Install etc/var files on postinstall.

Also, don't delete them after that. This means that `brew postinstall`
becomes a way to easily reinstall configuration files for any formula
without needing any changes to any bottles or requiring a reinstall.

Library/Homebrew/cmd/postinstall.rb

@@ -7,7 +7,10 @@ module Homebrew
   module_function
 
   def postinstall
-    ARGV.resolved_formulae.each { |f| run_post_install(f) if f.post_install_defined? }
+    ARGV.resolved_formulae.each do |f|
+      ohai "Postinstalling #{f}"
+      run_post_install(f)
+    end
   end
 
   def run_post_install(formula)

Library/Homebrew/formula.rb

@@ -1004,11 +1004,6 @@ class Formula
   # Can be overridden to run commands on both source and bottle installation.
   def post_install; end
 
-  # @private
-  def post_install_defined?
-    method(:post_install).owner == self.class
-  end
-
   # @private
   def run_post_install
     @prefix_returns_versioned_prefix = true
@@ -1025,6 +1020,11 @@ class Formula
 
     ENV.clear_sensitive_environment!
 
+    Pathname.glob("#{bottle_prefix}/{etc,var}/**/*") do |path|
+      path.extend(InstallRenamed)
+      path.cp_path_sub(bottle_prefix, HOMEBREW_PREFIX)
+    end
+
     with_logging("post_install") do
       post_install
     end

Library/Homebrew/formula_installer.rb

@@ -564,14 +564,12 @@ class FormulaInstaller
       fix_dynamic_linkage(keg)
     end
 
-    if formula.post_install_defined?
     if build_bottle?
       ohai "Not running post_install as we're building a bottle"
       puts "You can run it manually using `brew postinstall #{formula.full_name}`"
     else
       post_install
     end
-    end
 
     caveats
 
@@ -832,12 +830,6 @@ class FormulaInstaller
     skip_linkage = formula.bottle_specification.skip_relocation?
     keg.replace_placeholders_with_locations tab.changed_files, skip_linkage: skip_linkage
 
-    Pathname.glob("#{formula.bottle_prefix}/{etc,var}/**/*") do |path|
-      path.extend(InstallRenamed)
-      path.cp_path_sub(formula.bottle_prefix, HOMEBREW_PREFIX)
-    end
-    FileUtils.rm_rf formula.bottle_prefix
-
     tab = Tab.for_keg(keg)
 
     CxxStdlib.check_compatibility(

Library/Homebrew/test/formula_spec.rb

@@ -10,7 +10,6 @@ RSpec::Matchers.alias_matcher :supersede_an_installed_formula, :be_supersedes_an
 RSpec::Matchers.alias_matcher :have_changed_alias, :be_alias_changed
 
 RSpec::Matchers.alias_matcher :have_option_defined, :be_option_defined
-RSpec::Matchers.alias_matcher :have_post_install_defined, :be_post_install_defined
 RSpec::Matchers.alias_matcher :have_test_defined, :be_test_defined
 RSpec::Matchers.alias_matcher :pour_bottle, :be_pour_bottle
 
@@ -624,23 +623,6 @@ describe Formula do
     expect(f.desc).to eq("a formula")
   end
 
-  specify "#post_install_defined?" do
-    f1 = formula do
-      url "foo-1.0"
-
-      def post_install
-        # do nothing
-      end
-    end
-
-    f2 = formula do
-      url "foo-1.0"
-    end
-
-    expect(f1).to have_post_install_defined
-    expect(f2).not_to have_post_install_defined
-  end
-
   specify "#test_defined?" do
     f1 = formula do
       url "foo-1.0"