[SR-1941] Dispatch object creation can return nil despite non-nil type annotations

[swift-ci]

Previous ID	SR-1941
Radar	None
Original Reporter	miken (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug
Assignee	mww (JIRA)
Priority	Medium

md5: d1490ba04e755f83c340b8874532c6bf

Issue Description:

Attempting to create a dispatch source or I/O channel is a fallible operation. In Swift 2.2 the functions used for creating them are annotated as returning implicitly-unwrapped optionals, and nil is returned when creation fails. This can be checked for and handled appropriately.

In Swift 2.3 and 3.0 the functions/initializers are annotated as returning non-nil values, but nil can still be returned. This can be reproduced by providing invalid parameters.

The code below will run successfully under Swift 2.2, but will crash at runtime under 2.3 and 3.0 despite the lack of any forced or implicit unwrapping.

#if swift(>=3.0)
    
    let channel = DispatchIO(type: .stream, path: "../bad.path", oflag: O_RDONLY, mode: S_IRUSR, queue: .main) { (error) in
        print(error)
    }
    print(channel) // Crash
    
    let source = DispatchSource.signal(signal: Int32.min)
    print(source) // Crash
    
#elseif swift(>=2.3)
    
    let channel = dispatch_io_create_with_path(DISPATCH_IO_STREAM, "../bad.path", O_RDONLY, S_IRUSR, dispatch_get_main_queue()) { (error) in
        print(error)
    }
    print(channel) // Crash
    
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, UInt.max, 0, dispatch_get_main_queue())
    print(source) // Crash
    
#else
    
    let channel = dispatch_io_create_with_path(DISPATCH_IO_STREAM, "../bad.path", O_RDONLY, S_IRUSR, dispatch_get_main_queue()) { (error) in
        print(error)
    }
    print(channel ?? "Nil") // Nil
    
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, UInt.max, 0, dispatch_get_main_queue())
    print(source ?? "Nil") // Nil
    
#endif

[belkadan]

cc das (JIRA User)

Workaround: immediately assign the result of the initializer into an optional.

[swift-ci]

Comment by Daniel A. Steffen (JIRA)

This was intentional and follows the rules established by Foundation for nullability annotation of return values.

The only time source object creation can return nil is on programmer error by passing bad input arguments that are non-sensical or contradictory (like in your example with a nonsensical signal number). It never returns nil for conditions that cannot be controlled by the programmer locally at the call site.

Similarly for dispatch_io, path-based creation requires an absolute path and so when that is not the case we treat that input as programmer error. The only other case treated this way is a bad channel type (c.f. source code, all these cases are expressed as DISPATCH_BAD_INPUT)