update-report: migrate /usr/local repository.

Migrate a HOMEBREW_REPOSITORY and HOMEBREW_PREFIX of `/usr/local` so
that the HOMEBREW_REPOSITORY is now in `/usr/local/Homebrew`. This gives
us more flexibility on changing the repository layout in future and
avoids putting junk in `/usr/local` when not necessary (e.g. our
`README.md`).

Library/Homebrew/cmd/update-report.rb

@@ -98,6 +98,9 @@ module Homebrew
     Tap.each(&:link_manpages)
 
     Homebrew.failed = true if ENV["HOMEBREW_UPDATE_FAILED"]
+
+    # This should always be the last thing to run
+    migrate_legacy_repository_if_necessary
   end
 
   private
@@ -169,16 +172,125 @@ module Homebrew
     end
   end
 
-  def link_completions_and_docs
-    return if HOMEBREW_PREFIX.to_s == HOMEBREW_REPOSITORY.to_s
+  def migrate_legacy_repository_if_necessary
+    return unless HOMEBREW_PREFIX.to_s == "/usr/local"
+    return unless HOMEBREW_REPOSITORY.to_s == "/usr/local"
+    return unless ARGV.homebrew_developer?
+
+    ohai "Migrating HOMEBREW_REPOSITORY (please wait)..."
+
+    unless HOMEBREW_PREFIX.writable_real?
+      ofail <<-EOS.undent
+        #{HOMEBREW_PREFIX} is not writable.
+
+        You should change the ownership and permissions of #{HOMEBREW_PREFIX}
+        temporarily back to your user account so we can complete the Homebrew
+        repository migration:
+          sudo chown -R $(whoami) #{HOMEBREW_PREFIX}
+      EOS
+      return
+    end
+
+    Keg::ALL_TOP_LEVEL_DIRECTORIES.each do |dir|
+      FileUtils.mkdir_p "#{HOMEBREW_PREFIX}/#{dir}"
+    end
+
+    new_homebrew_repository = Pathname.new "/usr/local/Homebrew"
+    if new_homebrew_repository.exist?
+      ofail <<-EOS.undent
+        #{new_homebrew_repository} already exists.
+        Please remove it manually or uninstall and reinstall Homebrew into a new
+        location as the migration cannot be done automatically.
+      EOS
+      return
+    end
+    new_homebrew_repository.mkpath
+
+    repo_files = HOMEBREW_REPOSITORY.cd do
+      Utils.popen_read("git ls-files").lines.map(&:chomp)
+    end
+
+    unless Utils.popen_read("git status --untracked-files=all --porcelain").empty?
+      HOMEBREW_REPOSITORY.cd do
+        safe_system "git", "-c", "user.email=brew-update@localhost",
+                           "-c", "user.name=brew update",
+                           "stash", "save", "--include-untracked"
+      end
+      stashed = true
+    end
+
+    FileUtils.cp_r "#{HOMEBREW_REPOSITORY}/.git", "#{new_homebrew_repository}/.git"
+    new_homebrew_repository.cd do
+      safe_system "git", "checkout", "--force", "."
+      safe_system "git", "stash", "pop" if stashed
+    end
+
+    if (HOMEBREW_REPOSITORY/"Library/Locks").exist?
+      FileUtils.cp_r "#{HOMEBREW_REPOSITORY}/Library/Locks", "#{new_homebrew_repository}/Library/Locks"
+    end
+
+    if (HOMEBREW_REPOSITORY/"Library/Taps").exist?
+      FileUtils.cp_r "#{HOMEBREW_REPOSITORY}/Library/Taps", "#{new_homebrew_repository}/Library/Taps"
+    end
+
+    unremovable_paths = []
+    extra_remove_paths = [".git", "Library/Locks", "Library/Taps",
+                          "Library/Homebrew/test"]
+    (repo_files + extra_remove_paths).each do |file|
+      path = Pathname.new "#{HOMEBREW_REPOSITORY}/#{file}"
+      begin
+        FileUtils.rm_rf path
+      rescue Errno::EACCES
+        unremovable_paths << path
+      end
+      quiet_system "rmdir", "-p", path.parent if path.parent.exist?
+    end
+
+    unless unremovable_paths.empty?
+      ofail <<-EOS.undent
+        Could not remove old HOMEBREW_REPOSITORY paths!
+        Please do this manually with:
+          sudo rm -rf #{unremovable_paths.join " "}
+      EOS
+    end
+
+    src = Pathname.new("#{new_homebrew_repository}/bin/brew")
+    dst = Pathname.new("#{HOMEBREW_PREFIX}/bin/brew")
+    begin
+      FileUtils.ln_s(src.relative_path_from(dst.parent), dst)
+    rescue Errno::EACCES
+      ofail <<-EOS.undent
+        Could not create symlink at #{dst}!
+        Please do this manually with:
+          sudo ln -sf #{src} #{dst}
+          sudo chown $(whoami) #{dst}
+      EOS
+    end
+
+    link_completions_and_docs(new_homebrew_repository)
+
+    ohai "Migrated HOMEBREW_REPOSITORY to #{new_homebrew_repository}!"
+    puts "You may now chown #{HOMEBREW_PREFIX} back to root:wheel."
+  rescue => e
+    ofail <<-EOS.undent
+      #{e}
+      #{Tty.white}Failed to migrate HOMEBREW_REPOSITORY to #{new_homebrew_repository}!
+      Please comment with your (new) error in issue:
+        #{Tty.em}https://github.com/Homebrew/brew/issues/987#{Tty.reset}
+    EOS
+    $stderr.puts e.backtrace
+  end
+
+  def link_completions_and_docs(repository = HOMEBREW_REPOSITORY)
+    return if HOMEBREW_PREFIX.to_s == repository.to_s
     command = "brew update"
-    link_src_dst_dirs(HOMEBREW_REPOSITORY/"etc/bash_completion.d",
+    link_src_dst_dirs(repository/"etc/bash_completion.d",
                       HOMEBREW_PREFIX/"etc/bash_completion.d", command)
-    link_src_dst_dirs(HOMEBREW_REPOSITORY/"share/doc/homebrew",
+    link_src_dst_dirs(repository/"share/doc/homebrew",
                       HOMEBREW_PREFIX/"share/doc/homebrew", command, link_dir: true)
-    link_src_dst_dirs(HOMEBREW_REPOSITORY/"share/zsh/site-functions",
+    link_src_dst_dirs(repository/"share/zsh/site-functions",
                       HOMEBREW_PREFIX/"share/zsh/site-functions", command)
-    link_path_manpages(HOMEBREW_REPOSITORY/"share", command)
+    link_path_manpages(repository/"share", command)
   end
 end