"pgbench -i -s 50; pgbench -S -j2 -c16 -T900 -P5" freezes consistently on
Cygwin 2.2.1 and Cygwin 2.6.0.  (I suspect most other versions are affected.)
I've pinged[1] the Cygwin bug thread with some additional detail.  If a Cygwin
buildfarm member starts using --enable-tap-tests, you may see failures in the
pgbench test suite.  (lorikeet used --enable-tap-tests from 2017-03-18 to
2017-03-20, but it failed before reaching the pgbench test suite.)  Curious
that "make check" has too little concurrency to see more effects from this.

Frozen backends show a stack trace like this:

#0  0x000000007710139a in ntdll!ZwWriteFile () from 
#1  0x000007fefd851b2b in WriteFile () from 
#2  0x0000000076fb3576 in WriteFile () from 
#3  0x0000000180160c6c in transport_layer_pipes::write (this=<optimized out>, 
buf=<optimized out>, len=<optimized out>)
    at /usr/src/debug/cygwin-2.6.0-1/winsup/cygserver/transport_pipes.cc:224
#4  0x000000018015feb6 in client_request::send (this=0xffffa930, 
conn=0x6000e8290) at 
#5  0x0000000180160591 in client_request::make_request 
(this=this@entry=0xffffa930) at 
#6  0x0000000180114f79 in semop (semid=65540, sops=0xffffaa00, nsops=1) at 
#7  0x0000000180117a4b in _sigfe () at sigfe.s:35
#8  0x000000010063c81a in PGSemaphoreLock (sema=sema@entry=0x6ffffe06a18) at 
#9  0x00000001006a962b in LWLockAcquire (lock=lock@entry=0x6fff6774d80, 
mode=mode@entry=LW_SHARED) at lwlock.c:1286
#10 0x0000000100687d46 in BufferAlloc (foundPtr=0xffffab0b <incomplete sequence 
\367\377\006>, strategy=0x0, blockNum=290, forkNum=MAIN_FORKNUM, 
    relpersistence=112 'p', smgr=0x6000ea588) at bufmgr.c:1012

The postmaster, also frozen, shows a stack trace like this:

#0  0x00000000771018ca in ntdll!ZwWaitForMultipleObjects () from 
#1  0x000007fefd851420 in KERNELBASE!GetCurrentProcess () from 
#2  0x0000000076fa1220 in WaitForMultipleObjects () from 
#3  0x0000000180120173 in child_info::sync (this=this@entry=0xffffc008, 
pid=4692, hProcess=@0xffffc1b0: 0x4b8, howlong=howlong@entry=300000)
    at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/sigproc.cc:1010
#4  0x00000001800aa163 in frok::parent (this=0xffffc000, stack_here=0xffffbfa0 
"") at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/fork.cc:501
#5  0x00000001800aaa05 in fork () at 
#6  0x0000000180117a4b in _sigfe () at sigfe.s:35
#7  0x0000000100641618 in fork_process () at fork_process.c:61
#8  0x000000010063e80a in StartAutoVacWorker () at autovacuum.c:1436

The postmaster log eventually has:

     28 [main] postgres 4408 child_info::sync: wait failed, pid 4692, Win32 
error 183
    292 [main] postgres 4408 fork: child 4692 - died waiting for dll loading, 
errno 11

[1] https://cygwin.com/ml/cygwin/2017-03/msg00218.html

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