Support content addressable gem builds / gem naming in PackageTask

Adds Gem::PackageTask#content_addressable. When true, the gem task builds
a content addressable gem for the single Ruby ABI identified by
gem_spec.required_ruby_version, passing content_addressable through to
Gem::Package.build.

A content addressable gem's file name contains a hash of its contents, so
it isn't known until the gem is built and can't be a Rake target. In this
mode the task uses a stamp file (pkg/<full_name>-<ruby abi>.gem-built) as
its Rake target, writes the built gem's path into it, and rebuilds when
the referenced gem no longer exists. The staging directory is suffixed
with the Ruby ABI (pkg/<full_name>-<ruby abi>) so per-ABI builds of the
same full_name don't share a directory.

When content_addressable is false, behaviour is unchanged.

Author: OughtPuts

lib/rubygems/content_address.rb

@@ -90,6 +90,25 @@ def self.eligible?(spec, validate_ruby_abi: true)
     !ruby_abi_for(spec.required_ruby_version).nil?
   end
 
+  ##
+  # Raises ArgumentError unless +spec+ is eligible for content addressing
+  # (see eligible?). The message explains which requirement is unmet so
+  # gem authors know what to change in their gemspec.
+
+  def self.eligible!(spec)
+    unless platform_eligible?(spec.platform)
+      raise ArgumentError,
+        "Cannot build a content-addressable gem as no platform or a Ruby platform has been set"
+    end
+
+    return unless ruby_abi_for(spec.required_ruby_version).nil?
+
+    raise ArgumentError,
+      "Cannot build a content-addressable gem because required_ruby_version is set to " \
+      "#{spec.required_ruby_version}. Please set required_ruby_version to \"~> X.Y.0\" " \
+      "so that it identifies a single Ruby ABI."
+  end
+
   ##
   # Whether +spec+ is content-addressed: it is eligible for content
   # addressing and has a valid content address set. See eligible? for

lib/rubygems/package.rb

@@ -385,7 +385,7 @@ def build(skip_validation = false, strict_validation = false)
   # +required_rubygems_version+ is constrained before building.
 
   def build_content_addressable_file(skip_validation = false, strict_validation = false)
-    validate_content_addressable_spec
+    Gem::ContentAddress.eligible!(@spec)
     ruby_abi = Gem::ContentAddress.ruby_abi_for(@spec.required_ruby_version)
     @spec.required_rubygems_version = normalized_required_rubygems_version(ruby_abi)
 
@@ -788,23 +788,6 @@ def satisfies_rubygems_floor?(requirement, floor)
     end
   end
 
-  ##
-  # Validates that the spec can be built as a content-addressable gem: it
-  # must declare a non-Ruby platform and its +required_ruby_version+ must
-  # identify a single Ruby ABI.
-
-  def validate_content_addressable_spec
-    return if Gem::ContentAddress.eligible?(@spec)
-
-    unless Gem::ContentAddress.platform_eligible?(@spec.platform)
-      raise ArgumentError, "Cannot build a gem scoped to a single Ruby ABI as no platform or a Ruby platform has been set"
-    end
-
-    raise ArgumentError,
-      "Cannot build a content-addressable gem because required_ruby_version is set to #{@spec.required_ruby_version}. " \
-      "Please set required_ruby_version to \"~> X.Y.0\" so that it identifies a single Ruby ABI."
-  end
-
   ##
   # Returns the full path for installing +filename+ into +destination_dir+,
   # which must already be resolved with File.realpath by the caller.

lib/rubygems/package_task.rb

@@ -65,6 +65,10 @@ class Gem::PackageTask < Rake::PackageTask
 
   attr_accessor :gem_spec
 
+  ##
+  # Indicates whether this package should be built as a content-addressable gem.
+  attr_accessor :content_addressable
+
   ##
   # Create a Gem Package task library.  Automatically define the gem if a
   # block is given.  If no block is supplied, then #define needs to be called
@@ -82,6 +86,7 @@ def initialize(gem_spec)
   def init(gem)
     super gem.full_name, :noversion
     @gem_spec = gem
+    @content_addressable = false
     @package_files += gem_spec.files if gem_spec.files
     @fileutils_output = $stdout
   end
@@ -91,33 +96,97 @@ def init(gem)
   # (+define+ is automatically called if a block is given to +new+).
 
   def define
+    Gem::ContentAddress.eligible!(gem_spec) if content_addressable
+
     super
 
     gem_file = File.basename gem_spec.cache_file
     gem_path = File.join package_dir, gem_file
-    gem_dir  = File.join package_dir, gem_spec.full_name
+    gem_dir = package_dir_path
+
+    if content_addressable
+      build_target = stamp_file
+      gem_description = "content-addressable gem #{gem_spec.name}-#{gem_spec.version} " \
+        "(Platform: #{gem_spec.platform}, Ruby ABI: #{content_addressable_ruby_abi})"
+
+      discard_stale_stamp_file
+    else
+      build_target = gem_path
+      gem_description = "gem file #{gem_file}"
+    end
 
     task package: [:gem]
 
     directory package_dir
     directory gem_dir
 
-    desc "Build the gem file #{gem_file}"
-    task gem: [gem_path]
+    desc "Build the #{gem_description}"
+    task gem: [build_target]
 
     trace = Rake.application.options.trace
     Gem.configuration.verbose = trace
 
-    file gem_path => [package_dir, gem_dir] + @gem_spec.files do
+    file build_target => [package_dir, gem_dir] + @gem_spec.files do
+      previous_gem = recorded_gem_path if content_addressable
+      built_gem_path = nil
+
       chdir(gem_dir) do
-        when_writing "Creating #{gem_spec.file_name}" do
-          built_gem_file = Gem::Package.build gem_spec
+        when_writing "Creating #{gem_description}" do
+          built_gem_file = Gem::Package.build gem_spec, false, false, nil, content_addressable
 
           verbose trace do
             mv built_gem_file, ".."
           end
+
+          if content_addressable
+            built_gem_path = File.join(package_dir, built_gem_file)
+            File.write File.join("..", File.basename(stamp_file)), built_gem_path
+          end
+        end
+      end
+
+      if previous_gem && built_gem_path && previous_gem != built_gem_path
+        verbose trace do
+          rm_f previous_gem
         end
       end
     end
   end
+
+  ##
+  # The name of this package. Content-addressable builds append the Ruby ABI
+  # so per-ABI builds of the same +full_name+ do not share a staging
+  # directory.
+
+  def package_name
+    name = super
+    ruby_abi = content_addressable_ruby_abi
+    ruby_abi ? "#{name}-#{ruby_abi}" : name
+  end
+
+  private
+
+  def content_addressable_ruby_abi
+    return unless content_addressable
+
+    Gem::ContentAddress.ruby_abi_for(gem_spec.required_ruby_version)
+  end
+
+  def stamp_file
+    File.join package_dir, "#{package_name}.gem-built"
+  end
+
+  def recorded_gem_path
+    return unless File.file?(stamp_file)
+
+    path = File.read(stamp_file).strip
+    path unless path.empty?
+  end
+
+  def discard_stale_stamp_file
+    return unless File.file?(stamp_file)
+
+    path = recorded_gem_path
+    File.delete(stamp_file) unless path && File.file?(path)
+  end
 end