Stop supporting `brew install --force'

Rationale: it breaks stuff. You should not be able to install over something already installed *and* linked. brew now prompts you to unlink the existing linked keg first.

I came to fix this because I discovered that `brew install foo` works without --force provided foo's formula version is different to that which is installed and linked. I'm not sure if this was intentionally broken, but it led to at least two tickets I found where people were getting crazy issues trying to install over the top of already installed previous versions.

So I also fixed a whole category of other issues, mostly by moving error handling into FormulaInstaller. Now error's can be caught in Homebrew.install and handled on a formula-by-formula basis. I will next port this behaviour to upgrade.

Library/Homebrew/cmd/install.rb

@@ -11,10 +11,6 @@ module Homebrew extend self
       raise "No available formula for #{name}\n#{msg}" if msg
     end unless ARGV.force?
 
-    ARGV.formulae.each do |f|
-      opoo "#{f} already installed" if f.linked_keg.directory?
-    end unless ARGV.force?
-
     if Process.uid.zero? and not File.stat(HOMEBREW_BREW_FILE).uid.zero?
       # note we only abort if Homebrew is *not* installed as sudo and the user
       # calls brew as root. The fix is to chown brew to root.
@@ -84,29 +80,16 @@ module Homebrew extend self
     unless formulae.empty?
       perform_preinstall_checks
       formulae.each do |f|
-        # Check formula status and skip if necessary---a formula passed on the
-        # command line may have been installed to satisfy a dependency.
-        next if f.installed? unless ARGV.force?
-
-        # Building head-only without --HEAD is an error
-        if not ARGV.build_head? and f.standard.nil?
-          raise "This is a head-only formula; install with `brew install --HEAD #{f.name}`"
-        end
-
-        # Building stable-only with --HEAD is an error
-        if ARGV.build_head? and f.unstable.nil?
-           raise "No head is defined for #{f.name}"
-        end
-
         begin
           fi = FormulaInstaller.new(f)
           fi.install
           fi.caveats
           fi.finish
-        rescue FormulaAlreadyInstalledError => e
-          opoo e.message
+        rescue CannotInstallFormulaError => e
+          onoe e.message
         end
       end
     end
   end
+
 end

Library/Homebrew/exceptions.rb

@@ -42,10 +42,7 @@ module Homebrew
   end
 end
 
-class FormulaAlreadyInstalledError < Homebrew::InstallationError
-  def message
-    "Formula already installed: #{formula}"
-  end
+class CannotInstallFormulaError < RuntimeError
 end
 
 class FormulaInstallationAlreadyAttemptedError < Homebrew::InstallationError

Library/Homebrew/formula_installer.rb

@@ -16,10 +16,40 @@ class FormulaInstaller
     @show_header = true
     @ignore_deps = ARGV.include? '--ignore-dependencies' || ARGV.interactive?
     @install_bottle = !ARGV.build_from_source? && ff.bottle_up_to_date?
+
+    check_install_sanity
+  end
+
+  def check_install_sanity
+    if f.installed?
+      raise CannotInstallFormulaError, "#{f}-#{f.version} already installed"
+    end
+
+    # Building head-only without --HEAD is an error
+    if not ARGV.build_head? and f.standard.nil?
+      raise CannotInstallFormulaError, <<-EOS.undent
+        #{f} is a head-only formula
+        Install with `brew install --HEAD #{f.name}
+      EOS
+    end
+
+    # Building stable-only with --HEAD is an error
+    if ARGV.build_head? and f.unstable.nil?
+      raise CannotInstallFormulaError, "No head is defined for #{f.name}"
+    end
   end
 
   def install
-    raise FormulaAlreadyInstalledError, f if f.installed? and not ARGV.force?
+    # not in initialize so upgrade can unlink the active keg before calling this
+    # function but after instantiating this class so that it can avoid having to
+    # relink the active keg if possible (because it is slow).
+    if f.linked_keg.directory?
+      # some other version is already installed *and* linked
+      raise CannotInstallFormulaError, <<-EOS.undent
+        #{f}-#{f.linked_keg.realpath.basename} already installed
+        To install this version, first `brew unlink #{f}'
+      EOS
+    end
 
     unless ignore_deps
       f.check_external_deps