In the Evergreen patch build in preparation for the vendoring of wiredtiger contents to mongodb, all sub tests of the dbtest task failed with segmentation fault accessing invalid address while downgrading wiredtiger datafiles as part of engine shutdown, the clean up stage of those tests. 

[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I - [testsuite] **************************************************
[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I - [testsuite] basic | tests: 12 | fails: 0 | assert calls: 0 | time secs: 5.000
[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I - [testsuite] TOTALS | tests: 12 | fails: 0 | assert calls: 0 | time secs: 5.000
[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I - [testsuite] SUCCESS - All tests in all suites passed
[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] Deregistering all the catalog entries and collection objects
[db_test:basic] 2019-05-27T04:18:33.408+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] Timestamp monitor shutting down
[db_test:basic] 2019-05-27T04:18:33.409+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] WiredTigerKVEngine shutting down
[db_test:basic] 2019-05-27T04:18:33.409+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] Shutting down session sweeper thread
[db_test:basic] 2019-05-27T04:18:33.409+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] Finished shutting down session sweeper thread
[db_test:basic] 2019-05-27T04:18:33.409+0000 2019-05-27T04:18:33.408+0000 I STORAGE [testsuite] Downgrading WiredTiger datafiles.
[db_test:basic] 2019-05-27T04:18:33.414+0000 2019-05-27T04:18:33.413+0000 F - [testsuite] Invalid access at address: 0x190
[db_test:basic] 2019-05-27T04:18:33.425+0000 2019-05-27T04:18:33.425+0000 F - [testsuite] Got signal: 11 (Segmentation fault).
[db_test:basic] 2019-05-27T04:18:33.425+0000 0x7f872fb4feb1 0x7f872fb4f6ae 0x7f872fb4f88c 0x7f872b6277e0 0x7f872e5194da 0x7f872e45268e 0x7f872e447b13 0x7f872e412dc0 0x7f872e7ee412 0x7f872fb4b766 0x7f872de03d19 0x7f872e222206 0x7f872debad9b 0x7f872de0e3b9 0x7f872b2a2d20 0x7f872de8b211
[db_test:basic] 2019-05-27T04:18:33.425+0000 ----- BEGIN BACKTRACE -----
[db_test:basic] 2019-05-27T04:18:33.425+0000 {"backtrace":[
 
(...)
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(_ZN5mongo15printStackTraceERSo+0x41) [0x7f872fb4feb1]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(+0x2B776AE) [0x7f872fb4f6ae]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(+0x2B7788C) [0x7f872fb4f88c]
[db_test:basic] 2019-05-27T04:18:33.425+0000 libpthread.so.0(+0xF7E0) [0x7f872b6277e0]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(__wt_las_config+0x5A) [0x7f872e5194da]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(__wt_conn_reconfig+0x2BE) [0x7f872e45268e]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(+0x146FB13) [0x7f872e447b13]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(_ZN5mongo18WiredTigerKVEngine13cleanShutdownEv+0x6F0) [0x7f872e412dc0]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(_ZN5mongo34shutdownGlobalStorageEngineCleanlyEPNS_14ServiceContextE+0x22) [0x7f872e7ee412]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(+0x2B73766) [0x7f872fb4b766]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(_ZN5mongo8shutdownENS_8ExitCodeERKNS_16ShutdownTaskArgsE+0x504) [0x7f872de03d19]
[db_test:basic] 2019-05-27T04:18:33.425+0000 dbtest(_ZN5mongo7dbtests10runDbTestsEiPPc+0x226) [0x7f872e222206]
[db_test:basic] 2019-05-27T04:18:33.426+0000 dbtest(Z11dbtestsMainiPPcS0+0x4DB) [0x7f872debad9b]
[db_test:basic] 2019-05-27T04:18:33.426+0000 dbtest(main+0x9) [0x7f872de0e3b9]
[db_test:basic] 2019-05-27T04:18:33.426+0000 libc.so.6(__libc_start_main+0x100) [0x7f872b2a2d20]
[db_test:basic] 2019-05-27T04:18:33.426+0000 dbtest(+0xEB3211) [0x7f872de8b211]
[db_test:basic] 2019-05-27T04:18:33.426+0000 ----- END BACKTRACE -----