Bug #128619 for Future-AsyncAwait: Failures with async foreach loop on temporaries

Another case turns up in Device-AVR-UPDI: This fails: foreach my $byte ( split //, $data ) { printf STDERR "Byte is %v02X (%s)\n", $byte, $byte; $fh->print( $byte ); $buf = ""; $buf .= await Future::IO->sysread( $fh, 2 - length $buf ) while length $buf < 2; substr( $buf, 1, 1 ) eq "\x40" or croak "Expected ACK to STR data"; } But this works: my @bytes = split //, $data; # Placate RT128619 foreach my $byte ( @bytes ) { printf STDERR "Byte is %v02X (%s)\n", $byte, $byte; $fh->print( $byte ); $buf = ""; $buf .= await Future::IO->sysread( $fh, 2 - length $buf ) while length $buf < 2; substr( $buf, 1, 1 ) eq "\x40" or croak "Expected ACK to STR data"; } What's interesting in the failure case is that the value doesn't just go missing, but seems to be replaced by an entirely different value - a CODEref: Byte is 19 () Byte is 43.4F.44.45.28.30.78.35.35.62.63.35.33.64.34.33.64.61.38.29 (CODE(0x55bc53d43da8)) Future::AsyncAwait panic: TODO: Unsure how to handle savestack entry of 10 Aborted This one isn't a silent failure, it complains with an abort. Quite different from the previous test case. I suspect this might be a stack-refcount issue, where the iteration-list temporaries get stomped on by other code.

On Mon Apr 15 13:16:51 2019, PEVANS wrote: Show quoted text
By adding some more debug printing into the suspend/resume logic around CXt_LOOP_LIST, we can observe that when this suspends, we have our values on the stack as expected: F:AA: suspend LOOP_LIST basesp=0 ix=1 SP=16 item=0x55dfc6f62638:SV{type=PV,refcnt=2,PVX=""} item=0x55dfc6f6a7d8:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6bf0b88:SV{type=PV,refcnt=1,PVX="3"} item=0x55dfc6f6af40:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f62620:SV{type=PV,refcnt=1,PVX="2"} item=0x55dfc6f6aac0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f6a940:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6bf06c0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f92260:SV{type=PV,refcnt=1,PVX="n"} item=0x55dfc6d6b8a0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f6abf8:SV{type=PV,refcnt=1,PVX="/"} item=0x55dfc6f62518:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f92140:SV{type=PV,refcnt=1,PVX="+"} item=0x55dfc6d6b9d8:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f920f8:SV{type=PV,refcnt=1,PVX="-"} item=0x55dfc6ee8db8:SV{type=PV,refcnt=1,PVX="�"} F:AA suspend STACK len=16 item=0x55dfc6f62638:SV{type=PV,refcnt=2,PVX=""} item=0x55dfc6f6a7d8:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6bf0b88:SV{type=PV,refcnt=1,PVX="3"} item=0x55dfc6f6af40:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f62620:SV{type=PV,refcnt=1,PVX="2"} item=0x55dfc6f6aac0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f6a940:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6bf06c0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f92260:SV{type=PV,refcnt=1,PVX="n"} item=0x55dfc6d6b8a0:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f6abf8:SV{type=PV,refcnt=1,PVX="/"} item=0x55dfc6f62518:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f92140:SV{type=PV,refcnt=1,PVX="+"} item=0x55dfc6d6b9d8:SV{type=PV,refcnt=1,PVX="�"} item=0x55dfc6f920f8:SV{type=PV,refcnt=1,PVX="-"} item=0x55dfc6ee8db8:SV{type=PV,refcnt=1,PVX="�"} F:AA suspend MORTALS len=2 item=0x55dfc6f62698:SV{type=IV,refcnt=1,IV=2} item=0x55dfc6f6af70:SV{type=RV,refcnt=1,ROK} But yet, when we resume again later while we put them same SV addresses back in place, they don't appear to contain the right values any more: F:AA: resume LOOP_LIST basesp=0 ix=1 SP=16 item=0x55dfc6f62638:SV{type=PV,refcnt=1,PVX=""} item=0x55dfc6f6a7d8:SV{type=(255),refcnt=0} item=0x55dfc6bf0b88:SV{type=(255),refcnt=0} item=0x55dfc6f6af40:SV{type=(255),refcnt=0} item=0x55dfc6f62620:SV{type=(255),refcnt=0} item=0x55dfc6f6aac0:SV{type=(255),refcnt=0} item=0x55dfc6f6a940:SV{type=(255),refcnt=0} item=0x55dfc6bf06c0:SV{type=(255),refcnt=0} item=0x55dfc6f92260:SV{type=(255),refcnt=0} item=0x55dfc6d6b8a0:SV{type=(255),refcnt=0} item=0x55dfc6f6abf8:SV{type=(255),refcnt=0} item=0x55dfc6f62518:SV{type=(255),refcnt=0} item=0x55dfc6f92140:SV{type=(255),refcnt=0} item=0x55dfc6d6b9d8:SV{type=(255),refcnt=0} item=0x55dfc6f920f8:SV{type=(255),refcnt=0} item=0x55dfc6ee8db8:SV{type=(255),refcnt=0} Use of freed value in iteration at /home/leo/src/perl/Device-AVR-UPDI/blib/lib/Device/AVR/UPDI.pm line 209, <$fh> line 155. Something has cleared them out inbetween time. -- Paul Evans