agent: Update the key file only if not changed.

* common/name-value.c (struct name_value_container): Add flag
"modified".
(nvc_modified): New.
(nvc_new): Set flag.
(_nvc_add): Set flag.
(nvc_delete): Set flag.
(nvc_set): Set flag unless value did not change.
(nve_set): Add arg PK.  Change the caller.
* agent/findkey.c (agent_write_private_key): Update only if modified.
--

This helps software which uses a file system watcher to track changes
to private keys.  In particular smartcard triggered changes are a
problem for such software because this may at worst trigger another
smartcard read.

GnuPG-bug-id: 6829

1 files changed, 10 insertions, 1 deletions

diff --git a/agent/findkey.c b/agent/findkey.c
index a5f022574..9e5550225 100644
--- a/agent/findkey.c
+++ b/agent/findkey.c
@@ -146,6 +146,8 @@ agent_write_private_key (const unsigned char *grip,
         }
     }
 
+  nvc_modified (pk, 1);  /* Clear that flag after a read.  */
+
   if (!pk)
     {
       /* Key is still in the old format or does not exist - create a
@@ -242,7 +244,7 @@ agent_write_private_key (const unsigned char *grip,
             ; /* No need to update Token entry.  */
           else
             {
-              err = nve_set (item, token);
+              err = nve_set (pk, item, token);
               if (err)
                 goto leave;
             }
@@ -263,6 +265,13 @@ agent_write_private_key (const unsigned char *grip,
         goto leave;
     }
 
+  /* Check whether we need to write the file at all.  */
+  if (!nvc_modified (pk, 0))
+    {
+      err = 0;
+      goto leave;
+    }
+
   /* Create a temporary file for writing.  */
   tmpfname = fname_from_keygrip (grip, 1);
   fp = tmpfname ? es_fopen (tmpfname, "wbx,mode=-rw") : NULL;