
+{
+  if (!s)
+    s = getenv ("SIMPLE_BACKUP_SUFFIX");
+  simple_backup_suffix = s && *s && s == last_component (s) ? s : "~";
+}
 
 /* If FILE (which was of length FILELEN before an extension was
    appended to it) is too long, replace the extension with the single
@@ -289,15 +299,8 @@ backupfile_internal (char const *file, enum backup_type 
backup_type, bool rename
   ptrdiff_t base_offset = last_component (file) - file;
   size_t filelen = base_offset + strlen (file + base_offset);
 
-  /* Initialize the default simple backup suffix.  */
   if (! simple_backup_suffix)
-    {
-      char const *env_suffix = getenv ("SIMPLE_BACKUP_SUFFIX");
-      if (env_suffix && *env_suffix)
-        simple_backup_suffix = env_suffix;
-      else
-        simple_backup_suffix = "~";
-    }
+    set_simple_backup_suffix (NULL);
 
   /* Allow room for simple or ".~N~" backups.  The guess must be at
      least sizeof ".~1~", but otherwise will be adjusted as needed.  */
diff --git a/lib/backupfile.h b/lib/backupfile.h
index d578a65..fe4d89d 100644
--- a/lib/backupfile.h
+++ b/lib/backupfile.h
@@ -46,6 +46,7 @@ enum backup_type
 
 extern char const *simple_backup_suffix;
 
+void set_simple_backup_suffix (char const *);
 char *backup_file_rename (char const *, enum backup_type);
 char *find_backup_file_name (char const *, enum backup_type);
 enum backup_type get_version (char const *context, char const *arg);
-- 
2.7.4