--prefetch --dry-run intermittently hangs with a large number of dependencies - Windows

[enolan]

Sometimes, when I run stack build --only-snapshot --prefetch --dry-run Stack hangs after the last download message is printed. Process Explorer and Resource Monitor report there are no network connections open, stack.exe is using ~12% CPU which corresponds to pegging one virtual core on my machine, and the vast majority of the CPU time is spent in kernel. The active thread, according to Process Explorer, is in ntdll.dll!ZwDeviceIoControlFile+0xa most of the time.

To reproduce:

• Grab the repo at https://github.com/enolan/stack-download-hang
• Clear %APPDATA%\Roaming\stack\indices\Hackage and %APPDATA%\Roaming\stack\snapshots
• stack build --only-snapshot --prefetch --dry-run --verbose

Here's a log https://gist.github.com/enolan/25aad1613c42fee5fcbf

[mgsloan]

Wow, thanks for figuring out the GHC issue and writing a blogpost about it!! http://www.echonolan.net/posts/2016-05-10-a-debugging-horror-story.html

Hopefully there will be a GHC release soon with the fix. Once released, I wonder if we should only release the windows version compiled with that GHC.

[enolan]

@mgsloan The fix didn't make it into GHC 8.0 unfortunately. :(

I wrote a workaround in network but that has its own problem - another crash. I might have added it when doing the workaround, or it may have always been there. Here's the network PR if anyone wants to take a look: haskell/network#192. If you can tell me why it's still crashing I'd be happy.

[mgsloan]

Closing as an upstream issue