emit_json Silently Discards Serialization and Write Errors

Problem Statement

emit_json in main.rs discards both the serialization result and the write result with let _. The BufWriter is dropped without explicit flush, so flush errors are also silently swallowed. A calling agent receives no JSON on stdout and has no way to distinguish this from a successful empty response vs a pipe error. The process exits with code 0 despite producing no output.

Findings

File: src/main.rs:122-127

fn emit_json(value: &serde_json::Value) {
    let stdout = std::io::stdout();
    let mut writer = BufWriter::new(stdout.lock());
    let _ = serde_json::to_writer(&mut writer, value);  // error discarded
    let _ = writer.write_all(b"\n");                     // error discarded
    // BufWriter drop: flush errors silently swallowed
}

On a broken pipe (agent killed the read end), this returns normally and main() exits 0. The agent gets no output and no error code.

Proposed Solutions

Option A: Flush explicitly and handle errors (Recommended)

fn emit_json(value: &serde_json::Value) {
    let stdout = std::io::stdout();
    let mut writer = BufWriter::new(stdout.lock());
    if serde_json::to_writer(&mut writer, value).is_err()
        || writer.write_all(b"\n").is_err()
        || writer.flush().is_err()
    {
        std::process::exit(3);  // distinct exit code for I/O failure
    }
}

Effort: Tiny
Risk: Low — explicit flush guarantees output or a known failure

Option B: Use writeln! and propagate errors to main

Return Result<(), io::Error> from emit_json, handle in main().

Effort: Small
Risk: Low

Recommended Action

Option A: explicit flush with a distinct exit code (3) for stdout I/O failure.

Technical Details

File: src/main.rs
Lines: 122–127
Component: JSON output path

Acceptance Criteria

emit_json flushes the BufWriter explicitly
Serialization/write errors result in non-zero exit code
Broken pipe does not produce exit code 0 with no output

Work Log

2026-02-19: Finding identified by data-integrity-guardian review agent

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