DRAFT: mariabackup --backup fails with missing file

Scenario:
--------------
There is the ongoing RQG test "table_stress" with concurrent
– simple shaped INSERT/UPDATE/DELETE
– ALTER TABLE ADD/DROP PRIMARY KEY/INDEX/FULLTEXT INDEX/COLUMN, TRUNCATE,
rename column for TABLE test.t1 but no CREATE/DROP/RENAME that table
– KILL QUERY/SESSION
In parallel runs all 30 seconds the RQG Reporter 'Mariabackup' which performs a mariabackup the data of that server to some other data directory followed mariabackup prepare, start
second server on them + check data. If all is fine destroy second server + its data.
Summary over ~ 600 RQG tests (~ 540 passed, ~60 failed)
~ 1000 "mariabackup --backup" were tried
~ 1140 worked well and also all the follow up operations of the RQG reporter.
~ 60 failed with missing files named
./test/FTS_0000000000006c2e_00000000000093a2_INDEX_2.ibd
./test/FTS_0000000000003ddd_DELETED_CACHE.ibd
and similar starting with "FTS_"
./test/t1.ibd
./test/#sql-ib8205-2347778651.ibd and similar
followed by a crash of mariabackup
mysqld got signal 6
/work_m/bb-10.2-marko/bld_debug/extra/mariabackup /mariabackup(my_print_stacktrace+0x38)[0x563f7fb2b2a0]
mysys/stacktrace.c:267(my_print_stacktrace)[0x563f7f4616f7]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x11670)[0x7fe188a09670]
linux/raise.c:58(__GI_raise)[0x7fe18756477f]
stdlib/abort.c:91(__GI_abort)[0x7fe18756637a]
/work_m/bb-10.2-marko/bld_debug/extra/mariabackup/mariabackup(+0xc2bebe)[0x563f7f774ebe]
os/os0file.cc:5271(os_file_handle_error_cond_exit(char const*, char const*, bool, bool))[0x563f7f76efdb]
os/os0file.cc:737(os_file_handle_error(char const*, char const*))[0x563f7f7730f4]
os/os0file.cc:3004(os_file_create_func(char const*, unsigned long, unsigned long, unsigned long, bool, bool*))[0x563f7f6514d3]
include/os0file.ic:170(pfs_os_file_create_func(unsigned int, char const*, unsigned long, unsigned long, unsigned long, bool, bool*, char const*, unsigned int))[0x563f7f653d1f]
fil/fil0fil.cc:734(fil_node_open_file(fil_node_t*))[0x563f7f65740d]
fil/fil0fil.cc:1818(fil_space_open(char const*))[0x563f7f0428c9]
mariabackup/xtrabackup.cc:3025(xb_load_single_table_tablespace(char const*, char const*, bool))[0x563f7f042cc8]
mariabackup/xtrabackup.cc:3140(enumerate_ibd_files(void (char const*, char const*, bool)))[0x563f7f0432ec]
mariabackup/xtrabackup.cc:3306(xb_load_tablespaces())[0x563f7f045395]
mariabackup/xtrabackup.cc:4229(xtrabackup_backup_func())[0x563f7f04c157]
mariabackup/xtrabackup.cc:6252(main_low(char**))[0x563f7f04b9a7]
csu/libc-start.c:325(__libc_start_main)[0x7fe18754f3f1]
/work_m/bb-10.2-marko/bld_debug/extra/mariabackup/mariabackup(_start+0x2a)[0x563f7f03bdda]

I will add more information soon.