brew: `brew upgrade` misbehaves when a formula's dependency is renamed

If gcc48 and gcc@4.8 both exist at the time update runs, then a migration will not be performed. So as we’ve folded the non-core taps into homebrew/core, every time the import PR into core was merged before the deletion PR in the original tap, anyone who updated in between will not have been migrated. In most cases, the import PR and deletion PR occurred nearly simultaneously, but there were cases where they were separated by hours or days. Also, anyone who did any ill-timed updates via git fetch and git reset, or via git pull, instead of via brew update, is potentially affected regardless of our timing of the merges.

So the poor man’s mitigation without changing any code, if you think you may have been impacted, is to run brew list | xargs -tn1 brew migrate and see if anything happens. In terms of code changes, we need to modify update so that it handles any migrations that were never attempted even though the target formulae already exist now.

The worst case scenario is that a user has installed both the source and target. This is entirely possible because if I had gcc48 installed, and it didn’t get migrated to gcc@4.8 for me, then I could have chosen to install gcc@4.8 at any time since then. I think we need a doctor check to detect this case, which manifests as both the left side and right side of a formula rename existing at the same time, with both left and right side being directories. When both exist, but the left side is a symlink, it was a normal migration. If the left side is a directory not a symlink, then it’s a double installation, or a failed migration, both of which would be problems doctor should report.

If/when https://github.com/Homebrew/brew/pull/2359 ships, it will also be possible for a double installation to simply be a migration that hasn’t been completed yet. Also, if that PR ships, double installations will be subject to migration attempts, possibly trying to move the exact same version on top of itself.

Ideally, we would also have something preventing double installations from ever happening by not allowing them to proceed, but currently there is nothing to prevent them regardless of whether the migration bug reported here gets fixed. Of course, they’ll be far less likely once this is fixed, but they will still currently be possible either way.

==> Renamed Formulae
antlr2 -> antlr@2                                  freetds091 -> freetds@0.91                         hdf4 -> hdf@4                                      phantomjs192 -> phantomjs@1.92
antlr3 -> antlr@3                                  gcc46 -> gcc@4.6                                   influxdb08 -> influxdb@0.8                         phantomjs198 -> phantomjs@1.98
bash-completion2 -> bash-completion@2              gcc47 -> gcc@4.7                                   jboss-as5 -> jboss-as@5                            play14 -> play@1.4
bazel02 -> bazel@0.2                               gcc5 -> gcc@5                                      jetty8 -> jetty@8                                  postgresql94 -> postgresql@9.4
berkeley-db4 -> berkeley-db@4                      giflib5 -> giflib@5                                jpeg6b -> jpeg@6                                   postgresql95 -> postgresql@9.5
bigdata -> blazegraph                              glfw2 -> glfw@2                                    jpeg9 -> jpeg@9                                    protobuf250 -> protobuf@2.5
bison27 -> bison@2.7                               gnupg21 -> gnupg@2.1                               juju2 -> juju                                      protobuf260 -> protobuf@2.6
boost-python159 -> boost-python@1.59               gnuplot4 -> gnuplot@4                              juju@2.0 -> juju                                   rebar3 -> rebar@3
boost155 -> boost@1.55                             go14 -> go@1.4                                     kafka080 -> kafka@0.80                             recipes -> gnome-recipes
boost157 -> boost@1.57                             go15 -> go@1.5                                     kibana41 -> kibana@4.1                             redis26 -> redis@2.6
boost159 -> boost@1.59                             go16 -> go@1.6                                     kibana44 -> kibana@4.4                             redis28 -> redis@2.8
boost160 -> boost@1.60                             gradle214 -> gradle@2.14                           kubernetes-cli13 -> kubernetes-cli@1.3             ruby187 -> ruby@1.8
cassandra21 -> cassandra@2.1                       grails25 -> grails@2.5                             lua51 -> lua@5.1 ✔                                 ruby193 -> ruby@1.9
cassandra22 -> cassandra@2.2                       gsl1 -> gsl@1                                      lua53 -> lua@5.3                                   ruby20 -> ruby@2.0
clang-format38 -> clang-format@3.8                 gst-ffmpeg010 -> gst-ffmpeg@0.10                   mariadb100 -> mariadb@10.0                         ruby21 -> ruby@2.1
docker111 -> docker@1.11                           gst-plugins-bad010 -> gst-plugins-bad@0.10         open-mpi16 -> open-mpi@1.6                         ruby22 -> ruby@2.2
docker171 -> docker@1.71                           gst-plugins-base010 -> gst-plugins-base@0.10       percona-server55 -> percona-server@5.5             ruby23 -> ruby@2.3
eigen32 -> eigen@3.2                               gst-plugins-good010 -> gst-plugins-good@0.10       percona-server56 -> percona-server@5.6             scala210 -> scala@2.10
erlang-r18 -> erlang@18                            gst-plugins-ugly010 -> gst-plugins-ugly@0.10       perl514 -> perl@5.14                               scala211 -> scala@2.11
ffmpeg28 -> ffmpeg@2.8                             gstreamer010 -> gstreamer@0.10                     perl518 -> perl@5.18

Note the gcc48 -> gcc@4.8 isn’t present in the update report.

Okay, I think I’ve got this:

brew update
git -C "$(brew --repository)/Library/Taps/homebrew/homebrew-core" reset --hard b366b398
git -C "$(brew --repository)/Library/Taps/homebrew/homebrew-versions" reset --hard 07b7501
HOMEBREW_NO_AUTO_UPDATE=1 brew install gcc48
brew upgrade