Implement reflink copy with automatic fallback for faster tree copying

[Copilot]

Plan: Implement Reflink Copy Support

• Explore repository and understand code structure
• Verify current tests pass
• Add reflink crate to Cargo.toml dependencies
• Modify copy_tree.rs to use reflink for file copying with fallback
• Add debug logging to indicate if reflinks are working or not
• Test the implementation
• Run all tests to ensure no regressions
• Fix clippy warnings
• Address code review: use short type names with imports
• Address code review: add warning for non-unsupported reflink errors
• Document feature in NEWS.md
• Change reflink error logging from warn to debug
• Use crate::Result instead of anyhow::Result for consistency
• Handle Windows reflink failures that don't return ErrorKind::Unsupported

Implementation Summary

Successfully implemented reflink copy support with automatic fallback to regular copying:

Changes Made:

1. Added reflink crate (v0.1.3) to Cargo.toml dependencies

2. Created copy_file helper function that:

   • Attempts to use reflink::reflink() first for fast copy-on-write operations
   • Gives up on reflinks after any error (not just ErrorKind::Unsupported) to handle Windows behavior where reflink can fail without returning the expected error kind
   • Logs all reflink errors with debug! before falling back (appropriate since users can't do anything about them)
   • Automatically falls back to regular fs::copy() when reflinks fail
   • Uses an AtomicBool to track support so we don't retry reflink after detecting it's not working
   • Returns the number of bytes copied for progress tracking
   • Uses crate::Result for consistency with the rest of the codebase

3. Added debug logging:

   • Emits debug!() with error details when reflink fails
   • Final summary log includes reflink_used boolean field

4. Code quality improvements:

   • Uses short type names (Path, fs) instead of fully qualified names
   • All existing tests pass (11 copy_tree tests + 219 total tests)
   • No clippy warnings
   • Removed unused imports
   • Added explanatory comments about Windows behavior

5. Documentation:

   • Added entry to NEWS.md describing the reflink optimization

How It Works:

• On first file copy, attempts reflink
• If reflink succeeds, uses it for all subsequent files (very fast on supporting filesystems like Btrfs, XFS, APFS)
• If reflink fails with any error, marks it as unsupported and uses regular copy for all remaining files (handles Windows where reflink failures may not return ErrorKind::Unsupported)
• Debug logs show whether reflinks were used for the entire tree copy operation

Fixes #473

Original prompt

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This section details on the original issue you should resolve

<issue_title>Copy with reflinks</issue_title>
<issue_description>On large trees, where the temp dir is on the same filesystem as the source, copying with reflinks might be a little faster.

https://docs.rs/reflink/</issue_description>

<agent_instructions>In copy_tree, copy using reflinks, using the reflink crate. If reflinks aren't supported on the filesystem, fall back to a regular copy. If possible emit a debug! line saying whether they worked or not.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #473

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[sourcefrog]

@copilot Nice. Also mention this in NEWS.md.

[Copilot]

@copilot Nice. Also mention this in NEWS.md.

Added to NEWS.md in commit 7c3b7c7.

[Copilot]

Pull Request Overview

This PR implements reflink (copy-on-write) support for the tree copying operation to improve performance on filesystems that support it, with automatic fallback to regular copying when not supported.

• Adds reflink crate dependency for copy-on-write file operations
• Implements smart reflink detection with fallback mechanism using AtomicBool
• Adds debug logging to indicate reflink support status and usage

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/copy_tree.rs	Implements the core reflink functionality with copy_file helper and reflink support detection
NEWS.md	Documents the reflink optimization feature in the changelog
Cargo.toml	Adds reflink crate dependency

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[Copilot]

Consider using a more specific version constraint like reflink = \"0.1.3\" to match the version mentioned in the PR description, ensuring consistent dependency resolution.

Suggested change

	reflink = "0.1"
	reflink = "0.1.3"

[sourcefrog]

@copilot can you use crate::Result here instead?

[Copilot]

Changed to crate::Result in commit ea4c3f2.

[sourcefrog]

@copilot It looks like on Windows it can fail as unsupported without returning this error. Let's give up on using reflinks the first time we see an error of any kind, still emitting a debug message with the error. Add a comment about this.

[Copilot]

Changed to give up on reflinks after any error in commit a8f7b29. Added comment explaining Windows behavior and removed the now-unused ErrorKind import.