NilAway

NilAway is a static analysis tool that seeks to help developers avoid nil panics in production by catching them at compile time rather than runtime. NilAway is similar to the standard nilness analyzer, however, it employs much more sophisticated and powerful static analysis techniques to track nil flows within a package as well across packages, and report errors providing users with the nilness flows for easier debugging.

NilAway enjoys three key properties that make it stand out:

• It is fully-automated: NilAway is equipped with an inference engine, making it require no any additional information from the developers (e.g., annotations) besides standard Go code.

• It is fast: we have designed NilAway to be fast and scalable, making it suitable for large codebases. In our measurements, we have observed less than 5% build-time overhead when NilAway is enabled. We are also constantly applying optimizations to further reduce its footprint.

• It is practical: it does not prevent all possible nil panics in your code, but it catches most of the potential nil panics we have observed in production, allowing NilAway to maintain a good balance between usefulness and build-time overhead.

Installation

NilAway is implemented using the standard go/analysis framework, making it easy to integrate with existing analyzer drivers (e.g., golangci-lint, nogo, or running as a standalone checker). Here, we list the instructions for running NilAway as a standalone checker. More integration supports will be added soon.

Standalone Checker

Install the binary from source by running:

go install go.uber.org/nilaway/cmd/nilaway@latest

Then, run the linter by:

nilaway ./...

Code Examples

Let's look at a few examples to see how NilAway can help prevent nil panics.

// Example 1:
var p *P
if someCondition {
      p = &P{}
}
print(p.f) // nilness reports NO error here, but NilAway does.

In this example, the local variable p is only initialized when someCondition is true. At the field access p.f, a panic may occur if someCondition is false. NilAway is able to catch this potential nil flow and reports the following error showing this nilness flow:

go.uber.org/example.go:12:9: error: Potential nil panic detected. Observed nil flow from source to dereference point:
    -> go.uber.org/example.go:12:9: unassigned variable `p` accessed field `f`

If we guard this dereference with a nilness check (if p != nil), the error goes away.

NilAway is also able to catch nil flows across functions. For example, consider the following code snippet:

// Example 2:
func foo() *int {
      return nil
}
func bar() {
     print(*foo()) // nilness reports NO error here, but NilAway does.
}

In this example, the function foo returns a nil pointer, which is directly dereferenced in bar, resulting in a panic whenever bar is called. NilAway is able to catch this potential nil flow and reports the following error, describing the nilness flow across function boundaries:

go.uber.org/example.go:23:13: error: Potential nil panic detected. Observed nil flow from source to dereference point:
    -> go.uber.org/example.go:20:14: literal `nil` returned from `foo()` in position 0
    -> go.uber.org/example.go:23:13: result 0 of `foo()` dereferenced

Note that in the above example, foo does not necessarily have to reside in the same package as bar. NilAway is able to track nil flows across packages as well. Moreover, NilAway handles Go-specific language constructs such as receivers, interfaces, type assertions, type switches, and more. For more detailed discussion, please check our paper.

Configurations

We expose a set of flags via the standard flag passing mechanism in go/analysis. Please check wiki/Configuration to see the available flags and how to pass them using different linter drivers.

Support

We follow the same version support policy as the Go project: we support and test the last two major versions of Go.

Please feel free to open a GitHub issue if you have any questions, bug reports, and feature requests.

Contributions

We'd love for you to contribute to NilAway! Please note that once you create a pull request, you will be asked to sign our Uber Contributor License Agreement.

License

This project is copyright 2023 Uber Technologies, Inc., and licensed under Apache 2.0.