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, 41 insertions, 6 deletions

diff --git a/common/name-value.c b/common/name-value.c
index 0dffc63b4..b7bc32844 100644
--- a/common/name-value.c
+++ b/common/name-value.c
@@ -48,6 +48,7 @@ struct name_value_container
   struct name_value_entry *first;
   struct name_value_entry *last;
   unsigned int private_key_mode:1;
+  unsigned int modified:1;
 };
 
 
@@ -87,11 +88,15 @@ my_error (gpg_err_code_t ec)
 
 /* Allocation and deallocation.  */
 
-/* Allocate a private key container structure.  */
+/* Allocate a name value container structure.  */
 nvc_t
 nvc_new (void)
 {
-  return xtrycalloc (1, sizeof (struct name_value_container));
+  nvc_t nvc;
+  nvc = xtrycalloc (1, sizeof (struct name_value_container));
+  if (nvc)
+    nvc->modified = 1;
+  return nvc;
 }
 
 
@@ -142,6 +147,24 @@ nvc_release (nvc_t pk)
   xfree (pk);
 }
 
+
+/* Return the modified-flag of the container and clear it if CLEAR is
+ * set.  That flag is set for a new container and set with each
+ * update.  */
+int
+nvc_modified (nvc_t pk, int clear)
+{
+  int modified;
+
+  if (!pk)
+    return 0;
+  modified = pk->modified;
+  if (clear)
+    pk->modified = 0;
+  return modified;
+}
+
+
 
 
 /* Dealing with names and values.  */
@@ -427,6 +450,8 @@ _nvc_add (nvc_t pk, char *name, char *value, strlist_t raw_value,
   else
     pk->first = pk->last = e;
 
+  pk->modified = 1;
+
  leave:
   if (err)
     {
@@ -470,6 +495,7 @@ gpg_error_t
 nvc_set (nvc_t pk, const char *name, const char *value)
 {
   nve_t e;
+  char *v;
 
   if (! valid_name (name))
     return GPG_ERR_INV_NAME;
@@ -477,7 +503,12 @@ nvc_set (nvc_t pk, const char *name, const char *value)
   e = nvc_lookup (pk, name);
   if (e)
     {
-      char *v;
+      if (e->value && value && !strcmp (e->value, value))
+        {
+          /* Setting same value - ignore this call and don't set the
+           * modified flag.  */
+          return 0;
+        }
 
       v = xtrystrdup (value);
       if (v == NULL)
@@ -489,7 +520,7 @@ nvc_set (nvc_t pk, const char *name, const char *value)
 	wipememory (e->value, strlen (e->value));
       xfree (e->value);
       e->value = v;
-
+      pk->modified = 1;
       return 0;
     }
   else
@@ -497,9 +528,10 @@ nvc_set (nvc_t pk, const char *name, const char *value)
 }
 
 
-/* Update entry E to VALUE.  */
+/* Update entry E to VALUE.  PK is optional; if given its modified
+ * flag will be updated.  */
 gpg_error_t
-nve_set (nve_t e, const char *value)
+nve_set (nvc_t pk, nve_t e, const char *value)
 {
   char *v;
 
@@ -516,6 +548,8 @@ nve_set (nve_t e, const char *value)
     wipememory (e->value, strlen (e->value));
   xfree (e->value);
   e->value = v;
+  if (pk)
+    pk->modified = 1;
 
   return 0;
 }
@@ -536,6 +570,7 @@ nvc_delete (nvc_t pk, nve_t entry)
     pk->last = entry->prev;
 
   nve_release (entry, pk->private_key_mode);
+  pk->modified = 1;
 }