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, 5 insertions, 2 deletions

diff --git a/common/name-value.h b/common/name-value.h
index b3fc2f63c..dfded6678 100644
--- a/common/name-value.h
+++ b/common/name-value.h
@@ -50,6 +50,9 @@ nvc_t nvc_new_private_key (void);
 /* Release a name value container structure.  */
 void nvc_release (nvc_t pk);
 
+/* Return the modified flag and optionally clear it.  */
+int nvc_modified (nvc_t pk, int clear);
+
 /* Get the name.  */
 char *nve_name (nve_t pke);
 
@@ -92,8 +95,8 @@ gpg_error_t nvc_add (nvc_t pk, const char *name, const char *value);
    first entry is updated.  */
 gpg_error_t nvc_set (nvc_t pk, const char *name, const char *value);
 
-/* Update entry E to VALUE.  */
-gpg_error_t nve_set (nve_t e, const char *value);
+/* Update entry E to VALUE.  PK is optional. */
+gpg_error_t nve_set (nvc_t pk, nve_t e, const char *value);
 
 /* Delete the given entry from PK.  */
 void nvc_delete (nvc_t pk, nve_t pke);