FileManager.copyItem() throws a permission error after having copied files (Ubuntu)

[sebsto]

When copying an entire directory where files are owned by root, FileManager.copyItem() throws a CocoaError.fileWriteNoPermission after having copied the files.

It works on macOS and it is reproductible on Ubuntu.

Version : Swift 6.0.3 on Ubuntu 24.04

Minimal code to reproduce

import FoundationEssentials

do {
    try FileManager.default.copyItem(
        atPath: "src",
        toPath: "dst"
    )
    print("Files copied successfully.")
} catch let error as CocoaError{
    print("Cocoa Error copying files: \(error)")
} catch {
    print("Other error copying files: \(error)")
}

and this shell script

#!/bin/bash 

sudo mkdir src 
sudo touch src/toto 
ls -al src
swift main.swift
ls -al dst

This produces

total 8
drwxr-xr-x 2 root   root   4096 Jan 20 07:24 .
drwxrwxr-x 3 ubuntu ubuntu 4096 Jan 20 07:24 ..
-rw-r--r-- 1 root   root      0 Jan 20 07:24 toto

Cocoa Error copying files: CocoaError(code: FoundationEssentials.CocoaError.Code(rawValue: 513), userInfo: ["NSURL": AnyHashable(dst -- file:///home/ubuntu/swift-aws-lambda-runtime/_REPRO/), "NSFilePath": AnyHashable("dst"), "NSUnderlyingError": AnyHashable(FoundationEssentials.POSIXError(code: Glibc.POSIXErrorCode.EPERM))])

total 8
drwxr-xr-x 2 ubuntu ubuntu 4096 Jan 20 07:24 .
drwxrwxr-x 4 ubuntu ubuntu 4096 Jan 20 07:24 ..
-rw-r--r-- 1 ubuntu ubuntu    0 Jan 20 07:24 toto

Note that the source files have perm 0o644, they are readable by non-root users. Copying them must succeed and the target files must be owned by the user running the swift script. This is the case here, but FileManager.copyItem() throws an error after the copy operation.

[yretenai]

copyItem eventually calls a method that invokes fchown which tries to set ownership back to root which is not permitted.

Main:

swift-foundation/Sources/FoundationEssentials/FileManager/FileOperations.swift

Lines 991 to 994 in 5ed6c72

	// Copy owner/group
	if fchown(dstFD, statInfo.st_uid, statInfo.st_gid) != 0 {
	try delegate.throwIfNecessary(errno, srcPath(), dstPath())
	}

Older:

swift-foundation/Sources/FoundationEssentials/FileManager/FileOperations.swift

Lines 968 to 973 in b0abbb4

	do {
	let attributes = try fileManager.attributesOfItem(atPath: String(cString: fts_path))
	try fileManager.setAttributes(attributes, ofItemAtPath: String(cString: buffer.baseAddress!))
	} catch {
	try delegate.throwIfNecessary(error, String(cString: fts_path), String(cString: buffer.baseAddress!))
	}

relates to swiftlang/swift-docc#1136 and swiftlang/swift-docc-plugin#102

[jmschonfeld]

Thanks @yretenai yeah that makes sense - the reason we don't hit this on Darwin is because on Darwin we use copyfile in order to copy the metadata over, and it looks like copyfile's behavior does not preserve the owner of the source in the destination copy, whereas the Linux implementation does attempt to preserve the source owner (at least from experimentation). We'll likely need to confirm the behavior of copyfile and update the Linux implementation to match the behavior of copyfile so we have parity across the platforms

[sebsto]

@jmschonfeld retaining file ownership during a copy is not how Linux behaves.

Other utilities, such as cp, transfer ownership of the files to the user issuing the cp command.
Here is an example.

$ id
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),4(adm),24(cdrom),27(sudo),30(dip),105(lxd),113(docker)

$ sudo mkdir src && sudo touch src/toto 

$ ls -al src 
total 8
drwxr-xr-x 2 root   root   4096 Jan 22 09:38 .
drwxrwxr-x 3 ubuntu ubuntu 4096 Jan 22 09:38 ..
-rw-r--r-- 1 root   root      0 Jan 22 09:38 toto

$ cp -r src dst 

$ ls -al dst 
total 8
drwxr-xr-x 2 ubuntu ubuntu 4096 Jan 22 09:39 .
drwxrwxr-x 4 ubuntu ubuntu 4096 Jan 22 09:39 ..
-rw-r--r-- 1 ubuntu ubuntu    0 Jan 22 09:39 toto

[yretenai]

In this context, Linux simply does not have a libc method to copy files. It has one for renaming which essentially is just a move but no copyfile equivalent. I do agree that Swift should follow the principle of least astonishment and keep it in line with expected behaviors.

Fortunately, that seems to line up with Darwin's libc copyfile.