Use HeadVersion for install/reinstall

Library/Homebrew/download_strategy.rb

@@ -135,6 +135,8 @@ def fetch
       clone_repo
     end
 
+    version.update_commit(last_commit) if head?
+
     if @ref_type == :tag && @revision && current_revision
       unless current_revision == @revision
         raise <<-EOS.undent

Library/Homebrew/formula.rb

@@ -287,6 +287,14 @@ def version
     active_spec.version
   end
 
+  def update_head_version
+    return unless head?
+    return unless head.downloader.is_a?(VCSDownloadStrategy)
+    return unless head.downloader.cached_location.exist?
+
+    head.version.update_commit(head.downloader.last_commit)
+  end
+
   # The {PkgVersion} for this formula with {version} and {#revision} information.
   def pkg_version
     PkgVersion.new(version, revision)
@@ -405,11 +413,23 @@ def linked_keg
     Pathname.new("#{HOMEBREW_LIBRARY}/LinkedKegs/#{name}")
   end
 
+  def latest_head_prefix
+    head_versions = installed_prefixes.map do |pn|
+      pn_pkgversion = PkgVersion.parse(pn.basename.to_s)
+      pn_pkgversion if pn_pkgversion.head?
+    end.compact
+
+    latest_head_version = head_versions.max_by do |pn_pkgversion|
+      [Tab.for_keg(prefix(pn_pkgversion)).source_modified_time, pn_pkgversion.revision]
+    end
+    prefix(latest_head_version) if latest_head_version
+  end
+
   # The latest prefix for this formula. Checks for {#head}, then {#devel}
   # and then {#stable}'s {#prefix}
   # @private
   def installed_prefix
-    if head && (head_prefix = prefix(PkgVersion.new(head.version, revision))).directory?
+    if head && (head_prefix = latest_head_prefix) && head_prefix.directory?
       head_prefix
     elsif devel && (devel_prefix = prefix(PkgVersion.new(devel.version, revision))).directory?
       devel_prefix

Library/Homebrew/formula_installer.rb

@@ -586,13 +586,16 @@ def build
       end
     end
 
+    formula.update_head_version
+
     if !formula.prefix.directory? || Keg.new(formula.prefix).empty_installation?
       raise "Empty installation"
     end
 
   rescue Exception
     ignore_interrupts do
       # any exceptions must leave us with nothing installed
+      formula.update_head_version
       formula.prefix.rmtree if formula.prefix.directory?
       formula.rack.rmdir_if_possible
     end

Library/Homebrew/pkg_version.rb

@@ -5,6 +5,8 @@ class PkgVersion
 
   RX = /\A(.+?)(?:_(\d+))?\z/
 
+  attr_reader :version, :revision
+
   def self.parse(path)
     _, version, revision = *path.match(RX)
     version = Version.create(version)
@@ -38,8 +40,4 @@ def <=>(other)
   def hash
     version.hash ^ revision.hash
   end
-
-  protected
-
-  attr_reader :version, :revision
 end

Library/Homebrew/software_spec.rb

@@ -29,6 +29,7 @@ class SoftwareSpec
   def_delegators :@resource, :cached_download, :clear_cache
   def_delegators :@resource, :checksum, :mirrors, :specs, :using
   def_delegators :@resource, :version, :mirror, *Checksum::TYPES
+  def_delegators :@resource, :downloader
 
   def initialize
     @resource = Resource.new
@@ -203,7 +204,7 @@ def add_dep_option(dep)
 class HeadSoftwareSpec < SoftwareSpec
   def initialize
     super
-    @resource.version = Version.new("HEAD")
+    @resource.version = HeadVersion.new("HEAD")
   end
 
   def verify_download_integrity(_fn)