facebook/flow · error

Failed to open log file '{}': {}

Error message

Failed to open log file '{}': {}

What it means

When the Flow server daemonizes, open_log_file() opens its log with create+append. try_open_log_file() first rotates an existing log to <file>.old (best-effort) and then opens the target; any open error is turned into this message and open_log_file panics on it. The log lives under the server temp dir, so failures here are almost always temp-dir permission or existence problems.

Source

Thrown at rust_port/crates/flow_server/src/server_daemon.rs:246

            }
            fs::rename(file, &old_file)?;
            Ok(())
        })() {
            eprintln!(
                "Log rotate: failed to move '{}' to '{}'\n{}",
                file, old_file, e
            );
        }
    }
    fs::OpenOptions::new()
        .create(true)
        .append(true)
        .open(file)
        .map_err(|e| format!("Failed to open log file '{}': {}", file, e))
}

pub fn open_log_file(file: &str) -> fs::File {
    try_open_log_file(file).unwrap_or_else(|e| panic!("{}", e))
}
pub fn daemonize(
    init_id: &str,
    log_file: &str,
    argv: &[String],
    lazy_mode: Option<String>,
    no_flowlib: bool,
    ignore_version: bool,
    options: Arc<Options>,
    file_watcher_pid: Option<u32>,
    start_cause: flow_server_env::server_status::StartCause,
    cli_overrides: &CliOverrides,
) -> Result<Handle<(), ()>, String> {
    let entry = registered_entry_point();

    let root = &options.root;
    let tmp_dir = &options.temp_dir;
    let flowconfig_name = &options.flowconfig_name;

View on GitHub (pinned to f88ac94bcf)

Solutions

1. From the panic message, check the log directory: mkdir -p the parent and chown/chmod it to the daemon user.
2. Remove stale root-owned log and .old files: rm <file> <file>.old under the temp dir.
3. Set temp_dir (or TMPDIR) to a location the daemon user can write and restart.
4. Check df -h and mount ro flags when permissions look correct.

Example fix

# before: temp dir missing / owned by another user
flow daemon start ...  # panic: Failed to open log file '/tmp/flow/x/log': ... 

# after
mkdir -p /tmp/flow/x && chown "$(id -un)" /tmp/flow/x && flow daemon start ...

// before daemonize()/open_log_file(file)
let dir = std::path::Path::new(file).parent().ok_or("log path has no parent")?;
std::fs::create_dir_all(dir)?;
let f = std::fs::OpenOptions::new().create(true).append(true).open(file)?;
drop(f); // proves the real open will succeed

match flow_server::server_daemon::try_open_log_file(file) {
    Ok(f) => f,
    Err(e) => {
        eprintln!("cannot open log, falling back to stderr logging: {e}");
        // fall back or abort with a clean message instead of the panic
        return Err(e);
    }
}

When it happens

Trigger: daemonize()/open_log_file() running when the log path's directory does not exist, the daemon user cannot write there, the path component is a directory, SELinux/AppArmor denies the append, or the filesystem is full/read-only.

Common situations: TMPDIR or the temp_dir option pointing to a deleted directory; running the daemon under a different user than the one that created the temp dir; root-owned log files left behind by a previous sudo run; read-only container mounts; full disks.

Related errors

• fd_of_path: mkdir_no_fail({:?}): {}
• fd_of_path: open({:?}): {}
• mkdir_no_fail({:?}): {}
• failed to create {}: {}
• mkdirp: mkdir {} failed: {}

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/3a48185c7c1255a1. Report an issue: GitHub.