Symlink race conditions in test executables under dune --force

[dkijania]

Summary

When running dune runtest --force (used for coverage builds to ensure full recompilation), several test executables hit symlink race conditions during parallel execution.

Failure Details

Affected executables:

• test_range_check.exe
• test_step.exe
• test_plonk_curve_ops.exe

Error: Unix.Unix_error "File exists" symlink

Observed in: Nightly build #1268 on branch dkijania/force-coverage-on-nightly

Log excerpt

test_range_check.exe: internal error, uncaught exception:
                      (Unix.Unix_error "File exists" symlink
test_step.exe: internal error, uncaught exception:
               (Unix.Unix_error "File exists" symlink
test_plonk_curve_ops.exe: internal error, uncaught exception:
                          (Sys_error

Context

The --force flag was added in PR #18751 to ensure bisect_ppx generates .coverage files for all source files during nightly builds. Without --force, dune's incremental compilation skips unchanged files, leading to incomplete coverage.

The symlink collisions occur because --force causes all targets to rebuild in parallel, and these test executables create symlinks that race with each other.

Impact

• These appear to be warnings/crashes of individual test processes, not test failures that block CI
• The affected tests (pickles-related) may still pass on retry since dune retries failed tests

Suggested Fix

• Investigate which symlinks are being created and whether they can use unique names or atomic operations
• Alternatively, clean the _build directory before --force runs to avoid stale symlinks
• Consider if dune clean before runtest --force is cheaper than debugging the race