build: store source modified time in tabfile

Library/Homebrew/build.rb

@@ -130,7 +130,7 @@ class Build
         formula.install
 
         stdlibs = detect_stdlibs(ENV.compiler)
-        Tab.create(formula, ENV.compiler, stdlibs.first, formula.build).write
+        Tab.create(formula, ENV.compiler, stdlibs.first, formula.build, formula.source_modified_time).write
 
         # Find and link metafiles
         formula.prefix.install_metafiles Pathname.pwd

Library/Homebrew/formula.rb

@@ -78,7 +78,6 @@ class Formula
   # This contains all the attributes (e.g. URL, checksum) that apply to the
   # stable version of this formula.
   # @private
-
   attr_reader :stable
 
   # The development {SoftwareSpec} for this {Formula}.
@@ -108,6 +107,10 @@ class Formula
   # @private
   attr_reader :active_spec_sym
 
+  # most recent modified time for source files
+  # @private
+  attr_reader :source_modified_time
+
   # Used for creating new Homebrew versions of software without new upstream
   # versions.
   # @see .revision
@@ -1550,6 +1553,7 @@ class Formula
 
   def stage
     active_spec.stage do
+      @source_modified_time = active_spec.source_modified_time
       @buildpath = Pathname.pwd
       env_home = buildpath/".brew_home"
       mkdir_p env_home

Library/Homebrew/resource.rb

@@ -8,7 +8,7 @@ require "version"
 class Resource
   include FileUtils
 
-  attr_reader :mirrors, :specs, :using
+  attr_reader :mirrors, :specs, :using, :source_modified_time
   attr_writer :version
   attr_accessor :download_strategy, :checksum
 
@@ -87,6 +87,7 @@ class Resource
   def unpack(target = nil)
     mktemp(download_name) do
       downloader.stage
+      @source_modified_time = downloader.source_modified_time
       if block_given?
         yield self
       elsif target

Library/Homebrew/software_spec.rb

@@ -25,7 +25,7 @@ class SoftwareSpec
   attr_reader :bottle_specification
   attr_reader :compiler_failures
 
-  def_delegators :@resource, :stage, :fetch, :verify_download_integrity
+  def_delegators :@resource, :stage, :fetch, :verify_download_integrity, :source_modified_time
   def_delegators :@resource, :cached_download, :clear_cache
   def_delegators :@resource, :checksum, :mirrors, :specs, :using
   def_delegators :@resource, :version, :mirror, *Checksum::TYPES