FormulaInstaller: implement installation locks

FormulaInstaller now attempts to take a lock on a "foo.brewing" file for
the formula and all of its dependencies before attempting installation.

The lock is an advisory lock implemented using flock(), and as such it
only locks out other processes that attempt to take the lock. It also
means that it is never necessary to manually remove the lock file,
because the lock is not enforced by I/O.

The uninstall, link, and unlink commands all learn to respect this lock
as well, so that the installation cannot be corrupted by a concurrent
Homebrew process, and keg operations cannot occur simultaneously.

Library/Homebrew/cmd/link.rb

@@ -35,11 +35,13 @@ module Homebrew extend self
         next
       end
 
+      keg.lock do
         print "Linking #{keg}... " do
           puts "#{keg.link(mode)} symlinks created"
         end
       end
     end
+  end
 
   private
 

Library/Homebrew/cmd/uninstall.rb

@@ -7,11 +7,13 @@ module Homebrew extend self
 
     if not ARGV.force?
       ARGV.kegs.each do |keg|
+        keg.lock do
           puts "Uninstalling #{keg}..."
           keg.unlink
           keg.uninstall
           rm_opt_link keg.fname
         end
+      end
     else
       ARGV.named.each do |name|
         name = Formula.canonical_name(name)

Library/Homebrew/cmd/unlink.rb

@@ -3,8 +3,10 @@ module Homebrew extend self
     raise KegUnspecifiedError if ARGV.named.empty?
 
     ARGV.kegs.each do |keg|
+      keg.lock do
         print "Unlinking #{keg}... "
         puts "#{keg.unlink} links removed"
       end
     end
   end
+end

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

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

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
 
 

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