15.1.3. 获取复杂错误场景的回溯
本实例展示了如何处理一个复杂的错误场景,并且打印出错误回溯。依赖于 chain_err
,通过附加新的错误来扩展错误信息。从而可以展开错误堆栈,这样提供了更好的上下文来理解错误的产生原因。
下述代码尝试将值 256
反序列化为 u8
。首先 Serde 产生错误,然后是 csv,最后是用户代码。
use error_chain::error_chain; use serde::Deserialize; use std::fmt; error_chain! { foreign_links { Reader(csv::Error); } } #[derive(Debug, Deserialize)] struct Rgb { red: u8, blue: u8, green: u8, } impl Rgb { fn from_reader(csv_data: &[u8]) -> Result<Rgb> { let color: Rgb = csv::Reader::from_reader(csv_data) .deserialize() .nth(0) .ok_or("Cannot deserialize the first CSV record")? .chain_err(|| "Cannot deserialize RGB color")?; Ok(color) } } impl fmt::UpperHex for Rgb { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { let hexa = u32::from(self.red) << 16 | u32::from(self.blue) << 8 | u32::from(self.green); write!(f, "{:X}", hexa) } } fn run() -> Result<()> { let csv = "red,blue,green 102,256,204"; let rgb = Rgb::from_reader(csv.as_bytes()).chain_err(|| "Cannot read CSV data")?; println!("{:?} to hexadecimal #{:X}", rgb, rgb); Ok(()) } fn main() { if let Err(ref errors) = run() { eprintln!("Error level - description"); errors .iter() .enumerate() .for_each(|(index, error)| eprintln!("└> {} - {}", index, error)); if let Some(backtrace) = errors.backtrace() { eprintln!("{:?}", backtrace); } // In a real use case, errors should handled. For example: // ::std::process::exit(1); } }
错误回溯信息如下:
Error level - description
└> 0 - Cannot read CSV data
└> 1 - Cannot deserialize RGB color
└> 2 - CSV deserialize error: record 1 (line: 2, byte: 15): field 1: number too large to fit in target type
└> 3 - field 1: number too large to fit in target type
可以通过附加命令参数 RUST_BACKTRACE=1
运行实例,以显示与此错误相关的详细回溯。