fix(tower-runtime): preserve error details in SpawnFailed and PackageUnpackFailed

[sammuti]

Summary

• Add detail: String field to Error::SpawnFailed and Error::PackageUnpackFailed so the root cause (IO error, missing uv, permission denied, etc.) flows through to termination logs and trace spans
• Add Display impl for tower_uv::Error
• Previously all tower_uv::Error variants were mapped to a bare Error::SpawnFailed with no context, making production spawn_failed errors impossible to diagnose without pod-level access

Context

Investigating TOW-2151/TOW-2160 (WaiterClosed) and recent spawn_failed errors on May 31, we found that while the structured AppCompletion::Failed pipeline (from #274) correctly surfaces the error kind, the actual error message was always just "SpawnFailed" because the From<tower_uv::Error> impl discarded the inner detail.

After this change, termination logs and trace spans will show messages like:

• spawning process: not found: uv binary not found in PATH
• spawning process: IO error: No such file or directory (os error 2)
• spawning process: permission denied: /app/.venv/bin/python
• package unpack failed: InvalidManifest

Test plan

• cargo check --workspace passes
• cargo test --workspace passes
• Deploy to staging and trigger a run with a broken pyproject.toml to verify the detail appears in traces

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved error messages with detailed context for spawn failures and package operations, helping users better understand what went wrong during runtime execution.

[github-actions]

⚠️ WARNING: This PR targets main instead of develop

This PR is targeting main which will trigger a production deployment when merged.

If this is a regular feature/fix PR, please change the base branch to develop.
If this is intentional (e.g., hotfix), you can ignore this warning.

Current base: main
Recommended base: develop

[coderabbitai]

📝 Walkthrough

Walkthrough

Error variants SpawnFailed and PackageUnpackFailed are extended with detail strings, their display messages are updated, From trait implementations are enriched to capture error details, a Display trait is added to tower_uv::Error, and subprocess error handling is updated to populate these detail fields with specific context.

Changes

Error Detail Enrichment

Layer / File(s)	Summary
Error variant definitions and display messages crates/tower-runtime/src/errors.rs	SpawnFailed and PackageUnpackFailed are changed from unit variants to struct variants carrying detail: String, with Snafu display strings updated to interpolate the detail.
Display trait for tower_uv::Error crates/tower-uv/src/lib.rs	Display is implemented for tower_uv::Error, formatting each variant into human-readable strings to support to_string() calls in error conversions.
From trait implementations to populate error details crates/tower-runtime/src/errors.rs	From<std::io::Error>, From<tower_uv::Error>, and From<tower_package::Error> implementations are updated to map upstream errors into struct variants with details captured via to_string().
Subprocess error handling enriched with detail crates/tower-runtime/src/subprocess.rs	receive_and_unpack_package and backend create methods populate PackageUnpackFailed detail with formatted error context for unpack failures and missing invariant states.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 Errors now carry their stories with care,
With details and strings beyond bare despair,
From IO to UV, each mishap now speaks,
The devils are caught in the error technique!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding detail fields to error variants to preserve error context.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ 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/preserve-error-details-in-runtime-errors

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[github-actions]

⚠️ WARNING: This PR targets main instead of develop

This PR is targeting main which will trigger a production deployment when merged.

If this is a regular feature/fix PR, please change the base branch to develop.
If this is intentional (e.g., hotfix), you can ignore this warning.

Current base: main
Recommended base: develop

[coderabbitai]

🧹 Nitpick comments (1)

crates/tower-runtime/src/errors.rs (1)

82-85: ⚡ Quick win

Standardize I/O detail formatting in spawn conversion.

Line 84 currently uses raw err.to_string(), so this path omits the IO error: prefix used by tower_uv::Error::IoError. Prefixing here keeps SpawnFailed messages consistent across conversion paths.

Suggested patch

 impl From<std::io::Error> for Error {
     fn from(err: std::io::Error) -> Self {
         Error::SpawnFailed {
-            detail: err.to_string(),
+            detail: format!("IO error: {err}"),
         }
     }
 }

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/tower-runtime/src/errors.rs` around lines 82 - 85, The SpawnFailed
conversion currently uses err.to_string() directly; change the conversion in the
From<std::io::Error> implementation so the detail string is prefixed with "IO
error: " (e.g., build the detail as format!("IO error: {}", err)) to match the
formatting used by tower_uv::Error::IoError and keep Error::SpawnFailed messages
consistent.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@crates/tower-runtime/src/errors.rs`:
- Around line 82-85: The SpawnFailed conversion currently uses err.to_string()
directly; change the conversion in the From<std::io::Error> implementation so
the detail string is prefixed with "IO error: " (e.g., build the detail as
format!("IO error: {}", err)) to match the formatting used by
tower_uv::Error::IoError and keep Error::SpawnFailed messages consistent.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 2fdfb143-4d30-4bf4-bda0-8a3665bba3ae

📥 Commits

Reviewing files that changed from the base of the PR and between c4e63f8 and 90a256b.

📒 Files selected for processing (3)

• crates/tower-runtime/src/errors.rs
• crates/tower-runtime/src/subprocess.rs
• crates/tower-uv/src/lib.rs

[bradhe]

Looks good. Just fix failing CI.