[swift-evolution] [Pitch] Make `errno`-setting functions more usable from Swift

[Johannes Weiß]

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