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.

1 files changed, 26 insertions, 0 deletions

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