What causes "Aborted from C" in windmill-labs/windmill?

Inspect the C# code/log immediately before the abort to find the failing assertion or runtime condition

Aborted from C

Error message

Aborted from C

What it means

The C# parser compiles C# to WASM using a minimal libc shim. This export implements abort() from stdlib.h: when compiled C# code (or its runtime/libc) calls abort(), the shim panics with 'Aborted from C' to unwind and surface the abort inside the WASM host. The message means the parsed/compiled program executed an abort path rather than a normal return.

Source

Thrown at backend/parsers/windmill-parser-csharp/src/wasm_libc.rs:15

use std::collections::BTreeMap;
use std::sync::{Mutex, OnceLock};
use std::{
    alloc::{self, Layout},
    ffi::{c_char, c_int, c_void},
    mem::align_of,
    ptr,
};
use wasm_bindgen::prelude::*;

/* -------------------------------- stdlib.h -------------------------------- */

#[no_mangle]
pub unsafe extern "C" fn abort() {
    panic!("Aborted from C");
}

macro_rules! console_log {
    ($($t:tt)*) => (unsafe { log(&format_args!($($t)*).to_string()) })
}

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(a: &str);
}

#[no_mangle]
pub unsafe extern "C" fn malloc(size: usize) -> *mut c_void {
    if size == 0 {
        return ptr::null_mut();
    }

View on GitHub (pinned to e474e8803c)

Solutions

Inspect the C# code/log immediately before the abort to find the failing assertion or runtime condition
Update the parser's WASM runtime dependencies if the abort comes from a known upstream runtime bug
Reproduce with a minimal C# script to isolate the code path that triggers abort()
Report/upgrade via windmill-parser-csharp if the abort originates in the vendored libc/runtime shim

Example fix

null

Defensive patterns

Strategy: try-catch

Validate before calling

// sanity-check C# input before handing it to the parser
if strings.TrimSpace(csSource) == "" {
    return errors.New("empty C# source will not compile")
}

Try / catch

// catch panics from the WASM parser boundary
func safeParse(src []byte) (out []byte, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("csharp parser aborted: %v", r)
        }
    }()
    return parseCSharp(src), nil
}

Prevention

Keep the vendored WASM runtime/libc shim up to date
Isolate failing C# snippets to find what triggers abort()
Catch panics at the WASM boundary in the host so aborts become errors, not crashes
Check parser logs immediately preceding the abort for assertion messages

When it happens

Trigger: C# code under windmill-parser-csharp invoking abort() — e.g. assert failures, unhandled fatal conditions in the Mono/runtime WASM runtime, or stdlib abort calls triggered during script parsing/compilation.

Common situations: Runtime assertions firing in WASM-compiled C# workers; memory/allocation failures inside the embedded runtime; malformed C# that drives a library into an abort path during preprocessing.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/ac18873515627952. Report an issue: GitHub.