Fix nested Tokio runtime panic in subprocess bridge

[zackees]

Summary

• allow blocking subprocess helpers to run from an existing multithreaded Tokio runtime via block_in_place
• keep plain sync callers on a small current-thread runtime
• add a regression test for the Tokio worker path

Closes#827

Tests

• soldr cargo fmt --all -- --check
• soldr cargo test -p fbuild-core run_command_blocking_works_from_tokio_worker -- --nocapture
• soldr cargo test -p fbuild-core

Summary by CodeRabbit

• Bug Fixes

  • Improved command execution when called from inside an active Tokio runtime, avoiding nested runtime issues.
  • run_command_blocking now works correctly from multithreaded Tokio worker tasks and returns expected output.

• Tests

  • Added coverage for invoking blocking command execution within a Tokio task.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: f415e433-2cf4-412c-90be-8760d0d65545

📥 Commits

Reviewing files that changed from the base of the PR and between 9445e7f and f50ec01.

📒 Files selected for processing (1)

• crates/fbuild-core/src/subprocess.rs

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📝 Walkthrough

Walkthrough

block_on in subprocess.rs is updated to detect an existing Tokio runtime via Handle::try_current() and use block_in_place when one is present, instead of always constructing a new current-thread runtime. Documentation is updated to describe both paths, and a new multi-thread Tokio test verifies run_command_blocking succeeds from a worker task.

Subprocess blocking shim fix

Layer / File(s)	Summary
block_on runtime detection and regression test crates/fbuild-core/src/subprocess.rs	block_on checks Handle::try_current() and runs via block_in_place when inside a multithreaded Tokio runtime; falls back to a new current-thread runtime for plain sync callers. Docs updated to describe both paths. New #[tokio::test(flavor = "multi_thread")] test asserts run_command_blocking completes without panicking from a worker task.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A rabbit once blocked in a loop,
Two runtimes caused quite a flop!
Now try_current saves the day,
block_in_place clears the way,
No more panics — hip hop hooray! 🎉

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/nested-runtime-blocking-subprocess

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