Processes with small and infrequent output doesn't get emitted in a timely manner when using sequence to capture the output

[rdingman]

Describe the bug
I have a process I want to monitor that is long running. It has small and infrequent output that I need to get in as soon as it is available. I'm running this process with swift-subprocess and monitoring its output by using SequenceOutput. In the custom execution closure, I only get the output either when the process exits or when the output size reaches readBufferSize defined in AsyncBufferSequence.

To Reproduce

Here is some sample code that demonstrates the problem:

let script = """
echo "[NOTICE] Important log line"
sleep 3600
"""

let catResult = try await Subprocess.run(
    .path("/bin/bash"),
    arguments: ["-c", script],
    output: .sequence,
    error: .discarded,
    body: { (execution, _) in
        for try await chunk in execution.standardOutput {
            let string = chunk.withUnsafeBytes { String(decoding: $0, as: UTF8.self) }

            if string.hasPrefix("[NOTICE]") {
                // Do Something interesting
                print(string)
            }
        }

        return ""
    }
)

Expected behavior

There should be a mechanism for getting the output as soon as it is available.

Environment (please complete the following information):

• macOS 15
• swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)

Additional context

I'll follow up with a PR that has the sketch for a proposed solution.

[marlimox]

+1

I've also run into this issue with a project I'm working on that uses Subprocess. Excited to see you're working on a fix 😊