Retry without Range when compact index returns 416

Bundler's downloader already recovers from a stale over-long cache by
dropping the Range header and refetching the whole file, but
Gem::CompactIndexClient::HTTPFetcher raised FetchError instead, so the
gem CLI could not self-heal. Align the behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

lib/rubygems/compact_index_client/http_fetcher.rb

@@ -41,6 +41,11 @@ def fetch(uri, headers, redirects_remaining)
         raise Gem::RemoteFetcher::FetchError.new("redirecting but no redirect location was given", uri) unless location
 
         fetch(uri + location, headers, redirects_remaining - 1)
+      when Gem::Net::HTTPRangeNotSatisfiable
+        raise Gem::RemoteFetcher::FetchError.new("bad response #{response.message} #{response.code}", uri) unless headers.key?("Range")
+
+        # The local cache is longer than the remote file, refetch it whole.
+        fetch(uri, headers.except("Range"), redirects_remaining)
       else
         raise Gem::RemoteFetcher::FetchError.new("bad response #{response.message} #{response.code}", uri)
       end

test/rubygems/test_gem_compact_index_client_http_fetcher.rb

@@ -34,6 +34,12 @@ def initialize
     end
   end
 
+  class FakeRangeNotSatisfiable < Gem::Net::HTTPRangeNotSatisfiable
+    def initialize
+      super("1.1", "416", "Range Not Satisfiable")
+    end
+  end
+
   class FakeRemoteFetcher
     attr_reader :requests
 
@@ -116,6 +122,35 @@ def test_call_raises_after_too_many_redirects
     assert_match(/too many redirects/, error.message)
   end
 
+  def test_call_retries_without_range_on_range_not_satisfiable
+    requests = []
+    remote = Object.new
+    remote.define_singleton_method(:request) do |uri, request_class, &block|
+      request = request_class.new(uri)
+      block&.call(request)
+      requests << request
+      request["Range"] ? FakeRangeNotSatisfiable.new : FakeResponse.new("full data")
+    end
+
+    fetcher = Gem::CompactIndexClient::HTTPFetcher.new("https://index.example", remote)
+    response = fetcher.call("versions", "Range" => "bytes=100-", "If-None-Match" => '"abc"')
+
+    assert_equal "full data", response.body
+    assert_equal 2, requests.size
+    assert_nil requests.last["Range"]
+    assert_equal '"abc"', requests.last["If-None-Match"]
+  end
+
+  def test_call_raises_on_range_not_satisfiable_without_range
+    fetcher, _remote = fetcher_for("https://index.example/versions" => FakeRangeNotSatisfiable.new)
+
+    error = assert_raise Gem::RemoteFetcher::FetchError do
+      fetcher.call("versions")
+    end
+
+    assert_match(/bad response Range Not Satisfiable 416/, error.message)
+  end
+
   def test_call_raises_fetch_error_on_failure_response
     fetcher, _remote = fetcher_for("https://index.example/versions" => FakeNotFound.new)