[rubygems/rubygems] Fix gem install does-not-exist being super slow

Every time a gem is not found in the Compact Index API, RubyGems will
fallback to the full index, which is very slow. This is unnecessary
because both indexes should be providing the same gems, so if a gem
can't be found in the Compact Index API, it won't be found in the full
index.

We _do_ want a fallback to the full index, whenever the Compact Index
API is not implemented. To detect that, we check that the API responds
to the "/versions" endpoint, just like Bundler does.

Before:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  20,77s user 0,59s system 96% cpu 22,017 total
```

After:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  5,02s user 0,09s system 91% cpu 5,568 total
```

https://github.com/rubygems/rubygems/commit/c0d6b9eea7

lib/rubygems/resolver/best_set.rb

@@ -29,10 +29,8 @@ class Gem::Resolver::BestSet < Gem::Resolver::ComposedSet
     pick_sets if @remote && @sets.empty?
 
     super
-  rescue Gem::RemoteFetcher::FetchError => e
-    replace_failed_api_set e
-
-    retry
+  rescue Gem::RemoteFetcher::FetchError
+    []
   end
 
   def prefetch(reqs) # :nodoc:
@@ -50,28 +48,4 @@ class Gem::Resolver::BestSet < Gem::Resolver::ComposedSet
       q.pp @sets
     end
   end
-
-  ##
-  # Replaces a failed APISet for the URI in +error+ with an IndexSet.
-  #
-  # If no matching APISet can be found the original +error+ is raised.
-  #
-  # The calling method must retry the exception to repeat the lookup.
-
-  def replace_failed_api_set(error) # :nodoc:
-    uri = error.original_uri
-    uri = Gem::URI uri unless Gem::URI === uri
-    uri += "."
-
-    raise error unless api_set = @sets.find do |set|
-      Gem::Resolver::APISet === set && set.dep_uri == uri
-    end
-
-    index_set = Gem::Resolver::IndexSet.new api_set.source
-
-    @sets.map! do |set|
-      next set unless set == api_set
-      index_set
-    end
-  end
 end

lib/rubygems/source.rb

@@ -79,7 +79,7 @@ class Gem::Source
       uri
     end
 
-    bundler_api_uri = enforce_trailing_slash(fetch_uri)
+    bundler_api_uri = enforce_trailing_slash(fetch_uri) + "./versions"
 
     begin
       fetcher = Gem::RemoteFetcher.fetcher

test/rubygems/test_gem_resolver_best_set.rb

@@ -9,32 +9,16 @@ class TestGemResolverBestSet < Gem::TestCase
     assert_empty set.sets
   end
 
-  def test_find_all_index
-    spec_fetcher do |fetcher|
-      fetcher.spec "a", 1
-      fetcher.spec "a", 2
-      fetcher.spec "b", 1
-    end
-
-    set = Gem::Resolver::BestSet.new
-
-    dependency = dep "a", "~> 1"
-
-    req = Gem::Resolver::DependencyRequest.new dependency, nil
-
-    found = set.find_all req
-
-    assert_equal %w[a-1], found.map(&:full_name)
-  end
-
-  def test_find_all_fallback
+  def test_find_all
     spec_fetcher do |fetcher|
       fetcher.spec "a", 1
     end
 
-    set = Gem::Resolver::BestSet.new
+    api_uri = Gem::URI "#{@gem_repo}info/"
 
-    api_uri = Gem::URI(@gem_repo)
+    @fetcher.data["#{api_uri}a"] = "---\n1  "
+
+    set = Gem::Resolver::BestSet.new
 
     set.sets << Gem::Resolver::APISet.new(api_uri)
 
@@ -90,64 +74,4 @@ class TestGemResolverBestSet < Gem::TestCase
 
     assert_empty set.sets
   end
-
-  def test_replace_failed_api_set
-    set = Gem::Resolver::BestSet.new
-
-    api_uri = Gem::URI(@gem_repo) + "./info/"
-    api_set = Gem::Resolver::APISet.new api_uri
-
-    set.sets << api_set
-
-    error_uri = api_uri + "a"
-
-    error = Gem::RemoteFetcher::FetchError.new "bogus", error_uri
-
-    set.replace_failed_api_set error
-
-    assert_equal 1, set.sets.size
-
-    refute_includes set.sets, api_set
-
-    assert_kind_of Gem::Resolver::IndexSet, set.sets.first
-  end
-
-  def test_replace_failed_api_set_no_api_set
-    set = Gem::Resolver::BestSet.new
-
-    index_set = Gem::Resolver::IndexSet.new Gem::Source.new @gem_repo
-
-    set.sets << index_set
-
-    error = Gem::RemoteFetcher::FetchError.new "bogus", @gem_repo
-
-    e = assert_raise Gem::RemoteFetcher::FetchError do
-      set.replace_failed_api_set error
-    end
-
-    assert_equal error, e
-  end
-
-  def test_replace_failed_api_set_uri_with_credentials
-    set = Gem::Resolver::BestSet.new
-
-    api_uri = Gem::URI(@gem_repo) + "./info/"
-    api_uri.user = "user"
-    api_uri.password = "pass"
-    api_set = Gem::Resolver::APISet.new api_uri
-
-    set.sets << api_set
-
-    error_uri = api_uri + "a"
-
-    error = Gem::RemoteFetcher::FetchError.new "bogus", error_uri
-
-    set.replace_failed_api_set error
-
-    assert_equal 1, set.sets.size
-
-    refute_includes set.sets, api_set
-
-    assert_kind_of Gem::Resolver::IndexSet, set.sets.first
-  end
 end

test/rubygems/test_gem_source.rb

@@ -46,7 +46,7 @@ class TestGemSource < Gem::TestCase
     response = Gem::Net::HTTPResponse.new "1.1", 200, "OK"
     response.uri = Gem::URI("http://example")
 
-    @fetcher.data[@gem_repo] = response
+    @fetcher.data["#{@gem_repo}versions"] = response
 
     set = @source.dependency_resolver_set
 

test/rubygems/test_gem_source_subpath_problem.rb

@@ -24,7 +24,7 @@ class TestGemSourceSubpathProblem < Gem::TestCase
     response = Gem::Net::HTTPResponse.new "1.1", 200, "OK"
     response.uri = Gem::URI("http://example")
 
-    @fetcher.data["#{@gem_repo}/"] = response
+    @fetcher.data["#{@gem_repo}/versions"] = response
 
     set = @source.dependency_resolver_set