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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