Use readData to read data from a pipe instead of availableData

[ahoppen]

Using availableData causes parallel XCTest execution to hang on Windows if the tests output significant data to stderr.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows

[dschaefer2]

I think we all have scars from using availableData to read stdout/stderr. Thanks @ahoppen!

[jakepetroules]

Are you sure this will work?. availableData just calls the same underlying API as read(upToCount:), and it still has the ability to block the calling thread.

    private func _checkFileHandle() {
        precondition(self._handle != INVALID_HANDLE_VALUE, "Invalid file handle")
    }

    open var availableData: Data {
        _checkFileHandle()
        do {
            let readResult = try _readDataOfLength(Int.max, untilEOF: false)
            return readResult.toData()
        } catch {
            fatalError("\(error)")
        }
    }

    public func read(upToCount count: Int) throws -> Data? {
        guard self != FileHandle._nulldeviceFileHandle else { return nil }
        
        let result = try _readDataOfLength(count, untilEOF: true)
        return result.length == 0 ? nil : result.toData()
    }

[ahoppen]

Yes, I tested that we no longer see the hang when using read(upToCount:). I don’t know why it fixes it but 🤷🏽 maybe it’s because one of them does untilEOF: true and the other one untilEOF: false.

[dschaefer2]

Yes, I tested that we no longer see the hang when using read(upToCount:). I don’t know why it fixes it but 🤷🏽 maybe it’s because one of them does untilEOF: true and the other one untilEOF: false.

That feels true. The hangs I've seen have been at the end of the stream where EOF isn't properly detected.