Do not raise during archive size validation when wait thread is finished

lib/gitlab/import_export/decompressed_archive_size_validator.rb

@@ -32,7 +32,16 @@ module Gitlab
         Timeout.timeout(TIMEOUT_LIMIT) do
           stdin, stdout, stderr, wait_thr = Open3.popen3(command, pgroup: true)
           stdin.close
-          pgrp = Process.getpgid(wait_thr[:pid])
+
+          # When validation is performed on a small archive (e.g. 100 bytes)
+          # `wait_thr` finishes before we can get process group id. Do not
+          # raise exception in this scenario.
+          pgrp = begin
+            Process.getpgid(wait_thr[:pid])
+          rescue Errno::ESRCH
+            nil
+          end
+
           status = wait_thr.value
 
           if status.success?

spec/lib/gitlab/import_export/decompressed_archive_size_validator_spec.rb

@@ -24,6 +24,14 @@ RSpec.describe Gitlab::ImportExport::DecompressedArchiveSizeValidator do
       it 'returns true' do
         expect(subject.valid?).to eq(true)
       end
+
+      context 'when waiter thread no longer exists' do
+        it 'does not raise exception' do
+          allow(Process).to receive(:getpgid).and_raise(Errno::ESRCH)
+
+          expect(subject.valid?).to eq(true)
+        end
+      end
     end
 
     context 'when file exceeds allowed decompressed size' do