7 files changed, 79 insertions, 8 deletions
diff --git a/Library/Homebrew/cmd/link.rb b/Library/Homebrew/cmd/link.rb
index a85690ecb..6371c41de 100644
--- a/Library/Homebrew/cmd/link.rb
+++ b/Library/Homebrew/cmd/link.rb
@@ -35,8 +35,10 @@ module Homebrew extend self
         next
       end
 
-      print "Linking #{keg}... " do
-        puts "#{keg.link(mode)} symlinks created"
+      keg.lock do
+        print "Linking #{keg}... " do
+          puts "#{keg.link(mode)} symlinks created"
+        end
       end
     end
   end
diff --git a/Library/Homebrew/cmd/uninstall.rb b/Library/Homebrew/cmd/uninstall.rb
index d719be96a..19ad6eeb5 100644
--- a/Library/Homebrew/cmd/uninstall.rb
+++ b/Library/Homebrew/cmd/uninstall.rb
@@ -7,10 +7,12 @@ module Homebrew extend self
 
     if not ARGV.force?
       ARGV.kegs.each do |keg|
-        puts "Uninstalling #{keg}..."
-        keg.unlink
-        keg.uninstall
-        rm_opt_link keg.fname
+        keg.lock do
+          puts "Uninstalling #{keg}..."
+          keg.unlink
+          keg.uninstall
+          rm_opt_link keg.fname
+        end
       end
     else
       ARGV.named.each do |name|
diff --git a/Library/Homebrew/cmd/unlink.rb b/Library/Homebrew/cmd/unlink.rb
index 12b037781..37ef8f1da 100644
--- a/Library/Homebrew/cmd/unlink.rb
+++ b/Library/Homebrew/cmd/unlink.rb
@@ -3,8 +3,10 @@ module Homebrew extend self
     raise KegUnspecifiedError if ARGV.named.empty?
 
     ARGV.kegs.each do |keg|
-      print "Unlinking #{keg}... "
-      puts "#{keg.unlink} links removed"
+      keg.lock do
+        print "Unlinking #{keg}... "
+        puts "#{keg.unlink} links removed"
+      end
     end
   end
 end
diff --git a/Library/Homebrew/exceptions.rb b/Library/Homebrew/exceptions.rb
index ae2dd20c4..edd5cf1d1 100644
--- a/Library/Homebrew/exceptions.rb
+++ b/Library/Homebrew/exceptions.rb
@@ -45,6 +45,18 @@ class FormulaUnavailableError < RuntimeError
   end
 end
 
+class OperationInProgressError < RuntimeError
+  def initialize name
+    message = <<-EOS.undent
+      Operation already in progress for #{name}
+      Another active Homebrew process is already using #{name}.
+      Please wait for it to finish or terminate it to continue.
+      EOS
+
+    super message
+  end
+end
+
 module Homebrew
   class InstallationError < RuntimeError
     attr :formula
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 2e072afff..e2fde2ef9 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -228,6 +228,21 @@ class Formula
     end
   end
 
+  def lock
+    lockpath = HOMEBREW_CACHE_FORMULA/"#{@name}.brewing"
+    @lockfile = lockpath.open(File::RDWR | File::CREAT)
+    unless @lockfile.flock(File::LOCK_EX | File::LOCK_NB)
+      raise OperationInProgressError, @name
+    end
+  end
+
+  def unlock
+    unless @lockfile.nil?
+      @lockfile.flock(File::LOCK_UN)
+      @lockfile.close
+    end
+  end
+
   def == b
     name == b.name
   end
diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index 373440ba8..b24641e55 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -22,6 +22,7 @@ class FormulaInstaller
 
     @@attempted ||= Set.new
 
+    lock
     check_install_sanity
   end
 
@@ -226,6 +227,8 @@ class FormulaInstaller
     print "#{f.prefix}: #{f.prefix.abv}"
     print ", built in #{pretty_duration build_time}" if build_time
     puts
+
+    unlock if hold_locks?
   end
 
   def build_time
@@ -463,6 +466,29 @@ class FormulaInstaller
     check_jars
     check_non_libraries
   end
+
+  private
+
+  def hold_locks?
+    @hold_locks || false
+  end
+
+  def lock
+    if (@@locked ||= []).empty?
+      f.recursive_deps.each { |d| @@locked << d } unless ignore_deps
+      @@locked.unshift(f)
+      @@locked.each(&:lock)
+      @hold_locks = true
+    end
+  end
+
+  def unlock
+    if hold_locks?
+      @@locked.each(&:unlock)
+      @@locked.clear
+      @hold_locks = false
+    end
+  end
 end
 
 
diff --git a/Library/Homebrew/keg.rb b/Library/Homebrew/keg.rb
index 3a4d01262..4949d24dc 100644
--- a/Library/Homebrew/keg.rb
+++ b/Library/Homebrew/keg.rb
@@ -61,6 +61,18 @@ class Keg < Pathname
     parent.basename.to_s
   end
 
+  def lock
+    path = HOMEBREW_CACHE_FORMULA/"#{fname}.brewing"
+    file = path.open(File::RDWR | File::CREAT)
+    unless file.flock(File::LOCK_EX | File::LOCK_NB)
+      raise OperationInProgressError, fname
+    end
+    yield
+  ensure
+    file.flock(File::LOCK_UN)
+    file.close
+  end
+
   def linked_keg_record
     @linked_keg_record ||= HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname
   end