Gitlab::Git::Popen can lazily hand output to a block

This allows input to start processing immediately without waiting for the process to complete.
This also allows long or infinite inputs to be partially processed,
which will termiate the process when reading stops with SIGPIPE.

lib/gitlab/git/popen.rb

@@ -7,7 +7,7 @@ module Gitlab
     module Popen
       FAST_GIT_PROCESS_TIMEOUT = 15.seconds
 
-      def popen(cmd, path, vars = {})
+      def popen(cmd, path, vars = {}, lazy_block: nil)
         unless cmd.is_a?(Array)
           raise "System commands must be given as an array of strings"
         end
@@ -22,7 +22,12 @@ module Gitlab
           yield(stdin) if block_given?
           stdin.close
 
-          @cmd_output << stdout.read
+          if lazy_block
+            return lazy_block.call(stdout.lazy)
+          else
+            @cmd_output << stdout.read
+          end
+
           @cmd_output << stderr.read
           @cmd_status = wait_thr.value.exitstatus
         end

spec/lib/gitlab/git/popen_spec.rb

@@ -53,6 +53,23 @@ describe 'Gitlab::Git::Popen' do
       it { expect(status).to be_zero }
       it { expect(output).to eq('hello') }
     end
+
+    context 'with lazy block' do
+      it 'yields a lazy io' do
+        expect_lazy_io = lambda do |io|
+          expect(io).to be_a Enumerator::Lazy
+          expect(io.inspect).to include('#<IO:fd')
+        end
+
+        klass.new.popen(%w[ls], path, lazy_block: expect_lazy_io)
+      end
+
+      it "doesn't wait for process exit" do
+        Timeout.timeout(2) do
+          klass.new.popen(%w[yes], path, lazy_block: ->(io) {})
+        end
+      end
+    end
   end
 
   context 'popen_with_timeout' do