[rubygems/rubygems] Emit progress to stderr when `--print` is passed to `bundle lock`

`bundle lock --print --update` can take a long time to fetch sources and
resolve the lock file.

Before, `--print` caused output to be completely silenced, so nothing
was printed at all until the resolved lock file is finally emitted to
stdout.

With this change, `--print` now prints progress to stderr. E.g.:

```
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
```

This provides a better user experience, especially when
`lock --print --update` takes several seconds or more.

The lock file is still printed to stdout, so tools consuming the lock
file on stdout will not be affected.

https://github.com/rubygems/rubygems/commit/6719baa700

lib/bundler/cli/lock.rb

@@ -15,8 +15,8 @@ module Bundler
       end
 
       print = options[:print]
-      previous_ui_level = Bundler.ui.level
-      Bundler.ui.level = "silent" if print
+      previous_output_stream = Bundler.ui.output_stream
+      Bundler.ui.output_stream = :stderr if print
 
       Bundler::Fetcher.disable_endpoint = options["full-index"]
 
@@ -68,7 +68,7 @@ module Bundler
         end
       end
 
-      Bundler.ui.level = previous_ui_level
+      Bundler.ui.output_stream = previous_output_stream
     end
   end
 end

lib/bundler/ui/shell.rb

@@ -6,14 +6,17 @@ module Bundler
   module UI
     class Shell
       LEVELS = %w[silent error warn confirm info debug].freeze
+      OUTPUT_STREAMS = [:stdout, :stderr].freeze
 
       attr_writer :shell
+      attr_reader :output_stream
 
       def initialize(options = {})
         Thor::Base.shell = options["no-color"] ? Thor::Shell::Basic : nil
         @shell = Thor::Base.shell.new
         @level = ENV["DEBUG"] ? "debug" : "info"
         @warning_history = []
+        @output_stream = :stdout
       end
 
       def add_color(string, *color)
@@ -101,6 +104,11 @@ module Bundler
         index <= LEVELS.index(@level)
       end
 
+      def output_stream=(symbol)
+        raise ArgumentError unless OUTPUT_STREAMS.include?(symbol)
+        @output_stream = symbol
+      end
+
       def trace(e, newline = nil, force = false)
         return unless debug? || force
         msg = "#{e.class}: #{e.message}\n#{e.backtrace.join("\n  ")}"
@@ -119,6 +127,8 @@ module Bundler
 
       # valimism
       def tell_me(msg, color = nil, newline = nil)
+        return tell_err(msg, color, newline) if output_stream == :stderr
+
         msg = word_wrap(msg) if newline.is_a?(Hash) && newline[:wrap]
         if newline.nil?
           @shell.say(msg, color)

lib/bundler/ui/silent.rb

@@ -53,6 +53,13 @@ module Bundler
         false
       end
 
+      def output_stream=(_symbol)
+      end
+
+      def output_stream
+        nil
+      end
+
       def ask(message)
       end
 

spec/bundler/bundler/ui/shell_spec.rb

@@ -10,6 +10,13 @@ RSpec.describe Bundler::UI::Shell do
     it "prints to stdout" do
       expect { subject.info("info") }.to output("info\n").to_stdout
     end
+
+    context "when output_stream is :stderr" do
+      before { subject.output_stream = :stderr }
+      it "prints to stderr" do
+        expect { subject.info("info") }.to output("info\n").to_stderr
+      end
+    end
   end
 
   describe "#confirm" do
@@ -17,6 +24,13 @@ RSpec.describe Bundler::UI::Shell do
     it "prints to stdout" do
       expect { subject.confirm("confirm") }.to output("confirm\n").to_stdout
     end
+
+    context "when output_stream is :stderr" do
+      before { subject.output_stream = :stderr }
+      it "prints to stderr" do
+        expect { subject.confirm("confirm") }.to output("confirm\n").to_stderr
+      end
+    end
   end
 
   describe "#warn" do
@@ -33,6 +47,13 @@ RSpec.describe Bundler::UI::Shell do
     it "prints to stdout" do
       expect { subject.debug("debug") }.to output("debug\n").to_stdout
     end
+
+    context "when output_stream is :stderr" do
+      before { subject.output_stream = :stderr }
+      it "prints to stderr" do
+        expect { subject.debug("debug") }.to output("debug\n").to_stderr
+      end
+    end
   end
 
   describe "#error" do

spec/bundler/commands/lock_spec.rb

@@ -202,6 +202,25 @@ RSpec.describe "bundle lock" do
     expect(read_lockfile).to eq(expected_lockfile)
   end
 
+  it "prints an updated lockfile when there is an outdated lockfile using --print --update" do
+    lockfile outdated_lockfile
+
+    bundle "lock --print --update"
+
+    expect(out).to eq(expected_lockfile.rstrip)
+  end
+
+  it "emits info messages to stderr when updating an outdated lockfile using --print --update" do
+    lockfile outdated_lockfile
+
+    bundle "lock --print --update"
+
+    expect(err).to eq(<<~STDERR.rstrip)
+      Fetching gem metadata from https://gem.repo4/...
+      Resolving dependencies...
+    STDERR
+  end
+
   it "writes a lockfile when there is an outdated lockfile and bundle is frozen" do
     lockfile outdated_lockfile