What causes "Error creating qlog file attempted path was {path:?}: {e}" in cloudflare/quiche?

Create the target directory first (mkdir -p ) and re-run.

Error creating qlog file attempted path was

Error message

Error creating qlog file attempted path was {path:?}: {e}

What it means

h3i panics when it cannot create the qlog file at the path the user supplied on the command line, right before writing session traces. std::fs::File::create failed, which means the directory doesn't exist, the path is invalid, or the process lacks write permission. This is a hard abort: without a qlog sink the session cannot be recorded.

Solutions

Create the target directory first (mkdir -p <dir>) and re-run.
Check write permissions on the directory, or run from a writable location.
Verify the --qlog filename argument for typos and use an absolute path to avoid working-directory surprises.

Example fix

// before
h3i --qlog out/trace.qlog ...
// after
mkdir -p out && h3i --qlog out/trace.qlog ...

Defensive patterns

Strategy: validation

Validate before calling

// shell
[ -d "$(dirname "$QLOG_PATH")" ] && [ -w "$(dirname "$QLOG_PATH")" ] || mkdir -p "$(dirname "$QLOG_PATH")"

Prevention

Always create the qlog output directory before running h3i.
Use absolute, writable paths in CI sandboxes.

When it happens

Trigger: Running the h3i binary with a --qlog-style filename whose parent directory is missing, unwritable, or whose path is otherwise invalid, causing File::create to return Err.

Common situations: Typo in output directory; running in a read-only container or restricted CI sandbox; relative path resolved from an unexpected working directory; filename containing characters invalid on the filesystem.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

AI-assisted analysis of cloudflare/quiche@9f96daa2c2 (2026-09-08). Data as JSON: /api/errors/3ddfc4872729abb2. Report an issue: GitHub.

Appendix: source

Thrown at h3i/src/main.rs:448

}

/// Makes a buffered writer for a qlog.
pub fn make_qlog_writer() -> std::io::BufWriter<std::fs::File> {
    let mut path = std::env::current_dir().unwrap();
    let now = time::SystemTime::now();
    let filename = format!(
        "{}-qlog.sqlog",
        now.duration_since(time::UNIX_EPOCH).unwrap().as_millis()
    );
    path.push(filename.clone());

    log::info!("Session will be recorded to {filename}");

    match std::fs::File::create(&path) {
        Ok(f) => std::io::BufWriter::new(f),

        Err(e) =>
            panic!("Error creating qlog file attempted path was {path:?}: {e}"),
    }
}

pub fn make_streamer(
    writer: Box<dyn std::io::Write + Send + Sync>,
) -> qlog::streamer::QlogStreamer {
    let vp = qlog::VantagePointType::Client;

    let trace = qlog::TraceSeq::new(
        Some("h3i".into()),
        Some("h3i".into()),
        None,
        Some(qlog::VantagePoint {
            name: None,
            ty: vp,
            flow: None,
        }),
        vec![QUIC_URI.to_string(), HTTP3_URI.to_string()],

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