Hackage security lock mechanism does not always release

[mgsloan]

Looking for .cabal or package.yaml files to use to init the project.
Using cabal packages:
- ./

Selecting the best among 12 snapshots...

Didn't see Yampa-0.10.7@sha256:4a7d94ebce33e9863b14e637cdb99fad313345dbe08a4d142a649ba45a40c38d,5009 in your package indices.
Updating and trying again.
Selected mirror https://s3.amazonaws.com/hackage.fpcomplete.com/                                 
Downloading timestamp                                                                            
Exception /home/mgsloan/.stack/indices/Hackage/hackage-security-lock: createDirectory: already exists (File exists) when using mirror https://s3.amazonaws.com/hackage.fpcomplete.com/
Selected mirror http://hackage.fpcomplete.com/                                                   
Downloading timestamp                                                                            
Exception /home/mgsloan/.stack/indices/Hackage/hackage-security-lock: createDirectory: already exists (File exists) when using mirror http://hackage.fpcomplete.com/
Selected mirror http://objects-us-west-1.dream.io/hackage-mirror/                                
Downloading timestamp                                                                            
Verification error: <repo>/timestamp.json is expired                                             
Downloading root                                                                                 
Unable to load cabal files for snapshot                                                          
                                                                                                 
----                                                                                             
Deleting cached snapshot file: /home/mgsloan/.stack/build-plan/lts-10.0.yaml                     
Recommendation: try running again. If this fails again, open an upstream issue at:               
https://github.com/fpco/lts-haskell/issues/new                                                   
----                                                                                             
                                                                                                 
Updating package index Hackage (mirrored at https://s3.amazonaws.com/hackage.fpcomplete.com/) .../home/mgsloan/.stack/indices/Hackage/hackage-security-lock: createDirectory: already exists (File exists)

I got this log after interrupting stack in the middle of fetching the hackage index. Pretty sure that I only used one Ctrl-C. I may have used two and so caused even the exception handlers to not run. Even in this case, ideally it would get unlocked. Couldn't there be a better mechanism here, perhaps based on exclusive write access to the index file?

[snoyberg]

This should be fixed by #203.