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.

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