Hey swift-evolution,

First of all apologies, this is not a full proposal yet, it's meant to kick off 
a discussion on how to resolve the issue.

# Make `errno`-setting functions more usable from Swift

## Introduction

This is a pitch to make [`errno`][1]-setting functions properly usable, as in 
having a guarantee to get the correct `errno` value on failure of a [system 
call][2]. Currently, functions which set `errno` are just exported in the 
Darwin/Glibc modules with (as far as I understand) no guaranteed correct way of 
handling errors as the correct `errno` value can't be retrieved.
This means that much of the Swift code which uses Darwin/Glibc out there relies 
on behaviour that isn't guaranteed.


## Motivation

In many Swift libraries that use the Darwin/Glibc modules there is code similar 
to:

```
/* import Darwin/Glibc */

let rv = some_system_call(some, parameters)
if rv < 0 {
   throw SomeError(errorCode: errno) /* <-- errno use */
}
```

That looks very innocent but please note that `errno` is used here. And `errno` 
is an interesting one as it's a thread-local variable which is written to by 
many functions. A thread-local variable is like a global variable except that 
setting it in one thread does not affect its value in any other thread. Pretty 
much all system calls and many library functions set `errno` if something went 
wrong.

The problem is that as far as I see (and Swift developers have confirmed), 
there is no guarantee that in between the call of `some_system_call` and the 
reading of `errno`, `errno` hasn't been overwritten by some other system call 
that has been call on the same thread.

To illustrate this further, let's consider this example

```
/* import Darwin/Glibc */
public class SomeClass {
  public let someValue: Int = 1
  deinit {
      /* call some failing syscall, for example */
      write(-1, nil, 0) /* should set errno to EBADF */
  }
}

public func foo() {
   let x = SomeClass()
   let rv = write(x.someValue, nil, 0)
   let errnoSave = errno
   if rv != 0 {
      throw SomeError(errorCode: errnoSave)
   }
}
```

as you see in function `foo`, the instance `x` of `SomeClass` isn't needed 
anymore as soon as `write` has been called. So (as far as I understand) there's 
no guarantee that ARC doesn't turn the above code into

```
let x = SomeClass()
let rv = write(x.someValue, nil, 42) /* should set errno to EFAULT */
/* ARC generated */ x.release()
let errnoSave = errno /* wrong errno value :( */
if rv != 0 {
  throw SomeError(errorCode: errnoSave)
}
```

And the ARC generated `x.release()` will cause `x` to be deallocated which will 
call the failing `write` in the `deinit` of `SomeClass`. So `errnoSave` might 
be `EBADF` instead of `EFAULT` depending on where ARC put the `x.release()` 
call.

What `errno` value we read will depend on the optimisation settings and the 
Swift compiler version. That's IMHO a big issue as it might make the lowest 
layers unstable with hard-to-debug issues.


## Proposed solution

I don't have a full story on how to actually resolve the issue but I see a few 
options:

### Option 1: always return errno

clang importer could be changed to make all `errno`-setting functions return a 
tuple of the actual return value and the `errno` value.

For example, currently write(2) is imported as:

```
public func write(_ __fd: Int32, _ __buf: UnsafeRawPointer!, _ __nbyte: Int) -> 
Int
```

which could be changed to

```
 public func write(_ __fd: Int32, _ __buf: UnsafeRawPointer!, _ __nbyte: Int) 
-> (Int, Int32 /* for errno */)
```

Correct code to use write would then look like this:

```
let (bytesWritten, writeErrno) = write(fd, buf, len)
if bytesWritten >= 0 {
   /* everything's fine */
} else {
   throw POSIXError(code: writeErrno)
}
```


### Option 2: make them throw

The second option is to teach clang importer to make the functions throwing. So 
write(2) would be imported as

```
public func write(_ __fd: Int32, _ __buf: UnsafeRawPointer!, _ __nbyte: Int) 
throws /* POSIXError */ -> Int
```

That would make these functions quite easy to use and would feel natural in 
Swift:

```
do {
   let bytesWritten = write(fd, buf, len)
} catch let e as POSIXError {
   /* handle error */
} catch {
   ...
}
```


### Discussion

The beauty of option 1 is simplicity. Clang importer would not need to know 
what exact values a system call returns on failure. Also very little additional 
code needs to be emitted for calling a system call. That seems to be the [way 
Go is going][3].

The downside of option 1 is that the API doesn't feel like idiomatic Swift. The 
returned `errno` value is only useful if the system call failed and is 
arbitrary in the case when it worked. (There is no guarantee that `errno` is 
set to `0` when a system call succeeds.)
Also there is a slight overhead in reading `errno` which would be paid for 
every `errno`-setting function, even if successful. Hence, option 2 looks nice 
as it brings these functions more in like with other Swift functions. However, 
as mentioned before, clang importer would need to learn what values are 
returned on success/failure for _every_ `errno`-setting function (and there's 
_many_ of them).


## Proposed Approach

Let's discuss what is a good solution and then I will volunteer to put together 
a full proposal.


## Source compatibility

it depends.

To retain source compatibility the Darwin/Glibc modules could also be left as 
is. The safe `errno` handling could then be implemented only in a new, unified 
module for Darwin/Glibc. There's already ongoing discussions/proposals about 
that on the list anyway. That new module could then be implemented in the 
spirit of options 1, 2, or some other solution. The benefits are guaranteed 
source compatibility for legacy applications and `errno` safety plus easier 
imports for new applications - win/win 🙂.


## Effect on ABI stability

Will most likely be additive, so probably none.


## Effect on API resilience

see source compatibility.


## Alternatives considered

Do nothing and workaround `errno` capturing being very hard. I discussed this 
previously elsewhere and Joe Groff came up with the code below which should 
convince the optimiser not to insert any release calls at the wrong place or 
inline the function:

```
@inline(never)
func callWithErrno(_ fn: () -> Int) -> (result: Int, errno: Int) {
   var result: Int
   var savedErrno: Int
   withExtendedLifetime(fn) {
       result = fn()
       savedErrno = errno
   }
   return (result, savedErrno)
}
```

An example use of that is

```
let (rv, writeErrno) = callWithErrno {
   write(-1, nil, 0)
}

if rv < 0 {
   throw SomeError(errorCode: writeErrno)
}
```

This makes it possible to retrieve the correct `errno` value in Swift but I 
think there remain too many ways to do it wrongly. First and foremost that the 
compiler doesn't complain if the programmer forgets to use `callWithErrno`.

===

Let me know what you think!

[1]: https://en.wikipedia.org/wiki/Errno.h
[2]: https://en.wikipedia.org/wiki/System_call
[3]: https://golang.org/pkg/syscall/#Write

Many thanks,
 Johannes
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