What causes "Failed to serialize health check: {}" in dmtrKovalenko/fff?

Retry the call — transient allocation failures may resolve.

Failed to serialize health check

Error message

Failed to serialize health check: {}

What it means

fff_health_check builds a health JSON object and serializes it with serde_json::to_string; if serialization fails, this error is returned. This is rare because the input is a constructed serde_json::Value, but it guards the FFI boundary against any serialization failure.

Solutions

Retry the call — transient allocation failures may resolve.
Check process memory availability if running in constrained environments.
Verify you are using a compatible serde_json version if linking statically.
Report to maintainers if reproducible — serializing a built Value should not fail.

Example fix

null

Defensive patterns

Strategy: try-catch

Try / catch

local res = fff.health_check(inst)
if res.is_err then health_json = '{}' end

Prevention

Ensure adequate process memory in constrained environments.
Retry once on transient serialization failures.
Report reproducible failures upstream — this should never fire.

When it happens

Trigger: Calling fff_health_check when serde_json cannot serialize the assembled health map — practically only due to a serde_json internal failure such as an exhausted allocator or a non-string map key that slipped into the construction.

Common situations: Extremely low-memory environments; custom/patched serde_json builds with strict settings; latent library bug putting invalid data into the health object.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

AI-assisted analysis of dmtrKovalenko/fff@7f8537e70f (2026-09-10). Data as JSON: /api/errors/146e729518f9396f. Report an issue: GitHub.

Appendix: source

Thrown at crates/fff-c/src/lib.rs:1338

                        serde_json::Value::Object(db_health),
                    );
                }
            }
            Err(_) => {
                query_info.insert("initialized".to_string(), serde_json::Value::Bool(false));
            }
        }
    } else {
        query_info.insert("initialized".to_string(), serde_json::Value::Bool(false));
    }
    health.insert(
        "query_tracker".to_string(),
        serde_json::Value::Object(query_info),
    );

    match serde_json::to_string(&health) {
        Ok(json) => FffResult::ok_string(&json),
        Err(e) => FffResult::err(&format!("Failed to serialize health check: {}", e)),
    }
}

/// Free a search result returned by `fff_search`: the struct, its `items`
/// and `scores` arrays, and every string within.
///
/// ## Safety
/// `result` must be a valid pointer previously returned via `FffResult.handle`
/// from `fff_search`, or null (no-op).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fff_free_search_result(result: *mut FffSearchResult) {
    if result.is_null() {
        return;
    }

    unsafe {
        let result = Box::from_raw(result);
        let count = result.count as usize;

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