[GHC] #8591: Concurrent executions of ghc-pkg can cause inconstant package.cache files

[GHC]

#8591: Concurrent executions of ghc-pkg can cause inconstant package.cache files
-------------------------------------+-------------------------------------
        Reporter:  janm              |                   Owner:  pgj
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Package system    |                 Version:  7.6.3
      Resolution:                    |                Keywords:  ghc-pkg
Operating System:  Unknown/Multiple  |  race
 Type of failure:  Other             |            Architecture:
      Blocked By:                    |  Unknown/Multiple
 Related Tickets:  #10205            |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by thomie):

 * os:  FreeBSD => Unknown/Multiple
 * related:   => #10205


Comment:

 janm: are you installing the packages with cabal or manually?

 The GHC build system (in ghc.mk) contains the following comment:
 {{{
 # register the boot packages in strict sequence, because running
 # multiple ghc-pkgs in parallel doesn't work (registrations may get
 # lost).
 }}}

 Presumably `cabal -j` also registers the packages sequentially, or there'd
 be many more reported issues.

 Changing just `withFileAtomic` as you propose in comment:2 and comment:5
 wouldn't solve the problem. Two concurrent ghc-pkg processes still
 wouldn't know about each other's modifications to the package database, so
 the last one to call `withFileAtomic` would still win.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8591#comment:11>
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