I have no objections to the patch, but please push into a development tree first.

As it turns out, the patch requires amendments.

First, it causes ERROR: wait_any failed when tests are run with testcase-timeout > 20. It is currently being fixed in https://github.com/MariaDB/server/pull/709#issuecomment-383030848.

Another, and more trickier problem, is a race condition / non-determinism in processing actual crashes.
It presents like this (for example, the test case from MDEV-15878 can be used to reproduce it):

worker[1] Using MTR_BUILD_THREAD 300, with reserved ports 16000..16019

CREATE TABLE t1 (f INT) ENGINE=InnoDB;

INSERT INTO t1 VALUES (1),(2);

ALTER TABLE t1 ORDER BY unknown_column;

ERROR 42S22: Unknown column 'unknown_column' in 'order clause'

CREATE TABLE t2 ENGINE=Aria SELECT * FROM t1;

SELECT * FROM t2;

worker[1] Trying to dump core for [mysqltest - pid: 2476, winpid: 2476, exit: 256]

worker[1] Trying to dump core for [mysqld.1 - pid: 2444, winpid: 2444, exit: 256]

worker[1] mysql-test-run: *** ERROR: Unhandled process [mysqltest - pid: 2476, winpid: 2476, exit: 256] exited

mysql-test-run: *** ERROR: Test suite aborted

Possible reason is that SRVDIED logic lies outside the foreach $proc (keys(%keep_waiting_proc)) loop. so whenever the process remaining in $proc is not the server process, the crash doesn't get handled properly.

The version of pull request #709 of Apr 25 (with commit 9f0d9012) seems to be fixing problems observed locally and in buildbot, verified in buildbot on bb-10.2-mtr tree; however, please note review comments by serg, some changes have been requested.

https://github.com/MariaDB/server/pull/709