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.

3 files changed, 14 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