perf(agent): avoid preview allocations for non-truncated strings (fix #894)

[G7CNF]

Summary

Fixes #894 by eliminating unnecessary heap allocation in truncate_for_preview when the input is already within the byte budget.

What changed

• src/agent/agentic_loop.rs
  • Changed return type from String to Cow<'_, str>.
  • Returns Cow::Borrowed(s) for non-truncated strings.
  • Returns Cow::Owned(...) only when truncation occurs.
  • Added tests proving borrowed vs owned behavior.
• src/worker/container.rs
  • Added .into() on one match arm so both arms unify as Cow<'_, str>.

Issue

• Closes [MEDIUM] truncate_for_preview allocates String even for non-truncated short strings #894

Before / After (explicit)

Before (staging):

pub fn truncate_for_preview(s: &str, max: usize) -> String {
    if s.len() <= max {
        s.to_string()
    } else {
        ...
    }
}

After (this PR):

pub fn truncate_for_preview(s: &str, max: usize) -> Cow<'_, str> {
    if s.len() <= max {
        Cow::Borrowed(s)
    } else {
        Cow::Owned(...)
    }
}

Behavioral verification added:

• test_truncate_short_string_borrowed -> verifies non-truncated result is borrowed
• test_truncate_long_string_owned -> verifies truncated result is owned

Validation

Targeted tests run locally:

cargo test -q agent::agentic_loop::tests::test_truncate_short_string_borrowed -- --nocapture
cargo test -q agent::agentic_loop::tests::test_truncate_long_string_owned -- --nocapture
cargo test -q worker::container::tests::test_truncate_multibyte_safe -- --nocapture

All passed.

Scope

Localized performance fix; no user-facing semantic change in preview content.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a performance improvement by optimizing the truncate_for_preview function. It refactors the function to use std::borrow::Cow<'_, str>, which allows it to return a borrowed reference to the input string when no truncation is necessary, thereby avoiding unnecessary heap allocations. When truncation is required, it still returns an owned string. This change reduces memory overhead and improves efficiency without altering the external behavior of the preview content.

Highlights

• Performance Optimization: Eliminated unnecessary heap allocations in truncate_for_preview by returning a borrowed string (Cow::Borrowed) when truncation is not needed.
• Type Signature Change: Updated the return type of truncate_for_preview from String to std::borrow::Cow<'_, str> to enable zero-copy optimization.
• Behavioral Verification: Added new unit tests to explicitly verify that truncate_for_preview returns a borrowed string when input is short and an owned string when truncation occurs.
• Type Coercion: Adjusted a match arm in src/worker/container.rs to correctly handle the new Cow<'_, str> return type by adding an .into() call.

Changelog

• src/agent/agentic_loop.rs
  • Imported the Cow type from std::borrow.
  • Modified the truncate_for_preview function to return Cow<'_, str> instead of String.
  • Updated the logic within truncate_for_preview to return Cow::Borrowed(s) for non-truncated strings and Cow::Owned(format!...) for truncated strings.
  • Added test_truncate_short_string_borrowed to assert that short strings result in a borrowed Cow.
  • Added test_truncate_long_string_owned to assert that truncated strings result in an owned Cow.
• src/worker/container.rs
  • Added .into() to the Err arm of a match expression to convert the format! result into a Cow<'_, str>, ensuring type compatibility with the Ok arm.

Activity

• Targeted tests were run locally and all passed.

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[gemini-code-assist]

Code Review

This pull request introduces a performance optimization by using Cow<'_ , str> in truncate_for_preview to avoid unnecessary string allocations. The changes are well-implemented and no security vulnerabilities were identified during the audit. However, an issue was found in src/worker/container.rs where a ToolError is passed to truncate_for_preview instead of a &str, which should be addressed for correctness.

[zmanian]

LGTM. Clean optimization that avoids allocating for non-truncated strings.

• Return type changes from String to Cow<'_, str> -- borrows the input when no truncation is needed, only allocates when the ellipsis suffix is appended.
• Cow<str> implements Serialize, Display, Deref<Target=str>, and PartialEq<str>, so all existing call sites (including serde_json::json!() macros, format!(), and string comparisons) continue to work.
• Note: this changes the truncate_for_preview in agentic_loop.rs only (used by container.rs and job.rs). The separate truncate_for_preview in agent_loop.rs is unchanged. The job.rs call sites were not modified in this diff but compile fine since Cow<str> serializes identically to String in JSON contexts.
• The container.rs fix correctly adds .into() on the format!() result to produce Cow::Owned.
• Good test additions verifying Cow::Borrowed vs Cow::Owned variants.

Approved.