Add read-only directory support

[shashankbrgowda]

Summary

Adds ReadOnlyDirectory for opening an immutable index without requiring write access to its directory.

• Passes read operations to the underlying directory.
• Rejects write and delete operations.
• Skips META_LOCK.
• Rejects other locks, including the writer lock.
• Disables filesystem watching.

Two convenience methods are included:

Index::open_read_only(directory)
Index::open_read_only_in_dir(path)

Motivation

IndexReader acquires META_LOCK while loading segment readers. This requires write access to the index directory, even when the index is immutable and used only for searching.

ReadOnlyDirectory skips this lock so an index can be used from a read-only filesystem.

A custom Directory wrapper can provide similar behaviour in Rust, but this workaround is not available to users of bindings such as tantivy-py, because the Directory interface is not exposed to Python. Providing ReadOnlyDirectory in Tantivy allows bindings to expose the same behaviour without maintaining their own implementation.

The underlying index must remain unchanged while it is open. This mode must not be used while another process is writing to or garbage-collecting the same index.

Addresses #557 #557 (comment)

Testing

Tests cover:

• Reading from the wrapped directory.
• Rejection of writes and deletes.
• Metadata and writer lock behaviour.
• Searching and manually reloading an index.
• Opening a real read-only filesystem directory.
• Opening the same index with multiple readers.
• Verifying that .tantivy-meta.lock is not created.

Built with GPT-5.6 Sol in the Codex desktop harness.

[fulmicoton]

This is a just a read only directory, and that functionality can leave outside of tantivy, so this is just about including batteries, correct?

I would call it immutable more than read only maybe.
Read only could mean "I do not write but there is a writer in a different process".
This is not compatible with this.

[shashankbrgowda]

Thanks for the feedback @fulmicoton .

Correct, this is mainly about including the batteries. Rust users can implement a custom directory wrapper outside tantivy, but tantivy-py users cannot provide their own directory implementation. Including this in tantivy would allow the python bindings to open a static index from a read only filesystem.

I agree Immutable communicates the api better than read only. Shall I rename the directory and methods accordingly ?

This is a just a read only directory, and that functionality can leave outside of tantivy, so this is just about including batteries, correct?

I would call it immutable more than read only maybe. Read only could mean "I do not write but there is a writer in a different process". This is not compatible with this.