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.
(nve_set): Add arg PK.  Change the caller.  Check whether to change at
all.
* 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

agent/findkey.c

@@ -117,6 +117,8 @@ 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
@@ -212,7 +214,7 @@ 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;
             }
@@ -232,6 +234,13 @@ 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.  */
   fname = fname_from_keygrip (grip, 1);
   fp = fname ? es_fopen (fname, "wbx,mode=-rw") : NULL;

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)
     {
@@ -476,21 +501,29 @@ nvc_set (nvc_t pk, const char *name, const char *value)
 
   e = nvc_lookup (pk, name);
   if (e)
-    return nve_set (e, value);
+    return nve_set (pk, e, value);
   else
     return nvc_add (pk, name, 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;
 
   if (!e)
     return GPG_ERR_INV_ARG;
 
+  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? value:"");
   if (!v)
     return my_error_from_syserror ();
@@ -501,6 +534,8 @@ 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;
 }
@@ -521,6 +556,7 @@ nvc_delete (nvc_t pk, nve_t entry)
     pk->last = entry->prev;
 
   nve_release (entry, pk->private_key_mode);
+  pk->modified = 1;
 }
 
 /* Delete the entries with NAME from PK.  */

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);
 
@@ -91,8 +94,8 @@ 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);