"Invalid cross-device link" when generating query files

[lheckemann]

Bug Description

When CARGO_TARGET_DIR is not on the same filesystem as the project source, cargo sqlx prepare fails:

error: failed to move query file: PersistError(Os { code: 18, kind: CrossesDevices, message: "Invalid cross-device link" })

Minimal Reproduction

(on linux)

1. Mount a tmpfs somewhere, e.g. sudo mount -o X-mount.mkdir /tmp/cargo-target-dir
2. Use this as CARGO_TARGET_DIR while running cargo sqlx prepare.

Info

• SQLx version: 0.7.0
• SQLx features enabled: features = ["postgres", "runtime-tokio-rustls", "chrono", "uuid"]
• Database server and version: postgres (PostgreSQL) 14.8
• Operating system: NixOS
• rustc --version: rustc 1.71.0 (8ede3aae2 2023-07-12)

[lheckemann]

Similar to #2395.

[mattfbacon]

I'm also encountering this issue. The whole idea of using a temporary file and then persisting it doesn't make any sense to me. Why not just serialize the data into memory and then write it out directly to the "persisted" location? I'll look into making a PR to do that instead.

[abonander]

@mattfbacon

Why not just serialize the data into memory and then write it out directly to the "persisted" location?

Because it is the recommended way to ensure the write is atomic. If the same query appears in multiple places, then multiple instances of the macro may attempt to write the data file at the same time. This may be a non-issue for invocations within the same crate as macros are expanded serially, but could be an issue for concurrent compiler invocations when building a multi-crate project.

[mattfbacon]

That can be handled with the O_EXCL flag to open.

[abonander]

The manpage for open() says of O_EXCL:

When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then open() fails regardless of where the symbolic link points.

Does this apply if the parent directory is a symbolic link as might also happen?

[mattfbacon]

No, it works fine in that case. Also, it's in the Rust stdlib as OpenOptions::create_new.

[mattfbacon]

I just pushed a PR that uses OpenOptions::create_new.