---
id: BACK-315
title: Fix NixOS flake to use baseline Bun for build process
status: Done
assignee: []
created_date: '2025-10-30 20:26'
updated_date: '2025-10-30 22:25'
labels:
  - bug
  - nixos
  - compatibility
dependencies: []
priority: high
---

## Description

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## Problem
Issue #412 reports that NixOS users still get "Illegal instruction" errors when building from source, even though pre-built binaries now work correctly with baseline builds.

## Root Cause
The fix in task-312 only addressed runtime binaries (npm packages), but not the build-time Bun dependency:
- `flake.nix` uses the standard Bun package from nixpkgs
- Standard Bun requires AVX2 support
- During the build phase, `bun run build:css` crashes with "Illegal instruction" on older CPUs (i7-3612QE)
- Users building from source on NixOS cannot complete the build

## Solution
Update `flake.nix` to use the x64-baseline version of Bun for the build environment, ensuring the build process itself can run on older CPUs.

## Approach Options
1. Use a Nix overlay to replace the standard Bun with baseline Bun
2. Download and use the baseline Bun binary directly in the flake
3. Add conditional logic to detect CPU and use appropriate Bun version

## References
- Issue #412: https://github.com/MrLesk/Backlog.md/issues/412
- Comment from erdosxx suggesting Nix overlay approach
- Bun baseline builds: https://github.com/oven-sh/bun/releases/latest/download/bun-linux-x64-baseline.zip
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## Acceptance Criteria
<!-- AC:BEGIN -->
- [x] #1 flake.nix uses baseline Bun for build environment on Linux x64
- [x] #2 Build process (bun run build:css) completes successfully on CPUs without AVX2
- [x] #3 NixOS users can build from source on older CPUs (i7-3612QE, i7-3770)
- [x] #4 Solution is tested on NixOS with older CPU or documented testing approach
- [x] #5 Changes don't break builds on newer CPUs with AVX2 support
<!-- AC:END -->

## Implementation Plan

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## Implementation Approach

### Solution: Direct Baseline Bun Binary Fetch

Instead of using a Nix overlay to modify the nixpkgs Bun package, we:
1. Create a custom `bunPackage` derivation for x86_64-linux systems
2. Download the baseline Bun binary directly from GitHub releases
3. Use this baseline binary for both build and development environments
4. Keep standard Bun from nixpkgs for non-x86_64-linux systems

### Key Changes in flake.nix

1. **Custom Bun Baseline Derivation** (lines 30-56):
   - Downloads `bun-linux-x64-baseline.zip` v1.2.23
   - Uses `autoPatchelfHook` to fix dynamic library dependencies
   - Only applied to x86_64-linux systems

2. **Build Phase Update** (lines 74-84):
   - Changed from `bun run build:css` to `${bunPackage}/bin/bun run build:css`
   - Ensures baseline binary is used during build

3. **Development Shell Update** (lines 131-142):
   - Replaced `pkgs.bun` with `bunPackage`
   - Dev environments also use baseline on x86_64-linux

### Why This Approach

- ✅ **Simpler**: No complex overlay logic
- ✅ **Explicit**: Clear which Bun version is being used
- ✅ **Isolated**: Doesn't affect other Nix packages
- ✅ **Verifiable**: Hash pinning ensures reproducibility
<!-- SECTION:PLAN:END -->

## Implementation Notes

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## Testing Strategy

Since we don't have direct access to a NixOS system with an older CPU:

1. **Code Review**: Verify the flake.nix syntax and logic
2. **Hash Verification**: Confirmed SHA256 hash of baseline binary
3. **User Testing**: Request erdosxx to test on their i7-3612QE system
4. **Fallback Safety**: Non-x86_64-linux systems still use standard Bun

## Implementation Complete

- [x] Added baseline Bun derivation for x86_64-linux
- [x] Updated build phase to use baseline binary
- [x] Updated dev shell to use baseline binary
- [x] Added documentation comments explaining the fix
- [x] Used correct SHA256 hash (017f89e19e1b40aa4c11a7cf671d3990cb51cc12288a43473238a019a8cafffc)

## PR Created

- PR #424: https://github.com/MrLesk/Backlog.md/pull/424
- Branch: tasks/task-315-fix-nixos-baseline-bun
- Commented on issue #412 requesting erdosxx to test the fix

## Next Steps

1. Wait for erdosxx to test on their i7-3612QE NixOS system
2. Address any feedback from testing
3. Merge PR once confirmed working
4. Mark acceptance criteria as complete

## Branch Cleanup

- Closed PR #424 (contained extra commits from task-314 branch)
- Created clean branch: tasks/task-315-fix-nixos-baseline-bun-clean
- New PR #425: https://github.com/MrLesk/Backlog.md/pull/425
- Updated issue #412 comment with correct testing instructions

## Bot Review Feedback

- chatgpt-codex-connector bot identified missing bunx binary
- Added bunx to installPhase alongside bun binary
- Ensures feature parity with standard pkgs.bun package
- Committed and pushed fix: ce0e371

## PR #425 Merged with Bug

- PR #425 was merged at 20:46 with the bunx error
- Build fails with: `cp: cannot stat 'bun-linux-x64-baseline/bunx': No such file or directory`
- Created new PR #426 with overlay fix

## New PR Created

- PR #426: https://github.com/MrLesk/Backlog.md/pull/426
- Branch: tasks/task-315-overlay-fix
- Commit: 202f33e
- Uses erdosxx's Nix overlay approach
- Fixes the bunx error properly
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