implement pinning of taps

Library/Homebrew/cmd/tap-pin.rb

@@ -0,0 +1,13 @@
+require "cmd/tap"
+
+module Homebrew
+  def tap_pin
+    taps = ARGV.named.map do |name|
+      Tap.new(*tap_args(name))
+    end
+    taps.each do |tap|
+      tap.pin
+      ohai "Pinned #{tap.name}"
+    end
+  end
+end

Library/Homebrew/cmd/tap-unpin.rb

@@ -0,0 +1,13 @@
+require "cmd/tap"
+
+module Homebrew
+  def tap_unpin
+    taps = ARGV.named.map do |name|
+      Tap.new(*tap_args(name))
+    end
+    taps.each do |tap|
+      tap.unpin
+      ohai "Unpinned #{tap.name}"
+    end
+  end
+end

Library/Homebrew/cmd/untap.rb

@@ -10,6 +10,8 @@ module Homebrew
       raise TapUnavailableError, tap.name unless tap.installed?
       puts "Untapping #{tap}... (#{tap.path.abv})"
 
+      tap.unpin if tap.pinned?
+
       formula_count = tap.formula_files.size
       tap.path.rmtree
       tap.path.dirname.rmdir_if_possible

Library/Homebrew/exceptions.rb

@@ -98,6 +98,17 @@ class TapUnavailableError < RuntimeError
   end
 end
 
+class TapPinStatusError < RuntimeError
+  attr_reader :name, :pinned
+
+  def initialize name, pinned
+    @name = name
+    @pinned = pinned
+
+    super pinned ? "#{name} is already pinned." : "#{name} is already unpinned."
+  end
+end
+
 class OperationInProgressError < RuntimeError
   def initialize(name)
     message = <<-EOS.undent

Library/Homebrew/tap.rb

@@ -99,6 +99,27 @@ class Tap
     Pathname.glob("#{path}/cmd/brew-*").select(&:executable?)
   end
 
+  def pinned_symlink_path
+    HOMEBREW_LIBRARY/"PinnedTaps/#{@name}"
+  end
+
+  def pinned?
+    @pinned ||= pinned_symlink_path.directory?
+  end
+
+  def pin
+    raise TapUnavailableError, name unless installed?
+    raise TapPinStatusError.new(name, true) if pinned?
+    pinned_symlink_path.make_relative_symlink(@path)
+  end
+
+  def unpin
+    raise TapUnavailableError, name unless installed?
+    raise TapPinStatusError.new(name, false) unless pinned?
+    pinned_symlink_path.delete
+    pinned_symlink_path.dirname.rmdir_if_possible
+  end
+
   def to_hash
     {
       "name" => @name,
@@ -111,7 +132,8 @@ class Tap
       "custom_remote" => custom_remote?,
       "formula_names" => formula_names,
       "formula_files" => formula_files.map(&:to_s),
-      "command_files" => command_files.map(&:to_s)
+      "command_files" => command_files.map(&:to_s),
+      "pinned" => pinned?
     }
   end