Make usage of ObserverPathnameExtension more obvious

Remove use of globals.

Closes Homebrew/homebrew#21795.

Library/Homebrew/cmd/prune.rb

@@ -2,12 +2,9 @@ require 'keg'
 require 'cmd/tap'
 
 module Homebrew extend self
-  # $n and $d are used by the ObserverPathnameExtension to keep track of
-  # certain filesystem actions.
-
   def prune
-    $n = 0
-    $d = 0
+    ObserverPathnameExtension.reset_counts!
+
     dirs = []
 
     Keg::PRUNEABLE_DIRECTORIES.select(&:directory?).each do |dir|
@@ -41,11 +38,13 @@ module Homebrew extend self
 
     repair_taps unless ARGV.dry_run?
 
-    if $n == 0 && $d == 0
+    n, d = ObserverPathnameExtension.counts
+
+    if ObserverPathnameExtension.total.zero?
       puts "Nothing pruned" if ARGV.verbose?
     else
-      print "Pruned #{$n} symbolic links "
-      print "and #{$d} directories " if $d > 0
+      print "Pruned #{n} symbolic links "
+      print "and #{d} directories " if d > 0
       puts  "from #{HOMEBREW_PREFIX}"
     end unless ARGV.dry_run?
   end

Library/Homebrew/extend/pathname.rb

@@ -433,21 +433,36 @@ class Pathname
   end
 end
 
-# sets $n and $d so you can observe creation of stuff
 module ObserverPathnameExtension
+  class << self
+    attr_accessor :n, :d
+
+    def reset_counts!
+      @n = @d = 0
+    end
+
+    def total
+      n + d
+    end
+
+    def counts
+      [n, d]
+    end
+  end
+
   def unlink
     super
     puts "rm #{to_s}" if ARGV.verbose?
-    $n+=1
+    ObserverPathnameExtension.n += 1
   end
   def rmdir
     super
     puts "rmdir #{to_s}" if ARGV.verbose?
-    $d+=1
+    ObserverPathnameExtension.d += 1
   end
   def make_relative_symlink src
     super
-    $n+=1
+    ObserverPathnameExtension.n += 1
   end
   def install_info
     super
@@ -458,6 +473,3 @@ module ObserverPathnameExtension
     puts "uninfo #{to_s}" if ARGV.verbose?
   end
 end
-
-$n=0
-$d=0

Library/Homebrew/keg.rb

@@ -33,9 +33,7 @@ class Keg < Pathname
   end
 
   def unlink
-    # these are used by the ObserverPathnameExtension to count the number
-    # of files and directories linked
-    $n=$d=0
+    ObserverPathnameExtension.reset_counts!
 
     dirs = []
 
@@ -62,7 +60,7 @@ class Keg < Pathname
 
     dirs.reverse_each(&:rmdir_if_possible)
 
-    $n+$d
+    ObserverPathnameExtension.total
   end
 
   def fname
@@ -108,8 +106,7 @@ class Keg < Pathname
   def link mode=OpenStruct.new
     raise "Cannot link #{fname}\nAnother version is already linked: #{linked_keg_record.realpath}" if linked_keg_record.directory?
 
-    $n=0
-    $d=0
+    ObserverPathnameExtension.reset_counts!
 
     share_mkpaths = %w[aclocal doc info locale man]
     share_mkpaths.concat((1..8).map { |i| "man/man#{i}" })
@@ -175,7 +172,7 @@ class Keg < Pathname
       optlink
     end
 
-    return $n + $d
+    ObserverPathnameExtension.total
   rescue Exception
     opoo "Could not link #{fname}. Unlinking..."
     unlink