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