[GHC] #11124: GHC does not shadow -package-name/-this-package-key

[GHC]

#11124: GHC does not shadow -package-name/-this-package-key
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Package        |           Version:  7.11
  system                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Steps to reproduce:

 1. Install a package named `a` with these contents:

    **a.cabal**
 {{{
 name:                a
 version:             0.1.0.0
 cabal-version:       >=1.10

 library
   exposed-modules:     A
   build-depends:       base
 }}}

    **A.hs**
 {{{
 module A where
 data A = A String
 }}}

    Note what unit ID/package key/package name the package installs as. On
 GHC 7.8 it will be something like `a-0.1.0.0`, on GHC 7.10 it will be
 something like `a_LKCPrTJwOTOLk4OU37YmeN`, on GHC 8.0 it will be something
 like `a-0.1.0.0-LKCPrTJwOTOLk4OU37Yme`.

 2. Install a package named `b` with these contents:

    **b.cabal**
 {{{
 name:                b
 version:             0.1.0.0
 cabal-version:       >=1.10

 library
   exposed-modules:     B
   build-depends:       base, a
 }}}

    **b.hs**
 {{{
 module B where
 import A
 b = A "foo"
 }}}

 3. Go back to `a`, and load `A.hs` in GHCi with `ghci A.hs -package-name
 a-0.1.0.0 -package-db ../db`. Notice that we can `import B` from this GHCi
 session:

 {{{
 ezyang at sabre:~/Dev/labs/ii/a$ ghci A.hs -package-name a-0.1.0.0 -package-
 db ../db
 GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling A                ( A.hs, interpreted )
 Ok, modules loaded: A.
 *A> import B
 *A B> b
 Loading package a-0.1.0.0 ... linking ... done.
 Loading package b-0.1.0.0 ... linking ... done.
 A "asdfsdf"
 *A B>
 Leaving GHCi.
 }}}

 3. Edit `A.hs` so that now contains:

 {{{
 module A where
 data A = A Int
 }}}

 4. Load in GHCi again:

 {{{
 ezyang at sabre:~/Dev/labs/ii/a$ ghci A.hs -package-name a-0.1.0.0 -package-
 db ../db
 GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling A                ( A.hs, interpreted )
 Ok, modules loaded: A.
 *A> import B
 *A B> b
 Loading package a-0.1.0.0 ... linking ... done.
 Loading package b-0.1.0.0 ... linking ... done.
 A 3458765062984598387
 }}}

 Disaster!

 Actually, this bug is relatively harmless:

 1. If you try to actually reinstall package a, that's a "dangerous"
 reinstall and it's known that reinstalls can break things.
 2. It's hard to convince Cabal to trigger this, since importing `B`
 requires `b` in the build-depends, and that would constitute a circular
 package dependency. (You can't separate it out because of (1))
 3. If you don't specify `-package-name` (or similar) the local `A` and the
 remote `A` will have different identities, no problem.
 4. When you do specify `-package-name`, you can't build a main executable
 (so the only way to run is by loading into GHCi, as shown above.)
 5. Sometimes, GHCi will notice that something is amiss if it tries to load
 the object file for both the remote A and the local A.

 But the fix seems simple: if a user specifies `-package-name`, it should
 be AS IF they also specified `-ignore-package`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11124>
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