0.7.0-alpha.1: `error: failed to move query file` when running `cargo sqlx prepare`

[aschey]

This issue appears on the latest commit to main at the time of writing this.

Bug Description

The PR to implement one file per query: #2363 implemented logic to write queries to a temp file and then persist them to SQLX_OFFLINE_DIR here:

sqlx/sqlx-macros-core/src/query/data.rs

Lines 163 to 170 in c03926c

	let mut tmp_file = tempfile::NamedTempFile::new()
	.map_err(|err| format!("failed to create query file: {:?}", err))?;
	serde_json::to_writer_pretty(tmp_file.as_file_mut(), self)
	.map_err(|err| format!("failed to serialize query data to file: {:?}", err))?;
	tmp_file
	.persist(dir.as_ref().join(format!("query-{}.json", self.hash)))
	.map_err(|err| format!("failed to move query file: {:?}", err))?;

However, the docs for NamedTempFile::persist mention the following:

Note: Temporary files cannot be persisted across filesystems.

On Linux, the /tmp directory is commonly mounted as tmpfs which means the call to tmp_file.persist will result in the following error: failed to move query file: PersistError(Os { code: 18, kind: CrossesDevices, message: "Invalid cross-device link" }).

Perhaps we could use tempfile::NamedTempFile::new_in(dir.as_ref()) to create the temporary files directly in SQLX_OFFLINE_DIR to ensure we're not crossing any filesystem boundaries?

Minimal Reproduction

Run cargo sqlx migrate on a Linux system with /tmp mounted separately from the main OS.

Info

• SQLx version: 0.7.0-alpha.1
• SQLx features enabled: "sqlite", "any", "macros", "runtime-tokio"
• Database server and version: SQLite
• Operating system: Arch Linux
• rustc --version: rustc 1.68.0 (2c8cc3432 2023-03-06)

[cycraig]

Darn. Temporary query files were previously stored under a subdirectory of target; we could also switch back to that to prevent potentially polluting a git project?

To give some context here, I used tempfile in #2363 for two reasons:

1. The target path was sometimes relative instead of absolute, causing errors. This was partially solved by overriding CARGO_TARGET_DIR when invoking cargo manually in sqlx prepare. Some developers specify the target manually, so to avoid conflicts I replaced that behaviour with tempfile.
2. The previous temporary query-{}.json filenames used a hash of the input span. This was meant to be unique but conflicted when compiling multiple targets as once, e.g. test or --all-targets. tempfile generates random names anyway so I chose not to reinvent the wheel.

Point being, using tempfile::NamedTempFile::new_in sounds good to me.

For a workaround in the interim, setting the TMPDIR environment variable to somewhere other than /tmp should control where tempfile outputs.

[aschey]

@cycraig thanks for the background info. I took a stab at a fix here: #2398. Please let me know if this is what you had in mind or if there's a better way, thanks!