quota: Fix negative quota_warnings with count backend

For example this never triggered:

  quota_warning = -messages=100%% quota-warning %u -100

The change to quota_alloc() to update the count_used doesn't seem to
actually fix anything right now, but it makes the code more correct.

src/plugins/quota/quota-private.h

@@ -135,7 +135,9 @@ struct quota_root {
 	/* don't enforce quota when saving */
 	unsigned int no_enforcing:1;
 	/* quota is automatically updated. update() should be called but the
-	   bytes/count won't be used. */
+	   bytes won't be changed. count is still changed, because it's cheap
+	   to do and it's internally used to figure out whether there have
+	   been some changes and that quota_warnings should be checked. */
 	unsigned int auto_updating:1;
 	/* If user has unlimited quota, disable quota tracking */
 	unsigned int disable_unlimited_tracking:1;

src/plugins/quota/quota-storage.c

@@ -398,8 +398,13 @@ static void quota_mailbox_sync_notify(struct mailbox *box, uint32_t uid,
 		qbox->expunge_qt->sync_transaction =
 			qbox->sync_transaction_expunge;
 	}
-	if (qbox->expunge_qt->auto_updating)
+	if (qbox->expunge_qt->auto_updating) {
+		/* even though backend doesn't care about size/count changes,
+		   make sure count_used changes so quota_warnings are
+		   executed */
+		quota_free_bytes(qbox->expunge_qt, 0);
 		return;
+	}
 
 	/* we're in the middle of syncing the mailbox, so it's a bad idea to
 	   try and get the message sizes at this point. Rely on sizes that

src/plugins/quota/quota.c

@@ -1331,10 +1331,10 @@ void quota_alloc(struct quota_transaction_context *ctx, struct mail *mail)
 {
 	uoff_t size;
 
-	if (ctx->auto_updating)
-		return;
-	if (mail_get_physical_size(mail, &size) == 0)
-		ctx->bytes_used += size;
+	if (!ctx->auto_updating) {
+		if (mail_get_physical_size(mail, &size) == 0)
+			ctx->bytes_used += size;
+	}
 
 	ctx->bytes_ceil = ctx->bytes_ceil2;
 	ctx->count_used++;