nostrdb: add ability to register a subscription callback

Since Damus iOS is not an immediate-mode UI like android, we would
rather not poll for results. Instead we need a way to register a
callback function that is called when we get new subscription results.

This is also useful on the android side, allowing us to request a new
frame to draw when we have new results, instead of drawing every second.

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/nostrdb.c

@@ -191,6 +191,8 @@ struct ndb_subscription {
 
 struct ndb_monitor {
 	struct ndb_subscription subscriptions[MAX_SUBSCRIPTIONS];
+	ndb_sub_fn sub_cb;
+	void *sub_cb_ctx;
 	int num_subscriptions;
 };
 
@@ -3738,6 +3740,7 @@ static void ndb_notify_subscriptions(struct ndb_monitor *monitor,
 				     struct written_note *wrote, int num_notes)
 {
 	int i, k;
+	int pushed;
 	struct written_note *written;
 	struct ndb_note *note;
 	struct ndb_subscription *sub;
@@ -3746,20 +3749,31 @@ static void ndb_notify_subscriptions(struct ndb_monitor *monitor,
 		sub = &monitor->subscriptions[i];
 		ndb_debug("checking subscription %d, %d notes\n", i, num_notes);
 
+		pushed = 0;
 		for (k = 0; k < num_notes; k++) {
 			written = &wrote[k];
 			note = written->note->note;
 
 			if (ndb_filter_group_matches(&sub->group, note)) {
 				ndb_debug("pushing note\n");
+
 				if (!prot_queue_push(&sub->inbox, &written->note_id)) {
 					ndb_debug("couldn't push note to subscriber");
+				} else {
+					pushed++;
 				}
 			} else {
 				ndb_debug("not pushing note\n");
 			}
 		}
 
+		// After pushing all of the matching notes, check to see if we
+		// have a registered subscription callback. If so, we call it.
+		// The callback needs to call ndb_poll_for_notes to pull data
+		// that was just pushed to the queue in the for loop above.
+		if (monitor->sub_cb != NULL && pushed > 0) {
+			monitor->sub_cb(monitor->sub_cb_ctx, sub->subid);
+		}
 	}
 }
 
@@ -4234,9 +4248,12 @@ static int ndb_run_migrations(struct ndb *ndb)
 	return 1;
 }
 
-static void ndb_monitor_init(struct ndb_monitor *monitor)
+static void ndb_monitor_init(struct ndb_monitor *monitor, ndb_sub_fn cb,
+			     void *sub_cb_ctx)
 {
-	memset(monitor, 0, sizeof(*monitor));
+	monitor->num_subscriptions = 0;
+	monitor->sub_cb = cb;
+	monitor->sub_cb_ctx = sub_cb_ctx;
 }
 
 void ndb_filter_group_destroy(struct ndb_filter_group *group)
@@ -4281,7 +4298,7 @@ int ndb_init(struct ndb **pndb, const char *filename, const struct ndb_config *c
 	if (!ndb_init_lmdb(filename, &ndb->lmdb, config->mapsize))
 		return 0;
 
-	ndb_monitor_init(&ndb->monitor);
+	ndb_monitor_init(&ndb->monitor, config->sub_cb, config->sub_cb_ctx);
 
 	if (!ndb_writer_init(&ndb->writer, &ndb->lmdb, &ndb->monitor)) {
 		fprintf(stderr, "ndb_writer_init failed\n");
@@ -5496,6 +5513,14 @@ void ndb_default_config(struct ndb_config *config)
 	config->flags = 0;
 	config->ingest_filter = NULL;
 	config->filter_context = NULL;
+	config->sub_cb_ctx = NULL;
+	config->sub_cb = NULL;
+}
+
+void ndb_config_set_subscription_callback(struct ndb_config *config, ndb_sub_fn fn, void *context)
+{
+	config->sub_cb_ctx = context;
+	config->sub_cb = fn;
 }
 
 void ndb_config_set_ingest_threads(struct ndb_config *config, int threads)

nostrdb/src/nostrdb.h

@@ -64,6 +64,9 @@ struct ndb_keypair {
 // function pointer for controlling what to do after we parse an id
 typedef enum ndb_idres (*ndb_id_fn)(void *, const char *);
 
+// callback function for when we receive new subscription results
+typedef void (*ndb_sub_fn)(void *, uint64_t subid);
+
 // id callback + closure data
 struct ndb_id_cb {
 	ndb_id_fn fn;
@@ -256,6 +259,8 @@ struct ndb_config {
 	size_t mapsize;
 	void *filter_context;
 	ndb_ingest_filter_fn ingest_filter;
+	void *sub_cb_ctx;
+	ndb_sub_fn sub_cb;
 };
 
 struct ndb_text_search_config {
@@ -430,6 +435,7 @@ void ndb_config_set_ingest_threads(struct ndb_config *config, int threads);
 void ndb_config_set_flags(struct ndb_config *config, int flags);
 void ndb_config_set_mapsize(struct ndb_config *config, size_t mapsize);
 void ndb_config_set_ingest_filter(struct ndb_config *config, ndb_ingest_filter_fn fn, void *);
+void ndb_config_set_subscription_callback(struct ndb_config *config, ndb_sub_fn fn, void *ctx);
 
 // HELPERS
 int ndb_calculate_id(struct ndb_note *note, unsigned char *buf, int buflen);