Look up gems for gem fetch through the compact index

`gem fetch` looks gems up through Gem::SpecFetcher, which asks a source for its
entire index -- the compact index `versions` file when the source serves one,
23MB of it for rubygems.org on a cold cache, and the Marshal index otherwise --
and then fetches a Marshal gemspec for every version that matches. `gem install`
resolves through Gem::Resolver::BestSet, which reads only the `info` file of the
gem it is looking for.

Look gems up through a BestSet instead, so `gem fetch a` reads `/info/a` rather
than the whole index, and fetches only the gemspec of the version it downloads.
Platform preference and the reporting of specs rejected for another platform are
unchanged.

Gem::Source uses Gem::RemoteFetcher without requiring it, relying on its callers
to have loaded it, and it cannot require it eagerly: Bundler loads
rubygems/source on boot, where pulling in net/http breaks bundles that do not
include its dependencies. Require it from the command instead, the way the
sources command already does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: segiddins

lib/rubygems/commands/fetch_command.rb

@@ -2,6 +2,8 @@
 
 require_relative "../command"
 require_relative "../local_remote_options"
+require_relative "../remote_fetcher"
+require_relative "../resolver"
 require_relative "../version_option"
 
 class Gem::Commands::FetchCommand < Gem::Command
@@ -79,31 +81,59 @@ def fetch_gems
     platform  = Gem.platforms.last
     gem_names = get_all_gem_names_and_versions
 
+    # A BestSet reads a source's compact index when it serves one, the same way
+    # gems are looked up when installing them.
+    remote_set = Gem::Resolver::BestSet.new
+
     gem_names.each do |gem_name, gem_version|
       gem_version ||= version
       dep = Gem::Dependency.new gem_name, gem_version
       dep.prerelease = options[:prerelease]
       suppress_suggestions = !options[:suggest_alternate]
 
-      specs_and_sources, errors =
-        Gem::SpecFetcher.fetcher.spec_for_dependency dep
+      remote_specs, errors = find_remote_specs dep, remote_set
 
       if platform
-        filtered = specs_and_sources.select {|s,| s.platform == platform }
-        specs_and_sources = filtered unless filtered.empty?
+        filtered = remote_specs.select {|s| s.platform == platform }
+        remote_specs = filtered unless filtered.empty?
       end
 
-      spec, source = specs_and_sources.max_by {|s,| s }
+      remote_spec = remote_specs.max_by {|s| [s.version, Gem::Platform.sort_priority(s.platform)] }
 
-      if spec.nil?
+      if remote_spec.nil?
         show_lookup_failure gem_name, gem_version, errors, suppress_suggestions, options[:domain]
         exit_code |= 2
         next
       end
-      source.download spec
+
+      spec = remote_spec.spec
+      remote_spec.source.download spec
       say "Downloaded #{spec.full_name}"
     end
 
     exit_code
   end
+
+  # Find specs in +set+ that match +dep+ and can be used on this platform,
+  # along with the reasons any other spec was rejected.
+
+  def find_remote_specs(dep, set)
+    set.prerelease = dep.prerelease?
+
+    request = Gem::Resolver::DependencyRequest.new dep, nil
+
+    matching, mismatched = set.find_all(request).partition do |spec|
+      Gem::Platform.match_spec? spec
+    end
+
+    [matching, set.errors + platform_mismatches(mismatched)]
+  end
+
+  def platform_mismatches(specs)
+    specs.group_by {|spec| [spec.name, spec.version] }.map do |(name, version), group|
+      mismatch = Gem::PlatformMismatch.new name, version
+      group.each {|spec| mismatch.add_platform spec.platform.to_s }
+      mismatch
+    end
+  end
 end

test/rubygems/test_gem_commands_fetch_command.rb

@@ -81,7 +81,7 @@ def test_execute_platform
 
     @cmd.options[:args] = %w[a]
 
-    @fetcher.data["#{@gem_repo}latest_specs.#{Gem.marshal_version}.gz"] = util_gzip(Marshal.dump([
+    @fetcher.data["#{@gem_repo}specs.#{Gem.marshal_version}.gz"] = util_gzip(Marshal.dump([
       Gem::NameTuple.new(a2_spec.name, a2_spec.version, a2_spec.platform),
       Gem::NameTuple.new(a2_universal_darwin_spec.name, a2_universal_darwin_spec.version, a2_universal_darwin_spec.platform),
     ]))
@@ -100,6 +100,66 @@ def test_execute_platform
                        "#{a2_universal_darwin_spec.full_name} not fetched")
   end
 
+  def test_execute_compact_index
+    specs = spec_fetcher do |fetcher|
+      fetcher.gem "a", 1
+      fetcher.gem "a", 2
+    end
+
+    util_setup_compact_index(*specs.values)
+
+    @cmd.options[:args] = %w[a]
+
+    execute_with_exit_code
+
+    a2 = specs["a-2"]
+
+    assert_path_exist(File.join(@tempdir, a2.file_name),
+                      "#{a2.full_name} not fetched")
+
+    assert_includes @fetcher.paths, "#{@gem_repo}info/a"
+    refute_includes @fetcher.paths, "#{@gem_repo}specs.#{Gem.marshal_version}.gz"
+  end
+
+  def test_execute_compact_index_platform
+    specs = spec_fetcher do |fetcher|
+      fetcher.gem "a", 2
+      fetcher.gem("a", 2) {|s| s.platform = "universal-darwin" }
+    end
+
+    util_setup_compact_index(*specs.values)
+
+    @cmd.options[:args] = %w[a]
+
+    util_set_arch "arm64-darwin20" do
+      execute_with_exit_code
+    end
+
+    a2_universal_darwin = specs["a-2-universal-darwin"]
+
+    assert_path_exist(File.join(@tempdir, a2_universal_darwin.file_name),
+                      "#{a2_universal_darwin.full_name} not fetched")
+  end
+
+  def test_execute_compact_index_platform_mismatch
+    specs = spec_fetcher do |fetcher|
+      fetcher.spec("a", 2) {|s| s.platform = "java" }
+    end
+
+    util_setup_compact_index(*specs.values)
+
+    @cmd.options[:args] = %w[a]
+
+    execute_with_term_error
+
+    expected = <<-EXPECTED
+ERROR:  Could not find a valid gem 'a' (>= 0), here is why:
+          Found a (2), but was for platform java
+    EXPECTED
+
+    assert_equal expected, @ui.error
+  end
+
   def test_execute_specific_prerelease
     specs = spec_fetcher do |fetcher|
       fetcher.gem "a", 2