Earlier today I reported on the cygwin-patches list a fork problem with a 
cygwin1.dll built from the master branch of the Cygwin source repo.  See 
https://cygwin.com/ml/cygwin-patches/2019-q2/msg00155.html.  A bisection showed 
that commit f03ea8e1 was the first bad commit for the problem being discussed there.

I've just run into a second fork problem for which I think the same commit is 
the first bad one.  I can't be positive, because the problem is sporadic, so I 
might have marked some bad commits as good if I didn't run the test enough times.

The test case for this (attached) is one that I used when testing my new FIFO 
code.  Normally I run this program in one terminal and type

   echo blah > /tmp/myfifo

in a second terminal.  For the present purposes, however, you can skip the 
second part and simply terminate fifo_fork_test with C-c.

In my testing, I found that running the test program would yield "read: Bad 
address" about 1 out of 10 times.  Occasionally I would get "read: Communication 
error on send" instead.  Both error messages indicate a problem with the child 
process reading from an fd inherited from the parent.

Ken