perf: use CoW (copy-on-write) cloning for directory copies

[arwtyxouymz]

Summary

• Add _fast_copy_dir() to lib/copy.sh that uses CoW file cloning when supported by the filesystem
  • macOS APFS: cp -cRP (clone) with fallback to cp -RP
  • Linux Btrfs/XFS: cp --reflink=auto -RP (auto-fallback built in)
  • Other filesystems: standard cp -RP (no behavior change)
• Replace cp -RP in copy_directories() with _fast_copy_dir
• Add 3 BATS tests for _fast_copy_dir (contents, symlinks, error handling)

Motivation

Copying large directories like node_modules or .venv via gtr.copy.includeDirs can be slow. On CoW-capable filesystems (APFS, Btrfs, XFS), file cloning is near-instant regardless of directory size, as only metadata is copied.

Risk

• Zero regression on non-CoW filesystems (ext4, NTFS, etc.) — all paths fall back to the current cp -RP behavior
• Individual file copies (copy_patterns) are intentionally unchanged, as CoW overhead outweighs benefit for small config files

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores

  • Optimized directory copying to use copy-on-write where supported, with safe fallbacks and cleanup for partial copies.
  • Improved symlink preservation and logging during copy operations.

• Tests

  • Added safety tests verifying directory copy behavior, symlink preservation, and handling of missing sources.

[coderabbitai]

Walkthrough

Adds _fast_copy_dir() to perform Copy-on-Write-capable directory copies with OS-specific commands and fallbacks, updates copy_directories() to use it, and adds tests validating content copying, symlink preservation, and error handling.

Changes

Cohort / File(s)	Summary
Copy-on-Write Implementation lib/copy.sh	Adds _fast_copy_dir() which detects OS once (cached), attempts CoW copies (macOS: cp -cRP then fallback; Linux: cp --reflink=auto -RP; others: cp -RP), cleans partial macOS targets on failure, and preserves symlinks. Replaces direct cp -RP call in copy_directories() with _fast_copy_dir.
Testing tests/copy_safety.bats	Adds tests sourcing platform script, teardown cleanup, and three _fast_copy_dir() tests: directory/structure copying, symlink target preservation, and failure when source missing.

Sequence Diagram(s)

sequenceDiagram
  participant Caller as Script (copy_directories)
  participant Helper as _fast_copy_dir()
  participant OS as OS `cp` command
  participant FS as Filesystem

  Caller->>Helper: request copy(src, dst, excludes)
  Helper->>Helper: detect/cached OS
  Helper->>OS: run CoW-aware cp (platform-specific)
  OS->>FS: perform copy (reflink or full copy)
  alt macOS CoW failed
    Helper->>FS: remove partial dst
    Helper->>OS: run standard cp fallback
    OS->>FS: perform full copy
  end
  Helper->>Caller: return success/failure, log outcome

Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 I hopped and nudged a copy bright,

CoW magic in the pale moonlight.
Symlinks safe and folders neat,
Fast and tidy—what a treat! 🥕

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: introducing Copy-on-Write cloning for directory copies as a performance optimization.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@lib/copy.sh`:
- Around line 88-93: The darwin branch can leave a partially-populated
destination when cp -cRP "$src" "$dest" fails, so modify the fallback to clean
the destination before retrying: after the failed cp -cRP "$src" "$dest"
attempt, detect and remove the partially-created destination (e.g., rm -rf
"$dest" only when it was created by the failed attempt) or perform the initial
clone into a temporary directory and then atomically rename/move it into
"$dest", then run cp -RP "$src" "$dest" as the fallback; update the darwin case
where cp -cRP and cp -RP are invoked to implement this cleanup/temporal-staging
logic.

🧹 Nitpick comments (2)

lib/copy.sh (1)

84-85: detect_os is invoked on every call; consider caching for loop-heavy callers.

copy_directories calls _fast_copy_dir once per matched directory. Each invocation spawns a subshell for detect_os. You could cache the result in a module-level variable on first call.

Proposed optimization

+# Cached OS value for _fast_copy_dir; set on first call.
+_fast_copy_os=""
+
 _fast_copy_dir() {
   local src="$1" dest="$2"
-  local os
-  os=$(detect_os)
+  if [ -z "$_fast_copy_os" ]; then
+    _fast_copy_os=$(detect_os)
+  fi
+  local os="$_fast_copy_os"

tests/copy_safety.bats (1)

89-101: Temp directory cleanup won't run if an assertion fails.

If [ -f "$dst/mydir/sub/file.txt" ] fails on line 98, lines 100 (rm -rf) is never reached. Same pattern in the other two tests. Consider using a BATS teardown function or the $BATS_TEST_TMPDIR built-in (BATS 1.4+) for automatic cleanup.

Example using teardown

+teardown() {
+  # Clean up any temp dirs created during tests
+  [ -n "${_test_tmpdir:-}" ] && rm -rf "$_test_tmpdir"
+}
+
 `@test` "_fast_copy_dir copies directory contents" {
-  local src dst
-  src=$(mktemp -d)
-  dst=$(mktemp -d)
+  _test_tmpdir=$(mktemp -d)
+  local src="$_test_tmpdir/src" dst="$_test_tmpdir/dst"
+  mkdir -p "$src" "$dst"
   mkdir -p "$src/mydir/sub"
   echo "hello" > "$src/mydir/sub/file.txt"
 
   _fast_copy_dir "$src/mydir" "$dst/"
 
   [ -f "$dst/mydir/sub/file.txt" ]
   [ "$(cat "$dst/mydir/sub/file.txt")" = "hello" ]
-  rm -rf "$src" "$dst"
 }