lib-index: Add mail_index_unset_fscked()

This can be used to easily remove MAIL_INDEX_HDR_FLAG_FSCKD. It takes a
transaction parameter instead of sync_ctx because some index rebuilds
are done with a separate transaction while the sync_ctx is rolled back.

src/lib-index/mail-index-transaction-update.c

@@ -1307,6 +1307,22 @@ void mail_index_reset(struct mail_index_transaction *t)
 	t->reset = TRUE;
 }
 
+void mail_index_unset_fscked(struct mail_index_transaction *t)
+{
+	struct mail_index_header new_hdr =
+		*mail_index_get_header(t->view);
+
+	i_assert(t->view->index->log_sync_locked);
+
+	/* remove fsck'd-flag if it exists. */
+	if ((new_hdr.flags & MAIL_INDEX_HDR_FLAG_FSCKD) != 0) {
+		new_hdr.flags &= ~MAIL_INDEX_HDR_FLAG_FSCKD;
+		mail_index_update_header(t,
+			offsetof(struct mail_index_header, flags),
+			&new_hdr.flags, sizeof(new_hdr.flags), FALSE);
+	}
+}
+
 void mail_index_set_deleted(struct mail_index_transaction *t)
 {
 	i_assert(!t->index_undeleted);

src/lib-index/mail-index.h

@@ -523,6 +523,9 @@ void mail_index_update_highest_modseq(struct mail_index_transaction *t,
 /* Reset the index before committing this transaction. This is usually done
    only when UIDVALIDITY changes. */
 void mail_index_reset(struct mail_index_transaction *t);
+/* Remove MAIL_INDEX_HDR_FLAG_FSCKD from header if it exists. This must be
+   called only during syncing so that the mailbox is locked. */
+void mail_index_unset_fscked(struct mail_index_transaction *t);
 /* Mark index deleted. No further changes will be possible after the
    transaction has been committed. */
 void mail_index_set_deleted(struct mail_index_transaction *t);