lib-storage: require MAIL_STORAGE_CLASS_FLAG_STUBS when caching

If we are going to be using the storage for caching, we should check that
the storage actually supports mail stubs.

src/lib-storage/index/dbox-single/sdbox-storage.c

@@ -405,7 +405,8 @@ struct mail_storage sdbox_storage = {
 	.class_flags = MAIL_STORAGE_CLASS_FLAG_FILE_PER_MSG |
 		MAIL_STORAGE_CLASS_FLAG_HAVE_MAIL_GUIDS |
 		MAIL_STORAGE_CLASS_FLAG_HAVE_MAIL_SAVE_GUIDS |
-		MAIL_STORAGE_CLASS_FLAG_BINARY_DATA,
+		MAIL_STORAGE_CLASS_FLAG_BINARY_DATA |
+		MAIL_STORAGE_CLASS_FLAG_STUBS,
 
 	.v = {
                 NULL,

src/lib-storage/mail-storage-private.h

@@ -81,7 +81,9 @@ enum mail_storage_class_flags {
 	MAIL_STORAGE_CLASS_FLAG_HAVE_MAIL_GUID128 = 0x200,
 	/* Storage deletes all files internally - mailbox list's
 	   delete_mailbox() shouldn't delete anything itself. */
-	MAIL_STORAGE_CLASS_FLAG_NO_LIST_DELETES	= 0x400
+	MAIL_STORAGE_CLASS_FLAG_NO_LIST_DELETES	= 0x400,
+	/* Storage supports stubs (used for caching purposes). */
+	MAIL_STORAGE_CLASS_FLAG_STUBS = 0x800,
 };
 
 struct mail_binary_cache {

src/lib-storage/mail-storage.c

@@ -1157,6 +1157,14 @@ static bool mailbox_try_undelete(struct mailbox *box)
 
 int mailbox_open(struct mailbox *box)
 {
+	/* check that the storage supports stubs if require them */
+	if (((box->flags & MAILBOX_FLAG_USE_STUBS) != 0) &&
+	    ((box->storage->storage_class->class_flags & MAIL_STORAGE_CLASS_FLAG_STUBS) == 0)) {
+		mail_storage_set_error(box->storage, MAIL_ERROR_NOTPOSSIBLE,
+				       "Mailbox does not support mail stubs");
+		return -1;
+	}
+
 	if (mailbox_open_full(box, NULL) < 0) {
 		if (!box->mailbox_deleted)
 			return -1;