ActiveSupport::Testing::Isolation: gracefully handle the subprocess dying

Right now if the subprocess exit uncleanly, it straight out bring the
parent down with it because it will fail to parse the (likely empty)
Marshal payload:

```
<internal:marshal>:34:in `load': marshal data too short (ArgumentError)
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:23:in `run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:1094:in `run_one_method'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:179:in `block in run'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:168:in `with_timestamps'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:178:in `run'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:229:in `block in run_from_queue'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/ci/queue/redis/worker.rb:55:in `poll'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:228:in `run_from_queue'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:213:in `__run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:162:in `run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:86:in `block in autorun'
 - XXXX::XXXXTest#test_xxxxx - /tmp/bundle/ruby/3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:52:in `write': closed stream (IOError)
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:52:in `puts'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:52:in `block (2 levels) in run_in_isolation'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:32:in `fork'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:32:in `block in run_in_isolation'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:28:in `pipe'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:28:in `run_in_isolation'
	from 3.3.0+0/bundler/gems/rails-488a7ce18880/activesupport/lib/active_support/testing/isolation.rb:19:in `run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:1094:in `run_one_method'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:179:in `block in run'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:168:in `with_timestamps'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:178:in `run'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:229:in `block in run_from_queue'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/ci/queue/redis/worker.rb:55:in `poll'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:228:in `run_from_queue'
	from 3.3.0+0/gems/ci-queue-0.38.0/lib/minitest/queue.rb:213:in `__run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:162:in `run'
	from 3.3.0+0/gems/minitest-5.20.0/lib/minitest.rb:86:in `block in autorun'
```

This breaks the Minitest contract that `run_one_method` shouldn't raise
ever, and return a `Minitest::Result`.

By properly checking the sub process status, we can turn this crash into
a test failure, allowing the original test process to go on.

Author: byroot

activesupport/lib/active_support/testing/isolation.rb

@@ -5,6 +5,8 @@ module Testing
     module Isolation
       require "thread"
 
+      SubprocessCrashed = Class.new(StandardError)
+
       def self.included(klass) # :nodoc:
         klass.class_eval do
           parallelize_me!
@@ -16,10 +18,17 @@ def self.forking_env?
       end
 
       def run
-        serialized = run_in_isolation do
+        status, serialized = run_in_isolation do
           super
         end
 
+        unless status&.success?
+          error = SubprocessCrashed.new("Subprocess exited with an error: #{status.inspect}\noutput: #{serialized.inspect}")
+          error.set_backtrace(caller)
+          self.failures << Minitest::UnexpectedError.new(error)
+          return defined?(Minitest::Result) ? Minitest::Result.from(self) : dup
+        end
+
         Marshal.load(serialized)
       end
 
@@ -50,13 +59,13 @@ def run_in_isolation(&blk)
               end
 
               write.puts [result].pack("m")
-              exit!
+              exit!(0)
             end
 
             write.close
             result = read.read
-            Process.wait2(pid)
-            result.unpack1("m")
+            _, status = Process.wait2(pid)
+            return status, result.unpack1("m")
           end
         end
       end
@@ -75,7 +84,7 @@ def run_in_isolation(&blk)
             File.open(ENV["ISOLATION_OUTPUT"], "w") do |file|
               file.puts [Marshal.dump(test_result)].pack("m")
             end
-            exit!
+            exit!(0)
           else
             Tempfile.open("isolation") do |tmpfile|
               env = {
@@ -93,13 +102,14 @@ def run_in_isolation(&blk)
 
               child = IO.popen([env, Gem.ruby, *load_path_args, $0, *ORIG_ARGV, test_opts])
 
+              status = nil
               begin
-                Process.wait(child.pid)
+                _, status = Process.wait2(child.pid)
               rescue Errno::ECHILD # The child process may exit before we wait
                 nil
               end
 
-              return tmpfile.read.unpack1("m")
+              return status, tmpfile.read.unpack1("m")
             end
           end
         end