Implement statfs with synthetic values

[c-hagem]

Description of change

This PR adds support for calling statfs on virtual file system created using mountpoint.
Some applications depend on the filesystem reporting non-zero available space; currently mountpoint reports 0 as number of available blocks, which can cause these applications to not work as expected.

This PR (building on #871) implements statfs with synthetic values (4611686018427387904 free blocks).
For example, the DF output now is:

mountpoint-s3  4611686018427387904        0 4611686018427387904   0% /local/home/chagem/mnt/bucket

Thus, checks for available space should no longer fail.

Relevant issues: #710.
This change impacts existing behaviour, as Mountpoint will report non-zero value for total blocks, free blocks, free inodes and maximum file name length.

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[dannycjones]

Nice! Awesome to see this working.

There are a few comments inline.

Can you add some tests for this, just to verify these values make it out of the file system? I'd recommend creating a new file in mountpoint-s3/tests/fuse_tests/ named statfs_test.rs, and write some tests that test end-to-end. statvfs isn't actually in the Rust standard library, but we have some example of using a crate named nix for querying this information.

mountpoint-s3/mountpoint-s3/src/data_cache/disk_data_cache.rs

Lines 308 to 321 in 9206ed4

	CacheLimit::AvailableSpace { min_ratio } => {
	let stats = match nix::sys::statvfs::statvfs(&self.cache_directory) {
	Ok(stats) if stats.blocks() == 0 => {
	warn!("unable to determine available space (0 blocks reported)");
	return false;
	}
	Ok(stats) => stats,
	Err(error) => {
	warn!(?error, "unable to determine available space");
	return false;
	}
	};
	(stats.blocks_free() as f64) < min_ratio * (stats.blocks() as f64)
	}

[dannycjones]

I'm not sure where any boundaries may lie, but I think we can take this down by 1024 at least. (In this case, I think we are fine to adjust these values in the future.)

[c-hagem]

Fixed it, assuming taking down means / 1024 instead of / 2.

[dannycjones]

nit: didn't catch this last time, but we always put Rustdoc comments above macros (so its clear what macros are applied to the struct)

Suggested change

	#[derive(Debug)]
	/// Reply to a 'statfs' call
	pub struct StatFs {
	/// Total number of blocks
	pub total_blocks: u64,
	/// Number of free blocks
	pub free_blocks: u64,
	/// Number of free blocks available to unprivileged user
	pub available_blocks: u64,
	/// Number of inodes in file system
	pub total_inodes: u64,
	/// Available inodes
	pub free_inodes: u64,
	/// Optimal transfer block size
	pub block_size: u32,
	/// Maximum name length
	pub maximum_name_length: u32,
	/// Fragement size
	pub fragment_size: u32,
	}
	/// Reply to a 'statfs' call
	#[derive(Debug)]
	pub struct StatFs {
	/// Total number of blocks
	pub total_blocks: u64,
	/// Number of free blocks
	pub free_blocks: u64,
	/// Number of free blocks available to unprivileged user
	pub available_blocks: u64,
	/// Number of inodes in file system
	pub total_inodes: u64,
	/// Available inodes
	pub free_inodes: u64,
	/// Optimal transfer block size
	pub block_size: u32,
	/// Maximum name length
	pub maximum_name_length: u32,
	/// Fragement size
	pub fragment_size: u32,
	}

non-blocking

[dannycjones]

What is mpst, is it typo for most?

[dannycjones]

I'd drop these, but actually maybe we just combine with the test above. No need to separate them.

[dannycjones]

drop commented out code

[dannycjones]

test case is wrong

Suggested change

	#[cfg(feature = "s3_tests")]
	#[test_case(""; "no prefix")]
	#[test_case("statfs_report_nonzero_test"; "prefix")]
	fn statfs_report_fuser_defaults_s3(prefix: &str) {
	statfs_test_available_nonzero(fuse::s3_session::new, prefix);
	}
	#[cfg(feature = "s3_tests")]
	#[test_case(""; "no prefix")]
	#[test_case("statfs_report_fuser_defaults"; "prefix")]
	fn statfs_report_fuser_defaults_s3(prefix: &str) {
	statfs_report_fuser_defaults(fuse::s3_session::new, prefix);
	}