fix(prepare): store temporary query files inside the workspace

[aschey]

Resolves #2395

Thanks to @cycraig for the advice here: #2395 (comment)

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" }).

To prevent this, we can't rely on using the default temp directory. This PR changes the logic to use NamedTempFile::new_in to create the temporary files in a new variable called SQLX_TMP. When run from cargo sqlx prepare, this is set to $TARGET_DIR/sqlx-tmp.

The point of the SQLX_TMP variable is to ensure the temporary files are written to a directory that isn't picked up by git. As long as everything completes successfully, this shouldn't really matter as the temp files will get cleaned up automatically on a clean exit. Ideally, we wouldn't need this and could instead just reference the target directory directly, but that would require a call to cargo metadata from sqlx-macros-core.

I'm not too familiar with this logic so please let me know if there's a better way to handle this. Just thought I'd take a stab at it, thanks!

[abonander]

I would definitely prefer if it defaulted to target/, I don't want to require users to have to set another temporary variable.

I believe the working directory for proc macro execution is the workspace root, so we could just default to target/sqlx/. I don't see any reason not to provide an environment variable override, I just don't want it to be required. If CARGO_TARGET_DIR is set we should also respect that.

[abonander]

The other option is to use the .sqlx directory or a subdirectory but default-generate a .gitignore file that excludes the temporary directory or files (using, e.g. .json.tmp as an extension). That might play better with non-Cargo build setups or weird configurations.

[aschey]

Okay, made the change to fall back to CARGO_TARGET_DIR and then to ./target/sqlx. As long as the process doesn't panic in the short time between creating the temp file and the variable going out of scope, I think there's a pretty small chance these temp files don't get cleaned up, so I think this is pretty low risk. With that in mind, I went with the simpler solution, but please let me know if you'd like me to change anything else.