vp run: output forwarding fails with EAGAIN when an inherited Node child sets stdout non-blocking

[naokihaba]

Context

If a write to parent stdout returns WouldBlock (EAGAIN), pipe_stdio fails the task and fast-fail kills the remaining tasks with exit code 137. This still happens even with ensureBlockingStdio() (voidzero-dev/vite-plus#2173), as an uncached task inherits the same stdout where Node sets O_NONBLOCK again after startup.

Reported in voidzero-dev/vite-plus#2824. Follow-up to #506 and #552.

Reproduction

• https://github.com/naokihaba/vp-eagain-repro
  • Based on https://github.com/korya/vp-eagain-repro, I confirmed that it can also be reproduced in v1.0.0.

[naokihaba]

I'll handle it.