gpgsm: Use a cache to speed up parent certificate lookup.

* sm/gpgsm.h (COMPAT_NO_CHAIN_CACHE): New.
(struct cert_cache_item_s, cert_cache_item_t): New.
(struct server_control_s): Add parent_cert_cache.
* sm/gpgsm.c (compatibility_flags): Add "no-chain-cache".
(parent_cache_stats): New.
(gpgsm_exit): Print the stats with --debug=memstat.
(gpgsm_deinit_default_ctrl): Release the cache.
* sm/certchain.c (gpgsm_walk_cert_chain): Cache the certificates.
(do_validate_chain): Ditto.
--

This gives another boost of 30% (from 6.5 to 4.0 seconds in the test
environment with ~1000 certs).  do_validate_chain actually brings us
the speedup becuase the gpgsm_walk_cert_chain is not used during a key
listing.  For the latter we actually cache all certificates because
that was easier.

GnuPG-bug-id: 7308

Adjusted for 2.2:
- Add gpgsm_deinit_default_ctrl
- Remove ctrl arg from keydb_new

sm/gpgsm.h

@@ -183,11 +183,23 @@ struct
  *  policies: 1.3.6.1.4.1.7924.1.1:N:
  */
 #define COMPAT_ALLOW_KA_TO_ENCR   1
-
+/* Not actually a compatibiliy flag but useful to limit the
+ * required memory for a validated key listing.  */
+#define COMPAT_NO_CHAIN_CACHE     2
 
 /* Forward declaration for an object defined in server.c */
 struct server_local_s;
 
+/* On object used to keep a track of already known certificates.  */
+struct cert_cache_item_s
+{
+  struct cert_cache_item_s *next;
+  unsigned char fpr[20]; /* The certificate's fingerprint.  */
+  ksba_cert_t result;    /* The resulting certificate (ie. the issuer).  */
+};
+typedef struct cert_cache_item_s *cert_cache_item_t;
+
+
 /* Session control object.  This object is passed down to most
    functions.  Note that the default values for it are set by
    gpgsm_init_default_ctrl(). */
@@ -236,6 +248,9 @@ struct server_control_s
 
   /* The current time.  Used as a helper in certchain.c.  */
   ksba_isotime_t current_time;
+
+  /* The cache used to find the parent cert.  */
+  cert_cache_item_t parent_cert_cache;
 };
 
 
@@ -271,6 +286,7 @@ extern int gpgsm_errors_seen;
 
 void gpgsm_exit (int rc);
 void gpgsm_init_default_ctrl (struct server_control_s *ctrl);
+void gpgsm_deinit_default_ctrl (ctrl_t ctrl);
 int  gpgsm_parse_validation_model (const char *model);
 
 /*-- server.c --*/