DF-5033 guest build session (DragonFly 6.5-DEVELOPMENT, cc 8.3 [DragonFly] Release/2019-02-22)
Captured untrimmed from the ssh session; four binaries built.

$ cc -O2 -g -Wall -pthread -I inc -o idr_race idr_race.c        [first attempt]
idr_test_core.c:239:11: error: 'EAGAIN' undeclared (first use in this function)
... (errno.h added to harness_prelude.h; rebuilt)

$ cc -O2 -g -Wall -pthread -I inc -o idr_race idr_race.c
$ cc -O2 -g -Wall -pthread -DIDR_FIXED_BUILD -I inc -o idr_race_fixed idr_race.c
REBUILD_OK

$ cc -O2 -g -Wall -pthread -I inc -o oracle oracle.c
$ cc -O2 -g -Wall -pthread -DIDR_FIXED_BUILD -I inc -o oracle_fixed oracle.c
$ cc -O2 -g -Wall -pthread -I inc -o idr_race idr_race.c
$ cc -O2 -g -Wall -pthread -DIDR_FIXED_BUILD -I inc -o idr_race_fixed idr_race.c
ALL_REBUILT
(one harmless unused-variable note for 'id, allocated, r' in the IDR_FIXED_BUILD branch of oracle.c)

Generation step (host, python3):
$ python3 gen_core.py idr_test_core.c
wrote idr_test_core.c (485 lines; USERLAND_TEST block 38..129 replaced)
$ diff <(algorithm portion of sys/libkern/linux_idr.c) <(algorithm portion of idr_test_core.c)
ALGORITHM BYTE-IDENTICAL