bensadeh/tailspin · error · anyhow::Error

Could not capture stdout of spawned process

Error message

Could not capture stdout of spawned process

What it means

This error is thrown when the spawned child process has no piped stdout to capture. The library sets stdout(Stdio::piped()) before spawning, so take_stdout() returning None indicates the pipe could not be established or was already consumed — without it the CommandReader cannot stream output.

Solutions

1. Verify .stdout(Stdio::piped()) is called on the Command before spawning
2. Ensure take_stdout() is called exactly once and before waiting on the child
3. Check no wrapper/indirection redirects the child's stdout away from the pipe
4. If the error persists, log the spawn configuration and upgrade/patch the shared-child usage

Example fix

// before
let mut sh = Command::new(shell); // no stdout configuration
// after
sh.arg("-c").arg(trap_command).stdout(Stdio::piped());

// Before constructing the reader, confirm the command will have a piped stdout:
let mut sh = Command::new("sh");
sh.arg("-c").arg(cmd).stdout(Stdio::piped());
assert!(sh.get_stdout().is_some() || std::env::var("CI").is_ok(),
        "stdout must be piped before spawn");

let reader = CommandReader::new(cmd).map_err(|e| {
    if e.to_string().contains("Could not capture stdout") {
        eprintln!("stdout pipe missing; check Stdio config");
    }
    e
})?;

When it happens

Trigger: Calling CommandReader::new (via spawn_command) when SharedChild::spawn succeeded but child.take_stdout() returns None. Practically this happens only if the stdout pipe was not set to piped before spawn or was taken elsewhere.

Common situations: Code changes that remove or reorder the .stdout(Stdio::piped()) call; spawning a command whose stdout is redirected to a file or /dev/null by an outer wrapper; duplicated take_stdout() calls in modified code.

Related errors

• --exec command failed

AI-assisted analysis of bensadeh/tailspin@8ecaa9a8a1 (2026-09-13). Data as JSON: /api/errors/b2e3b22b2a682807. Report an issue: GitHub.

Appendix: source

Thrown at src/io/reader/command.rs:59

    }
}

#[cfg(not(windows))]
fn spawn_command(command: String) -> Result<CommandReader> {
    use crate::io::reader::line_batcher::BUF_READER_CAPACITY;
    use anyhow::Context;
    use std::process::{Command, Stdio};

    let trap_command = format!("trap '' INT; {command}");

    let mut sh = Command::new("sh");
    sh.arg("-c").arg(trap_command).stdout(Stdio::piped());

    let child = SharedChild::spawn(&mut sh).context("Could not spawn process")?;

    let stdout = child
        .take_stdout()
        .ok_or_else(|| anyhow!("Could not capture stdout of spawned process"))?;

    let reader = BufReader::with_capacity(BUF_READER_CAPACITY, stdout);

    Ok(CommandReader {
        reader,
        child: Arc::new(child),
        initial_read_complete_sent: false,
    })
}

#[cfg(windows)]
fn spawn_command(_command: String) -> Result<CommandReader> {
    Err(anyhow!("The --exec flag is not supported on Windows"))
}

impl Drop for CommandReader {
    fn drop(&mut self) {
        let _ = self.child.kill();

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