imap: Allow plugins to hook into syncing.

Ideally all of the existing pieces in the syncing code would start using
this at some point, so their code could be moved to a more logical location.

src/imap/imap-client.c

@@ -610,6 +610,12 @@ client_default_send_tagline(struct client_command_context *cmd, const char *data
 	client->last_output = ioloop_time;
 }
 
+static int
+client_default_sync_notify_more(struct imap_sync_context *ctx ATTR_UNUSED)
+{
+	return 1;
+}
+
 void client_send_command_error(struct client_command_context *cmd,
 			       const char *msg)
 {
@@ -1432,4 +1438,5 @@ struct imap_client_vfuncs imap_client_vfuncs = {
 	imap_state_import_base,
 	client_default_destroy,
 	client_default_send_tagline,
+	client_default_sync_notify_more,
 };

src/imap/imap-client.h

@@ -133,6 +133,10 @@ struct imap_client_vfuncs {
 
 	void (*send_tagline)(struct client_command_context *cmd,
 			     const char *data);
+	/* Run "mailbox syncing". This can send any unsolicited untagged
+	   replies. Returns 1 = done, 0 = wait for more space in output buffer,
+	   -1 = failed. */
+	int (*sync_notify_more)(struct imap_sync_context *ctx);
 };
 
 struct client {

src/imap/imap-sync-private.h

@@ -25,6 +25,9 @@ struct imap_sync_context {
 
 	unsigned int messages_count;
 
+	/* Module-specific contexts. */
+	ARRAY(union imap_module_context *) module_contexts;
+
 	bool failed:1;
 	bool finished:1;
 	bool no_newmail:1;

src/imap/imap-sync.c

@@ -192,6 +192,7 @@ imap_sync_init(struct client *client, struct mailbox *box,
 	ctx->client = client;
 	ctx->box = box;
 	ctx->imap_flags = imap_flags;
+	i_array_init(&ctx->module_contexts, 5);
 
 	/* make sure user can't DoS the system by causing Dovecot to create
 	   tons of useless namespaces. */
@@ -341,6 +342,9 @@ static int imap_sync_notify_more(struct imap_sync_context *ctx)
 	   now it contains added/removed messages. */
 	if ((ret = imap_sync_send_search_updates(ctx, FALSE)) < 0)
 		ctx->failed = TRUE;
+
+	if (ret > 0)
+		ret = ctx->client->v.sync_notify_more(ctx);
 	return ret;
 }
 
@@ -362,6 +366,7 @@ int imap_sync_deinit(struct imap_sync_context *ctx,
 	}
 
 	array_free(&ctx->tmp_keywords);
+	array_free(&ctx->module_contexts);
 	i_free(ctx);
 	return ret;
 }