rust-lang/rust-analyzer · error

profiler failed to start

Error message

profiler failed to start

What it means

After atomically transitioning OFF -> PENDING, `start` invokes the gperftools `ProfilerStart` C function. If it returns 0 the native profiler failed to begin recording, and the wrapper panics with 'profiler failed to start'.

Source

Thrown at crates/profile/src/google_cpu_profiler.rs:33

}

const OFF: usize = 0;
const ON: usize = 1;
const PENDING: usize = 2;

fn transition(current: usize, new: usize) -> bool {
    static STATE: AtomicUsize = AtomicUsize::new(OFF);

    STATE.compare_exchange(current, new, Ordering::SeqCst, Ordering::SeqCst).is_ok()
}

pub(crate) fn start(path: &Path) {
    if !transition(OFF, PENDING) {
        panic!("profiler already started");
    }
    let path = CString::new(path.display().to_string()).unwrap();
    if unsafe { ProfilerStart(path.as_ptr()) } == 0 {
        panic!("profiler failed to start")
    }
    assert!(transition(PENDING, ON));
}

pub(crate) fn stop() {
    if !transition(ON, PENDING) {
        panic!("profiler is not started")
    }
    unsafe { ProfilerStop() };
    assert!(transition(PENDING, OFF));
}

View on GitHub (pinned to e8f7e90aa3)

Solutions

1. Verify the output directory exists and is writable before calling start
2. Check the path for invalid characters and that the filesystem has space
3. Wrap profiler startup in a check and fall back to running unprofiled if the environment disallows it

Example fix

// before
profiler::start(&Path::new("/proc/1/cpu.prof"));
// after
let path = Path::new("/tmp/cpu.prof");
if path.parent().map_or(false, |d| d.is_dir()) {
    profiler::start(path);
}

fn profiler_output_ok(path: &Path) -> bool {
    path.parent().map_or(false, |d| d.is_dir())
        && path.extension().is_some()
}

std::panic::catch_unwind(|| profiler::start(path))
    .err()
    .map(|_| eprintln!("profiling unavailable; continuing unprofiled"));

When it happens

Trigger: `ProfilerStart(path)` returning 0, typically because the output file at `path` cannot be created/written (bad directory, permissions, invalid path bytes).

Common situations: Passing a profile output path in a non-existent or read-only directory; running in a sandboxed/containerized environment without write access; path containing interior NUL would instead hit the CString unwrap earlier.

Related errors

• profiler already started
• profiler is not started
• We explicitly do not provide canonicalization API, as that i
• bad kind {other}
• bad spacing {other}

AI-assisted analysis of rust-lang/rust-analyzer@e8f7e90aa3 (2026-09-03). Data as JSON: /api/errors/31be6ad582dc565b. Report an issue: GitHub.