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https://github.com/bracesdev/errtrace
An alternative to stack traces for your Go errors
https://github.com/bracesdev/errtrace
golang
Last synced: about 1 month ago
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An alternative to stack traces for your Go errors
- Host: GitHub
- URL: https://github.com/bracesdev/errtrace
- Owner: bracesdev
- License: bsd-3-clause
- Created: 2023-11-03T15:18:42.000Z (about 1 year ago)
- Default Branch: main
- Last Pushed: 2024-02-21T06:46:03.000Z (10 months ago)
- Last Synced: 2024-04-08T22:04:09.997Z (8 months ago)
- Topics: golang
- Language: Go
- Homepage: https://pkg.go.dev/braces.dev/errtrace
- Size: 457 KB
- Stars: 687
- Watchers: 5
- Forks: 7
- Open Issues: 11
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- License: LICENSE
Awesome Lists containing this project
README
# errtrace
*What if every function added its location to returned errors?*
[](https://github.com/bracesdev/errtrace/actions/workflows/ci.yml)
[](https://pkg.go.dev/braces.dev/errtrace)
[](https://codecov.io/gh/bracesdev/errtrace)
- [Introduction](#introduction)
- [Features](#features)
- [Comparison with stack traces](#comparison-with-stack-traces)
- [Try it out](#try-it-out)
- [Why is this useful](#why-is-this-useful)
- [Installation](#installation)
- [Usage](#usage)
- [Manual instrumentation](#manual-instrumentation)
- [Automatic instrumentation](#automatic-instrumentation)
- [Performance](#performance)
- [Caveats](#caveats)
- [Error wrapping](#error-wrapping)
- [Safety](#safety)
- [Contributing](#contributing)
- [Acknowledgements](#acknowledgements)
- [License](#license)
## Introduction
errtrace is an **experimental** package to trace an error's return path —
the return trace — through a Go program.
Where a stack trace tracks the code path that led to an error,
a return trace tracks the code path that the error took to get to the user.
Often these are the same path, but in Go they can diverge,
since errors are values that can be transported across goroutines
(e.g. with channels).
When that happens, a return trace can be more useful than a stack trace.
This library is inspired by
[Zig's error return traces](https://ziglang.org/documentation/0.11.0/#Error-Return-Traces).
### Features
* **Lightweight**\
errtrace brings no other runtime dependencies with it.
* **[Simple](#manual-instrumentation)**\
The library API is simple, straightforward, and idiomatic.
* **[Easy](#automatic-instrumentation)**\
The errtrace CLI will automatically instrument your code.
* **[Fast](#performance)**\
On popular 64-bit systems,
errtrace is much faster than capturing a stack trace.
### Comparison with stack traces
With stack traces, caller information for the goroutine is
captured once when the error is created.
In constrast, errtrace records the caller information incrementally,
following the return path the error takes to get to the user.
This approach works even if the error isn't propagated directly
through function returns, and across goroutines.
Both approaches look similar when the error flows
through function calls within the same goroutine,
but can differ significantly when errors are passed
outside of functions and across goroutines (e.g., channels).
Here's a real-world example that shows the benefits
of errtrace tracing the return path
by comparing a custom dial error returned for a HTTP request,
which the net/http library uses a background goroutine for.
errtrace compared to a stack trace
errtracestack trace
```
Error: connect rate limited
braces.dev/errtrace_test.rateLimitDialer
/path/to/errtrace/example_http_test.go:72
braces.dev/errtrace_test.(*PackageStore).updateIndex
/path/to/errtrace/example_http_test.go:59
braces.dev/errtrace_test.(*PackageStore).Get
/path/to/errtrace/example_http_test.go:49
```
```
Error: connect rate limited
braces.dev/errtrace_test.rateLimitDialer
/errtrace/example_stack_test.go:81
net/http.(*Transport).dial
/goroot/src/net/http/transport.go:1190
net/http.(*Transport).dialConn
/goroot/src/net/http/transport.go:1625
net/http.(*Transport).dialConnFor
/goroot/src/net/http/transport.go:1467
runtime.goexit
/goroot/src/runtime/asm_arm64.s:1197
```
errtrace reports the method that triggered the HTTP request
stack trace shows details of how the HTTP client creates a connection
errtrace also reduces the performance impact
of capturing caller information for errors
that are handled rather than returned to the user,
as the information is captured incrementally.
Stack traces pay a fixed cost to capture caller information
even if the error is handled immediately by the caller
close to where the error is created.
### Try it out
Try out errtrace with your own code:
1. Install the CLI.
```bash
go install braces.dev/errtrace/cmd/errtrace@latest
```
2. Switch to your Git repository and instrument your code.
```bash
errtrace -w ./...
```
3. Let `go mod tidy` install the errtrace Go module for you.
```bash
go mod tidy
```
4. Run your tests to ensure everything still works.
You may see failures
if you're comparing errors with `==` on critical paths
or if you're type-casting errors directly.
See [Error wrapping](#error-wrapping) for more details.
```bash
go test ./...
```
5. Print return traces for errors in your code.
To do this, you can use the `errtrace.FormatString` function
or format the error with `%+v` in `fmt.Printf`-style functions.
```go
if err != nil {
fmt.Fprintf(os.Stderr, "%+v", err)
}
```
Return traces printed by errtrace
will include the error message
and the path the error took until it was printed.
The output will look roughly like this:
```
error message
example.com/myproject.MyFunc
/home/user/myproject/myfile.go:123
example.com/myproject.CallerOfMyFunc
/home/user/myproject/another_file.go:456
[...]
```
Here's a real-world example of errtrace in action:
Example
```
doc2go: parse file: /path/to/project/example/foo.go:3:1: expected declaration, found invalid
go.abhg.dev/doc2go/internal/gosrc.parseFiles
/path/to/project/internal/gosrc/parser.go:85
go.abhg.dev/doc2go/internal/gosrc.(*Parser).ParsePackage
/path/to/project/internal/gosrc/parser.go:44
main.(*Generator).renderPackage
/path/to/project/generate.go:193
main.(*Generator).renderTree
/path/to/project/generate.go:141
main.(*Generator).renderTrees
/path/to/project/generate.go:118
main.(*Generator).renderPackageIndex
/path/to/project/generate.go:149
main.(*Generator).renderTree
/path/to/project/generate.go:137
main.(*Generator).renderTrees
/path/to/project/generate.go:118
main.(*Generator).renderPackageIndex
/path/to/project/generate.go:149
main.(*Generator).renderTree
/path/to/project/generate.go:137
main.(*Generator).renderTrees
/path/to/project/generate.go:118
main.(*Generator).Generate
/path/to/project/generate.go:110
main.(*mainCmd).run
/path/to/project/main.go:199
```
Note the some functions repeat in this trace
because the functions are mutually recursive.
### Why is this useful?
In Go, [errors are values](https://go.dev/blog/errors-are-values).
This means that an error can be passed around like any other value.
You can store it in a struct, pass it through a channel, etc.
This level of flexibility is great,
but it can also make it difficult to track down the source of an error.
A stack trace stored in an error — recorded at the error site —
becomes less useful as the error moves through the program.
When it's eventually surfaced to the user,
we've lost a lot of context about its origin.
With errtrace,
we instead record the path the program took from the error site
to get to the user — the **return trace**.
Not only can this be more useful than a stack trace,
it tends to be much faster and more lightweight as well.
## Installation
Install errtrace with Go modules:
```bash
go get braces.dev/errtrace@latest
```
If you want to use the CLI, use `go install`.
```bash
go install braces.dev/errtrace/cmd/errtrace@latest
```
## Usage
errtrace offers the following modes of usage:
* [Manual instrumentation](#manual-instrumentation)
* [Automatic instrumentation](#automatic-instrumentation)
### Manual instrumentation
```go
import "braces.dev/errtrace"
```
Under manual instrumentation,
you're expected to import errtrace,
and wrap errors at all return sites like so:
```go
// ...
if err != nil {
return errtrace.Wrap(err)
}
```
Example
Given a function like the following:
```go
func writeToFile(path string, src io.Reader) error {
dst, err := os.Create(path)
if err != nil {
return err
}
defer dst.Close()
if _, err := io.Copy(dst, src); err != nil {
return err
}
return nil
}
```
With errtrace, you'd change it to:
```go
func writeToFile(path string, src io.Reader) error {
dst, err := os.Create(path)
if err != nil {
return errtrace.Wrap(err)
}
defer dst.Close()
if _, err := io.Copy(dst, src); err != nil {
return errtrace.Wrap(err)
}
return nil
}
```
It's important that the `errtrace.Wrap` function is called
inside the same function that's actually returning the error.
A helper function will not suffice.
### Automatic instrumentation
If manual instrumentation is too much work (we agree),
we've included a tool that will automatically instrument
all your code with errtrace.
First, [install the tool](#installation).
Then, run it on your code:
```bash
errtrace -w path/to/file.go path/to/another/file.go
```
Instead of specifying individual files,
you can also specify a Go package pattern.
For example:
```bash
errtrace -w example.com/path/to/package
errtrace -w ./...
```
errtrace can be set be setup as a custom formatter in your editor,
similar to gofmt or goimports.
#### Opting-out during automatic instrumentation
If you're relying on automatic instrumentation
and want to ignore specific lines from being instrumented,
you can add a comment in one of the following forms
on relevant lines:
```go
//errtrace:skip
//errtrace:skip(explanation)
//errtrace:skip // explanation
```
This can be especially useful if the returned error
has to match another error exactly because the caller still uses `==`.
For example, if you're implementing `io.Reader`,
you need to return `io.EOF` when you reach the end of the input.
Wrapping it will cause functions like `io.ReadAll` to misbehave.
```go
type myReader struct{/* ... */}
func (*myReader) Read(bs []byte) (int, error) {
// ...
return 0, io.EOF //errtrace:skip(io.Reader expects io.EOF)
}
```
## Performance
errtrace is designed to have very low overhead
on [supported systems](#supported-systems).
Benchmark results for linux/amd64 on an Intel Core i5-13600 (best of 10):
```
BenchmarkFmtErrorf 11574928 103.5 ns/op 40 B/op 2 allocs/op
# default build, uses Go assembly.
BenchmarkWrap 78173496 14.70 ns/op 24 B/op 0 allocs/op
# build with -tags safe to avoid assembly.
BenchmarkWrap 5958579 198.5 ns/op 24 B/op 0 allocs/op
# benchext compares capturing stacks using pkg/errors vs errtrace
# both tests capture ~10 frames,
BenchmarkErrtrace 6388651 188.4 ns/op 280 B/op 1 allocs/op
BenchmarkPkgErrors 1673145 716.8 ns/op 304 B/op 3 allocs/op
```
Stack traces have a large initial cost,
while errtrace scales with each frame that an error is returned through.
## Caveats
### Error wrapping
errtrace operates by wrapping your errors to add caller information.
As a result of this,
error comparisons and type-casting may not work as expected.
You can no longer use `==` to compare errors, or type-cast them directly.
You must use the standard library's
[errors.Is](https://pkg.go.dev/errors#Is) and
[errors.As](https://pkg.go.dev/errors#As) functions.
For example, if you have a function `readFile`
that wraps an `io.EOF` error with errtrace:
**Matching errors**
```go
err := readFile() // returns errtrace.Wrap(io.EOF)
// This will not work.
fmt.Println(err == io.EOF) // false
// Use errors.Is instead.
fmt.Println(errors.Is(err, io.EOF)) // true
```
Similarly, if you have a function `runCmd`
that wraps an `exec.ExitError` error with errtrace:
**Type-casting errors**
```go
err := runCmd() // returns errtrace.Wrap(&exec.ExitError{...})
// This will not work.
exitErr, ok := err.(*exec.ExitError) // ok = false
// Use errors.As instead.
var exitErr *exec.ExitError
ok := errors.As(err, &exitErr) // ok = true
```
#### Linting
You can use [go-errorlint](https://github.com/polyfloyd/go-errorlint)
to find places in your code
where you're comparing errors with `==` instead of using `errors.Is`
or type-casting them directly instead of using `errors.As`.
### Safety
To achieve the performance above on [supported systems](#supported-systems),
errtrace makes use of unsafe operations using Go assembly
to read the caller information directly from the stack.
This is part of the reason why we have the disclaimer on top.
errtrace includes an opt-in safe mode
that drops these unsafe operations in exchange for poorer performance.
To opt into safe mode,
use the `safe` build tag when compiling code that uses errtrace.
```bash
go build -tags safe
```
#### Supported systems
errtrace's unsafe operations are currently implemented
for `GOARCH=amd64` and `GOARCH=arm64` only.
Other systems are supported but they will use safe mode, which is slower.
Contributions to support unsafe mode for other architectures are welcome.
## Contributing
Contributions are welcome.
However, we ask that before contributing new features,
you [open an issue](https://github.com/bracesdev/errtrace/issues)
to discuss the feature with us.
## Acknowledgements
The idea of tracing return paths instead of stack traces
comes from [Zig's error return traces](https://ziglang.org/documentation/0.11.0/#Error-Return-Traces).
## License
This software is made available under the BSD3 license.
See LICENSE file for details.