Feature request: support in-memory XMP reading/writing or client-managed I/O

[finnbear]

I want to read/write XMP metadata from/to an in-memory buffer containing an image file, instead of writing my image to the file system and forcing this library to re-read parts of it.

Right now, the only way to create an XmpFile is to use a file path with XmpFile::open_file. The C++ SDK supports an aternative, for files not in the file system, called client-managed I/O. This is described on page 52 of the documentation.

This feature request is to either expose client-managed I/O or to internally use client-managed I/O to expose in-memory API's to read/write XMP metadata.

[scouten-adobe]

Two questions:

1. Would it suffice to have XMP_IO map to a rust Read + Seek trait pair or are there other features of XMP_IO that you would need.
2. Would you be interested in submitting a PR to implement this feature? I'm not saying we won't do this ourselves at some point, but it's not likely to get attention quickly, given other priorities at the moment.

[finnbear]

Thanks for the quick response!

1. I originally thought that could work but, due to how XMP_IO is expected to be able to create a new temporary file (indefinitely nested), a Read + Seek + Write implementation is not viable. I think the next best option is exposing the entire XMP_IO API as a Rust trait or similar.
2. I started working on a PR. Currently stuck on handling C-style memory management in Rust, Rust dynamic dispatch in C/C++, and Rust error handling in C/C++. I will continue working but I estimate a below 50% chance of success. (if I limited the API to assume use of in-memory buffers, I could do more of the implementation in C/C++ and avoid almost all these issues, but then Rust couldn't use, e.g., a network filesystem)

(I've also written a workaround for my use-case, that writes the buffer to a filesystem temp file, calls XMP, and then re-reads the temp file to memory. As such, this issue/PR would serve as a performance optimization)

[scouten-adobe]

OK, good catch. I look forward to the PR!