Commit fd10baf748aeb3b6d2394701d735098e64638a22 | zig

author	Motiejus Jakštys <motiejus@jakstys.lt> 2022-08-19 14:53:00+03:00
committer	Andrew Kelley <andrew@ziglang.org> 2022-10-18 12:57:21-04:00
log	fd10baf748aeb3b6d2394701d735098e64638a22
tree	cf68a2365532f691b47a4e8171747474db983ef8
parent	5ec76cf5c84f3863618e220ce41d09cc9781af8a

os.copy_file_range: save a syscall for most operations

Currenty copy_file_range always uses at least two syscalls:

1. As many as it needs to do the initial copy (always 1 during my
   testing)
2. The last one is always when offset is the size of the file.

The second syscall is used to detect the terminating condition. However,
because we do a stat for other reasons, we know the size of the file,
and we can skip the syscall.

Sparse files: since copy_file_range expands holes of sparse files, I
conclude that this layer was not intended to work with sparse files. In
other words, this commit does not make it worse for sparse file society.

Test program
------------

    const std = @import("std");

    pub fn main() !void {
        const arg1 = std.mem.span(std.os.argv[1]);
        const arg2 = std.mem.span(std.os.argv[2]);
        try std.fs.cwd().copyFile(arg1, std.fs.cwd(), arg2, .{});
    }

Test output (current master)
----------------------------

Observe two `copy_file_range` syscalls: one with 209 bytes, one with
zero:

    $ zig build-exe cp.zig
    $ strace ./cp ./cp.zig ./cp2.zig |& grep copy_file_range
    copy_file_range(3, [0], 5, [0], 4294967295, 0) = 209
    copy_file_range(3, [209], 5, [209], 4294967295, 0) = 0
    $

Test output (this diff)
-----------------------

Observe a single `copy_file_range` syscall with 209 bytes:

    $ /code/zig/build/zig build-exe cp.zig
    $ strace ./cp ./cp.zig ./cp2.zig |& grep copy_file_range
    copy_file_range(3, [0], 5, [0], 4294967295, 0) = 209
    $

1 files changed, 10 insertions(+), 4 deletions(-)

lib
- std
  - fs.zig

lib/std/fs.zig+10-4

...	...	@@ -2562,7 +2562,7 @@ pub const Dir = struct {
2562	2562	var atomic_file = try dest_dir.atomicFile(dest_path, .{ .mode = mode });
2563	2563	defer atomic_file.deinit();
2564	2564
2565		try copy_file(in_file.handle, atomic_file.file.handle);
	2565	try copy_file(in_file.handle, atomic_file.file.handle, size);
2566	2566	try atomic_file.finish();
2567	2567	}
2568	2568
...	...	@@ -3041,7 +3041,7 @@ const CopyFileRawError = error{SystemResources} \|\| os.CopyFileRangeError \|\| os.S
3041	3041	// Transfer all the data between two file descriptors in the most efficient way.
3042	3042	// The copy starts at offset 0, the initial offsets are preserved.
3043	3043	// No metadata is transferred over.
3044		fn copy_file(fd_in: os.fd_t, fd_out: os.fd_t) CopyFileRawError!void {
	3044	fn copy_file(fd_in: os.fd_t, fd_out: os.fd_t, maybe_size: ?u64) CopyFileRawError!void {
3045	3045	if (comptime builtin.target.isDarwin()) {
3046	3046	const rc = os.system.fcopyfile(fd_in, fd_out, null, os.system.COPYFILE_DATA);
3047	3047	switch (os.errno(rc)) {
...	...	@@ -3064,7 +3064,10 @@ fn copy_file(fd_in: os.fd_t, fd_out: os.fd_t) CopyFileRawError!void {
3064	3064	// a 32 bit value so that the syscall won't return EINVAL except for
3065	3065	// impossibly large files (> 2^64-1 - 2^32-1).
3066	3066	const amt = try os.copy_file_range(fd_in, offset, fd_out, offset, math.maxInt(u32), 0);
3067		// Terminate when no data was copied
	3067	// Terminate as soon as we have copied size bytes or no bytes
	3068	if (maybe_size) \|s\| {
	3069	if (s == amt) break :cfr_loop;
	3070	}
3068	3071	if (amt == 0) break :cfr_loop;
3069	3072	offset += amt;
3070	3073	}
...	...	@@ -3077,7 +3080,10 @@ fn copy_file(fd_in: os.fd_t, fd_out: os.fd_t) CopyFileRawError!void {
3077	3080	var offset: u64 = 0;
3078	3081	sendfile_loop: while (true) {
3079	3082	const amt = try os.sendfile(fd_out, fd_in, offset, 0, &empty_iovec, &empty_iovec, 0);
3080		// Terminate when no data was copied
	3083	// Terminate as soon as we have copied size bytes or no bytes
	3084	if (maybe_size) \|s\| {
	3085	if (s == amt) break :sendfile_loop;
	3086	}
3081	3087	if (amt == 0) break :sendfile_loop;
3082	3088	offset += amt;
3083	3089	}