TSAN warnings: destroy of a locked mutex

Building with ThreadSanitizer and running tests shows this error:

SUMMARY: ThreadSanitizer: destroy of a locked mutex src/platform_linux/platform.c:153 in platform_mutex_destroy
==================
==================
WARNING: ThreadSanitizer: destroy of a locked mutex (pid=205274)
    #0 pthread_mutex_destroy ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1237 (libtsan.so.0+0x3725c)
    #1 platform_mutex_destroy src/platform_linux/platform.c:153 (libsplinterdb.so+0x4312e)
    #2 task_group_deinit src/task.c:346 (libsplinterdb.so+0x465e2)
    #3 task_system_destroy src/task.c:646 (libsplinterdb.so+0x4790f)
    #4 kvstore_close src/kvstore.c:321 (libsplinterdb.so+0x174e1)
    #5 kvstore_basic_close src/kvstore_basic.c:338 (libsplinterdb.so+0x9e461)
    #6 test_kvstore_iterator_with_startkey tests/functional/kvstore_basic_test.c:705 (driver_test+0x1a5bd)
    #7 kvstore_basic_test tests/functional/kvstore_basic_test.c:1226 (driver_test+0x1d7bb)
    #8 test_dispatcher tests/functional/test_dispatcher.c:49 (driver_test+0x45d71)
    #9 main tests/functional/driver_test.c:13 (driver_test+0x45765)

  and:
    #0 pthread_mutex_lock ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:4165 (libtsan.so.0+0x526fc)
    #1 platform_mutex_lock src/platform_linux/platform_inline.h:158 (libsplinterdb.so+0x451bc)
    #2 task_group_stop_and_wait_for_threads src/task.c:317 (libsplinterdb.so+0x463f7)
    #3 task_group_deinit src/task.c:342 (libsplinterdb.so+0x465a1)
    #4 task_system_destroy src/task.c:646 (libsplinterdb.so+0x4790f)
    #5 kvstore_close src/kvstore.c:321 (libsplinterdb.so+0x174e1)
    #6 kvstore_basic_close src/kvstore_basic.c:338 (libsplinterdb.so+0x9e461)
    #7 test_kvstore_iterator_with_startkey tests/functional/kvstore_basic_test.c:705 (driver_test+0x1a5bd)
    #8 kvstore_basic_test tests/functional/kvstore_basic_test.c:1226 (driver_test+0x1d7bb)
    #9 test_dispatcher tests/functional/test_dispatcher.c:49 (driver_test+0x45d71)
    #10 main tests/functional/driver_test.c:13 (driver_test+0x45765)

  Location is heap block of size 1250240 at 0x7f9951cce000 allocated by main thread:
    #0 aligned_alloc ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:799 (libtsan.so.0+0x300a8)
    #1 platform_aligned_malloc src/platform_linux/platform_inline.h:390 (libsplinterdb.so+0x45372)
    #2 platform_aligned_zalloc src/platform_linux/platform.h:599 (libsplinterdb.so+0x4546e)
    #3 task_system_create src/task.c:595 (libsplinterdb.so+0x47629)
    #4 kvstore_create_or_open src/kvstore.c:191 (libsplinterdb.so+0x16d55)
    #5 kvstore_create src/kvstore.c:284 (libsplinterdb.so+0x173af)
    #6 kvstore_basic_create_or_open src/kvstore_basic.c:309 (libsplinterdb.so+0x9e2d7)
    #7 kvstore_basic_create src/kvstore_basic.c:324 (libsplinterdb.so+0x9e3cb)
    #8 setup_kvstore_basic tests/functional/kvstore_basic_test.c:63 (driver_test+0x16226)
    #9 test_kvstore_iterator_with_startkey tests/functional/kvstore_basic_test.c:677 (driver_test+0x1a1e5)
    #10 kvstore_basic_test tests/functional/kvstore_basic_test.c:1226 (driver_test+0x1d7bb)
    #11 test_dispatcher tests/functional/test_dispatcher.c:49 (driver_test+0x45d71)
    #12 main tests/functional/driver_test.c:13 (driver_test+0x45765)

  Mutex M1346 (0x7f9951ccf528) created at:
    #0 pthread_mutex_init ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1220 (libtsan.so.0+0x4a616)
    #1 platform_mutex_init src/platform_linux/platform.c:143 (libsplinterdb.so+0x430ef)
    #2 task_group_init src/task.c:380 (libsplinterdb.so+0x467c9)
    #3 task_system_create src/task.c:613 (libsplinterdb.so+0x47768)
    #4 kvstore_create_or_open src/kvstore.c:191 (libsplinterdb.so+0x16d55)
    #5 kvstore_create src/kvstore.c:284 (libsplinterdb.so+0x173af)
    #6 kvstore_basic_create_or_open src/kvstore_basic.c:309 (libsplinterdb.so+0x9e2d7)
    #7 kvstore_basic_create src/kvstore_basic.c:324 (libsplinterdb.so+0x9e3cb)
    #8 setup_kvstore_basic tests/functional/kvstore_basic_test.c:63 (driver_test+0x16226)
    #9 test_kvstore_iterator_with_startkey tests/functional/kvstore_basic_test.c:677 (driver_test+0x1a1e5)
    #10 kvstore_basic_test tests/functional/kvstore_basic_test.c:1226 (driver_test+0x1d7bb)
    #11 test_dispatcher tests/functional/test_dispatcher.c:49 (driver_test+0x45d71)
    #12 main tests/functional/driver_test.c:13 (driver_test+0x45765)

Apparently that's undefined behavior

TSAN also complains about lots of data races, which are too numerous to list here.

Start with the TSAN trace in src/platform_linux/platform.c:153 and the shutdown path through src/task.c:317, 342, and 346. Reproduce the warning using the functional test path in tests/functional/kvstore_basic_test.c, especially test_kvstore_iterator_with_startkey. Done means the locked-mutex warning and the relevant reported task-system races no longer appear under ThreadSanitizer.