fix: handle du exit code for missing paths in sandbox script (Issue #57)

[konard]

Summary

Fixes #57 — CI/CD "Measure Disk Space and Update README" fails with exit code 1 after the Issue #55 fix.

Root Cause

The Issue #55 fix changed Homebrew and Rust measurement to use du -sb on specific directories. When Homebrew installation fails (sandbox user cannot use sudo non-interactively), install_homebrew() returns 0 (due to || true), so the success branch runs. But then:

brew_bytes=$(du -sb /home/linuxbrew/.linuxbrew "$HOME/.linuxbrew" 2>/dev/null | awk ...)

$HOME/.linuxbrew doesn't exist → du exits with code 1 → set -euo pipefail kills the entire sandbox sub-script → CI step fails.

The 2>/dev/null suppresses the error message but not the exit code. This is the regression.

Fix

Before calling du, build an array of only the paths that actually exist:

brew_paths=()
[[ -d /home/linuxbrew/.linuxbrew ]] && brew_paths+=(/home/linuxbrew/.linuxbrew)
[[ -d "$HOME/.linuxbrew" ]] && brew_paths+=("$HOME/.linuxbrew")
if [[ ${#brew_paths[@]} -gt 0 ]]; then
  brew_bytes=$(du -sb "${brew_paths[@]}" 2>/dev/null | awk '{sum+=$1} END{print sum+0}')
else
  brew_bytes=0
fi

Applied to both Homebrew and Rust du-based measurements in the sandbox sub-script.

Files Changed

• scripts/measure-disk-space.sh — fix du to only measure existing paths
• docs/case-studies/issue-57/CASE-STUDY.md — full root cause analysis, timeline, reproducers
• docs/case-studies/issue-57/ci-job-64665886012.txt — CI failure log preserved

Verification

Local test confirms:

• Old code: du -sb /nonexistent 2>/dev/null → exit code 1 → kills script
• New code: only passes existing paths → never fails on missing dirs

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[konard]

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[konard]

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