[SR-14953] Data.write(to:) fails when writing to /dev/stdout

[weissi]

Previous ID	SR-14953
Radar	rdar://problem/80949531
Original Reporter	@weissi
Type	Bug

Environment

5.4/main docker image

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: 98334e709b9a35bb96b5b5e9f20f864e

Issue Description:

try Data("hello".utf8).write(to: URL(fileURLWithPath: "/dev/stdout"))

will fail on Linux with

NSError = domain: nil - code: 14987979559889010706 {
  Foundation.NSObject = {}
  _domain = "NSCocoaErrorDomain"
  _code = 512
  _userInfo = 1 key/value pair {
    [0] = {
      key = "NSUnderlyingError"
      value = domain: nil - code: 14987979559889010706 {
        Foundation.NSObject = {}
        _domain = "NSPOSIXErrorDomain"
        _code = 22
        _userInfo = nil
      }
    }
  }
}

Or a more comprehensive program

import Foundation

let target = CommandLine.arguments.dropFirst().first ?? "/dev/stdout"
do {
    try Data("hello".utf8).write(to: URL(fileURLWithPath: target))
} catch {
    print("ERROR: \(error)")
}

when run under Linux will print

helloERROR: Error Domain=NSCocoaErrorDomain Code=512 "(null)"

note how it actually succeeds in writing the data.

As of osstatus.com, the error is NSFileWriteUnknownError.

The problem seems to be that we call fsync on the file descriptor after the write which isn't valid for most devices:

openat(AT_FDCWD, "/dev/stdout", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
write(3, "hello", 5hello)                    = 5
fsync(3)                                = -1 EINVAL (Invalid argument)

probably originating from here: https://github.com/apple/swift-corelibs-foundation/blob/cc5fc93a9946e82d659f79e1bd991b96eba3fcdf/Sources/Foundation/NSData.swift#L446

[weissi]

CC @millenomi/@tomerd/@spevans

[weissi]

@swift-ci create

[spevans]

#3022

[weissi]

This is fixed in 5.6:

$ cat repro.swift 
import Foundation
try Data("hello".utf8).write(to: URL(fileURLWithPath: "/dev/stdout"))

johannes:/tmp
$ jw-docker-swift-5.6 swift repro.swift
hellojohannes:/tmp
$