Temporarily print standard output to a custom text output stream in Swift

printingStandardOutput.swift

import Foundation

/// Additionally writes any data written to standard output into the given output stream.
///
/// - Parameters:
///   - output: An output stream to receive the standard output text
///   - encoding: The encoding to use when converting standard output into text.
///   - body: A closure that is executed immediately.
/// - Returns: The return value, if any, of the `body` closure.
func printingStandardOutput<T>(to output: inout TextOutputStream, encoding: String.Encoding = .utf8, body: () -> T) async -> T {
    var result: T? = nil
    
    let consumer = Pipe()  // reads from stdout
    let producer = Pipe()  // writes to stdout
    
    let stream = AsyncStream<Data> { continuation in
        let clonedStandardOutput = dup(STDOUT_FILENO)
        defer {
            dup2(clonedStandardOutput, STDOUT_FILENO)
            close(clonedStandardOutput)
        }
        
        dup2(STDOUT_FILENO, producer.fileHandleForWriting.fileDescriptor)
        dup2(consumer.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
        
        consumer.fileHandleForReading.readabilityHandler = { fileHandle in
            let chunk = fileHandle.availableData
            if chunk.isEmpty {
                continuation.finish()
            } else {
                continuation.yield(chunk)
                producer.fileHandleForWriting.write(chunk)
            }
        }
        
        result = body()
        try! consumer.fileHandleForWriting.close()
    }
    
    for await chunk in stream {
        output.write(String(data: chunk, encoding: encoding)!)
    }
    
    return result!
}