Race condition in stdout/stderr piping

[lihaoyi]

mill.integration.SubprocessStdoutTests seems to fail flakily with the error below.

X mill.integration.SubprocessStdoutTests.0 8751ms 
  utest.AssertionError: res1.contains(
  res1: String = [info] compiling 1 Scala source to /home/runner/work/mill/mill/out/integration/feature/subprocess-stdout/local/workspaceDir.dest/out/mill-build/compile.dest/classes ...
  [info] done compiling
  print stdout1
  proc stdout1
  print stderr1
  print stdout2
  proc stderr1
  proc stdout2
  print stderr2
  proc stderr2
  print stdout3
  proc stdout3
  print stderr3
  proc stderr3
  print stdout4
  proc stdout4
  print stderr4
  proc stderr4
  print stdout5
  proc stdout5
  print stderr5
  proc stderr5
  print stdout6
  proc stdout6
  print stderr6
  proc stderr6
  print stdout7
  proc stdout7
  print stderr7
  proc stderr7
  print stdout8
  proc stdout8
  print stderr8
  proc stderr8
  print stdout9
  proc stdout9
  print stderr9
  proc stderr9

The first 5 lines seem to indicate the the stdout/stderr from println and os.proc is not being captured in order:

  print stdout1
  proc stdout1
  print stderr1
  print stdout2
  proc stderr1
  proc stdout2
  print stderr2
  proc stderr2

#3275 was meant to solve the issue with the stdout ordering, so something funny must be going on

[lihaoyi]

Maybe fixed by #3360? Let's see if it comes back

[lihaoyi]

I'm still seeing this happen in master, e.g. https://github.com/com-lihaoyi/mill/actions/runs/10437082287/job/28902913813?pr=3378

X mill.integration.SubprocessStdoutTests.0 9627ms 
  utest.AssertionError: res1.contains(
  res1: String = [info] compiling 1 Scala source to /home/runner/work/mill/mill/out/integration/feature/subprocess-stdout/local/workspaceDir.dest/out/mill-build/compile.dest/classes ...
  [info] done compiling
  print stdout1
  proc stdout1
  print stderr1
  print stdout2
  proc stderr1
  proc stdout2
  print stderr2
  proc stderr2
  print stdout3
  proc stdout3
  print stderr3
  proc stderr3
  print stdout4
  proc stdout4
  print stderr4
  proc stderr4
  print stdout5
  proc stdout5
  print stderr5
  proc stderr5
  print stdout6
  proc stdout6
  print stderr6
  proc stderr6
  print stdout7
  proc stdout7
  print stderr7
  proc stderr7
  print stdout8
  proc stdout8
  print stderr8
  proc stderr8
  print stdout9
  proc stdout9
  print stderr9
  proc stderr9
    utest.asserts.Asserts$.assertImpl(Asserts.scala:30)
    mill.integration.SubprocessStdoutTests$.$anonfun$tests$3(SubprocessStdoutTests.scala:15)
    scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
    utest.asserts.Asserts.retry(Asserts.scala:96)
    utest.asserts.Asserts.retry$(Asserts.scala:95)
    utest.package$.retry(package.scala:8)
    mill.integration.SubprocessStdoutTests$.$anonfun$tests$2(SubprocessStdoutTests.scala:15)

[lihaoyi]

Going to close this again since #3577 overhauls this area of the code, we can re-open if the issue persists