[perl #134235] MacOS X intermittent threads/t/blocks.t failure

# New Ticket Created by  Sergey Aleynikov 
# Please include the string:  [perl #134235]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=134235 >


This is a bug report for perl from sergey.aleynikov@gmail.com,
generated with the help of perlbug 1.41 running under perl 5.31.2.


-----------------------------------------------------------------
[Please describe your issue here]

While running ../dist/threads/t/blocks.t with env PERLIO=stdio set, i
rarely (less than 5% of total runs) get a test failure. It seems to
not happen without PERLIO set, though with such bugs you're never
sure.

../dist/threads/t/blocks.t ..
1..5
ok 1 - Loaded
ok 2 - Main thread
ok 3 - BEGIN thread
ok 4 - Main join
Failed 1/5 subtests

Test Summary Report
-------------------
../dist/threads/t/blocks.t (Wstat: 13 Tests: 4 Failed: 0)
  Non-zero wait status: 13
  Parse errors: Bad plan.  You planned 5 tests but ran 4.
Files=1, Tests=4,  0 wallclock secs ( 0.00 usr  0.01 sys +  0.05 cusr
0.01 csys =  0.07 CPU)
Result: FAIL

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=medium
---
Site configuration information for perl 5.31.2:

Configured by dur-randir at Thu Jun 27 08:46:43 MSK 2019.

Summary of my perl5 (revision 5 version 31 subversion 2) configuration:
  Commit id: 1bed9104d475eb126ab7d1df518bb894f66e1fab
  Platform:
    osname=darwin
    osvers=18.6.0
    archname=darwin-thread-multi-2level
    uname='darwin m-mojave.local 18.6.0 darwin kernel version 18.6.0:
thu apr 25 23:16:27 pdt 2019; root:xnu-4903.261.4~2release_x86_64
x86_64 '
    config_args='-de -Dusedevel -Duseithreads -Accflags=-DPERL_POISON'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.14
-DPERL_POISON -fno-strict-aliasing -pipe -fstack-protector-strong
-I/opt/local/include -DPERL_USE_SAFE_PUTENV'
    optimize='-O3'
    cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.14
-DPERL_POISON -fno-strict-aliasing -pipe -fstack-protector-strong
-I/opt/local/include'
    ccversion=''
    gccversion='4.2.1 Compatible Apple LLVM 10.0.1 (clang-1001.0.46.4)'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -mmacosx-version-min=10.14 -fstack-protector-strong
-L/opt/local/lib'
    libpth=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.1/lib
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib
/opt/local/lib /usr/lib
    libs=-lpthread -lgdbm -ldbm -ldl -lm -lutil -lc
    perllibs=-lpthread -ldl -lm -lutil -lc
    libc=
    so=dylib
    useshrplib=false
    libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=bundle
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags=' -mmacosx-version-min=10.14 -bundle -undefined
dynamic_lookup -L/opt/local/lib -fstack-protector-strong'


---
@INC for perl 5.31.2:
    lib
    /usr/local/lib/perl5/site_perl/5.31.2/darwin-thread-multi-2level
    /usr/local/lib/perl5/site_perl/5.31.2
    /usr/local/lib/perl5/5.31.2/darwin-thread-multi-2level
    /usr/local/lib/perl5/5.31.2

---
Environment for perl 5.31.2:
    DYLD_LIBRARY_PATH (unset)
    HOME=/Users/dur-randir
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LC_CTYPE=en_US.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/Users/dur-randir/perlbrew/bin:/Users/dur-randir/perlbrew/perls/perl-blead/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/local/bin
    PERLBREW_HOME=/Users/dur-randir/.perlbrew
    PERLBREW_MANPATH=/Users/dur-randir/perlbrew/perls/perl-blead/man
    PERLBREW_PATH=/Users/dur-randir/perlbrew/bin:/Users/dur-randir/perlbrew/perls/perl-blead/bin
    PERLBREW_PERL=perl-blead
    PERLBREW_ROOT=/Users/dur-randir/perlbrew
    PERLBREW_SHELLRC_VERSION=0.86
    PERLBREW_VERSION=0.86
    PERL_BADLANG (unset)
    SHELL=/opt/local/bin/zsh

• [perl #134235] MacOS X intermittent threads/t/blocks.t failure by Sergey Aleynikov